Topic 8 - Deans Community High School · Web viewMetal + Water Metal hydroxide + Hydrogen Potassium reacts vigorously, sodium very quickly, calcium quickly and magnesium slowly. Potassium

Topic 8 Acids and Alkalis1. Non metal elements can react with air or oxygen to form compounds

called oxides. C + O2 CO2 S + O2 SO2 N2 + O2 NO2

2. Non-metal oxides which dissolve in water produce acids e.g. CO2, NO2, SO2, SO3.

3. SO2 and NO2 react with water to form acid rain. H2O + SO2

H2SO3 (sulphurous acid)

4. Acid rain damages buildings, soil, and plant and animal life. 5. Oxides of non-metals which do not dissolve do not affect the pH. 6. Oxides of metals or hydroxides of metals which dissolve in water produce

alkaline solutions. The data book gives information about which ones dissolve or react.

Na2O + H2O

NaOH

7. The oxides of Group 1 metals and some of some Group 2 metals produce alkaline solutions with water.

 The pH scale

The pH scale is a continuous range of numbers from 1 to 14 which indicate the acidity or alkalinity of solutions.

Acids have a pH of less than 7. (pH<7) Alkalis have a pH of more than 7. (pH>7) Pure water and neutral solutions have a pH equal to 7. (pH=7)

Acids in the Laboratory and in the Home

Chemical Name

Formula pH Uses

Hydrochloric acid HCl 1-3 Common laboratory

acid

Nitric acid HNO3 1-3 Common laboratory acid

Sulphuric acid H2SO4 1-3 Common laboratory acid

Vinegar N/A 4-6 Common household acid

Lemon juice N/A 4-6 Common household acid

Car battery acid N/A 1-3 Common household acid

All acid solutions contain Hydrogen ions (H+(aq)) and have pH values < 7. If an electric current is passed through any acid solution, the H+ ions are changed into hydrogen gas at the negative electrode. Hydrogen gas pops when lit.

Alkalis in the Laboratory and in the Home

Chemical Name Formula pH Uses

Sodium hydroxide NaOH 11-14

Common laboratory alkali

Potassium hydroxide KOH 11-

14Common laboratory

alkali

Calcium hydroxide Ca(OH)211-14

Common laboratory alkali

Cleaning fluids N/A 8-14 Common household alkali

Toothpaste N/A 8-11 Common household alkali

Indigestion tablets N/A 8-11 Common household alkali

All alkaline solutions contain Hydroxide ions (OH-(aq)) and have pH values > 7

Water has a very low concentration of ions

 CREDIT

In water and any neutral solution, the concentration of H+ ions and OH- ions is the same and this gives a pH = 7 (neutral).

In an acid solution the concentration of hydrogen ions (H+(aq)) is more than that in pure water.

In an alkaline solution the concentration of hydroxide ions (OH-(aq)) is more than that in pure water.

 GENERAL

Diluting acids and alkalis

Adding more water to an acid increases the pH of the solution towards 7 making it less acidic. (The acidity decreases and the pH value increases).

Adding more water to an alkali decreases the pH of the solution towards 7 making it less alkaline. (The alkalinity decreases and the pH value decreases).

NB In both cases, the pH number moves towards 7

 CREDIT

Diluting an acid decreases the concentration H+(aq) ions. Diluting an alkali decreases the concentration OH-(aq) ions.

CREDIT CALCULATIONS

1. Formula mass is calculated by substituting relative atomic mass values from the data book and adding them together in the same numbers as in the formula.e.g. Calcium carbonate has the formula CaCO3 and contains 1 atom of calcium, 1 atom of carbon and 3 atoms of oxygen. Using relative atomic mass values :-

40 + 12 + (16 x 3) = 52 + 48 = 100

Formula mass = 100

2. If the formula mass is written in grams (g), it is known as the gram formula mass The gram formula mass of any substance is known as 1 mole.

e.g. Water, (H2O) has a formula mass of (1 x 2) + 16 = 18

Water, (H2O) has a gram formula mass of 18g

1 mole of water weighs 18g.

3. To change masses of substances into moles, divide the mass of the substance by the mass of 1 mole of the substance.

Number of moles = Mass of substance / Mass of 1 mole of substance.

e.g. How many moles are there in 4.5g of water?

Number of moles = 4.5g / 18g = 0.25 moles

4. The concentration of a solution is measured in mol / litre (mol / l)

The concentration, number of moles of substance and volume (in litres) are linked by the triangle shown on the right.

n = number of moles (mol)c = concentration (mol/l)

v = volume (l)

Some example questions 1. If 30g of sodium hydroxide is dissolved in 500 ml of water, what will be the concentration of the solution? If 1 mole of NaOH weighs (23 + 16 +1) = 40gThen number of moles in 30g = 30g / 40g = 0.75 molesFrom the moles / volume / concentration triangle - Concentration (in mol / l) = Number of moles / Volume of solution in litres500 ml = 500 / 1000 = 0.5 litresConcentration = 0.75 / 0.5 = 1.50 mol / l 2. If the concentration of a solution of hydrochloric acid is 0.5 mol / litre, what mass of acid will be present in 100 ml (0.1 litres) of this acid? Number of moles = Concentration x Volume

= 0.5 x 0.1= 0.05 moles of Hydrochloric acid

1 mole of HCl weighs 1 + 35.5 = 36.5gSo 0.05 moles weighs 36.5 x 0.05 = 1.825 g

 NEW WORDS AND THEIR MEANINGS

ACID RAIN - Formed when non-metal oxide gases dissolve in rain water.

PH SCALE - A continuous scale which tells us how acid or alkaline a solution is.

ACID - A substance with a pH value lower than 7.

ALKALI - A substance with a pH value greater than 7.

NEUTRAL - A substance with a pH value of exactly 7.

FORMULA MASS - The mass of a substance based on the total mass of each of the atoms that make it up.

GRAM FORMULA MASS - The formula mass of a substance expressed in grams

MOLE - 1 mole is the gram fomula mass of a substance.

Topic 9 - Reactions of Acids (General and Credit level)

 GENERAL

Neutralisation is the reaction of acids with neutralisers and at the same time the pH value moves towards 7.

In Topic 8 we learned that acids contain H+ ions and water contains few ions. When an acid is neutralised the number of H+ ions decreases.

Neutralisation in everyday life

1. Adding lime (an alkali) to soil to reduce acidity. 2. Adding lime to lakes to reduce the effect of acid rain.

3. Taking indigestion tablets to treat acid indigestion. 4. Using toothpaste to cancel mouth acidity. 5. Acid rain damaging building and carbonate rocks. 6. Acid rain damaging metal items (cars).

Reactions of Acids with Neutralisers (examples of neutralisation)

Acid + Alkali Salt + Water Acid + Metal Oxide Salt + Water Acid + Metal Carbonate

Salt + Water + Carbon dioxide

Acid + Metal Salt + Hydrogen

In the last reaction, hydrogen ions from the acid react with many metals to form hydrogen molecules. Some metals such as copper, silver and gold do not react with dilute acids. (Topic 11 has more details)

 CREDIT

What is a Salt? A salt is a substance in which the hydrogen ion of an acid has been replaced by a metal ion (or the ammonium ion).

When an acid is neutralised by an alkali, H+ ions from the acid join with OH- from the alkali to form water.

 GENERAL

Naming salts

1. Hydrochloric acid (HCl) always makes CHLORIDE salts 2. Nitric acid (HNO3) always makes NITRATE salts 3. Sulphuric acid (H2SO4) always makes SULPHATE salts

NB. Learn the formulae of these acids.NB. The acid forms the 'Surname' of the salt. e.g Sulphuric acid will form sulphate saltsNB. The neutraliser forms the 'Forename' of the salt. e.g. the neutraliser, sodium hydroxide (NaOH) will form sodium salts

e.g.

1. If hydrochloric acid is neutralised by potassium hydroxide, the salt potassium chloride will be formed (+ water)

2. If sulphuric acid is neutralised by sodium carbonate, the salt sodium sulphate will be formed (+ water + carbon dioxide)

3. If nitric acid is neutralised by calcium carbonate, the salt calcium nitrate will be formed (+ water + carbon dioxide)

4. If hydrochloric acid is neutralised by zinc metal, the salt zinc chloride will be formed (+ hydrogen)

 CREDIT

Making salts

Several methods are available

1. Volumetric Titration - In this process, acid is added in measured quantities from a burette to a measured volume of alkali in a flask containing indicator until the indicator shows that neutralisation has occurred.A pipette is used to accurately measure the volume of alkali.To obtain a pure salt, the experiment is repeated using the same exact volume but without the indicator.The salt can be obtained from the solution by evaporating it. This method is slow.

2. Add excess of an insoluble metal oxide or metal carbonate to an acid, and remove the unreacted substance by filtration, followed by evaporation of the filtrate to obtain the salt.This method is quick and doesn't require careful measuring. e.g. to make copper (II) sulphate, add excess copper (II) oxide to sulphuric acid or excess copper (II) carbonate to sulphuric acid.

3. Insoluble salts can be formed by precipitation. To make insoluble lead (II) iodide, mix together two separate solutions of soluble salts one containing lead (II) ions and the other containing iodide ions e.g. lead (II) nitrate solution and sodium iodide solution.

Pb(NO3)2(aq) + 2NaI(aq) PbI2(s) + 2NaNO3(aq)

Lead iodide is formed as a yellow precipitate which can be removed easily by filtering, followed by washing the precipitate with distilled water and then drying it. The method is quick and doesn't need careful measurement.

 GENERAL

Precipitation is the reaction of two solutions to form an insoluble product called a precipitate. The data book indicates the solubility of many substances.

e.g. Will a precipitate be formed if sodium carbonate solution is added to copper (II) sulphate solution?

As these two chemicals are both soluble salts, there will only be a precipitate formed if they can react to give an insoluble substance.

Possible products are sodium sulphate and copper (II) carbonate. The data book indicates that copper (II) carbonate is insoluble and so copper (II) carbonate will be formed as a precipitate and sodium sulphate will be left in the solution.

In general, to make the insoluble salt XY, mix solutions of X nitrate and sodium Y.

e.g. Will a precipitate be formed if potassium nitrate solution is added to sodium chloride solution?

As these two chemicals are both soluble salts (they could not be present as solutions if they were insoluble), there will only be a precipitate formed if they can react to give an insoluble substance.

Possible products are potassium chloride and sodium nitrate. The data book indicates that as both of these possible products are soluble in water, no precipitate will be formed. So there will be no reaction at all and the solution will contain a mixture of sodium particles, potassium particles, chloride particles and nitrate particles.

 CREDIT

A base is a substance which neutralises an acid. This means that alkalis (metal hydroxides), metal oxides, metal carbonates and metals are all examples of bases.

An alkali is made if the base dissolves in water. The data book indicates which of the bases previously mentioned dissolve in water. The list is small but included oxides, carbonates and hydroxides of Group 1 elements, of some Group 2 elements, and ammonium compounds e.g. ammonium hydroxide and ammonium carbonate. Metals do not dissolve in water.

At credit level you will need to be able to perform calculations of the concentration of acids/alkalis from volumetric titration. Your teacher will have explained this to you.

What is the concentration of sodium hydroxide solution if 25.0 ml of it is neutralised by 20.0 ml of 0.1 mol/l sulphuric acid?

Firstly, you should write a balanced chemical equation for the neutralisation reaction.

2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + H2O(l) Since we know the concentration (0.1 mol/l) and the volume of the acid used (20 ml, which is 0.02 l), we can calculate the number of moles of sulphuric acid used in the titration

For the sulphuric acid solution - H2SO4(aq)

number of moles of acid used =

concentration (mol/l)   x   volume (l)0.1 mol/l   x   0.02 l0.002 mol

From the balanced chemical equation, 2 moles of NaOH(aq) react with 1 mole of H2SO4(aq)Therefore, 0.004 mol of NaOH(aq) will react with 0.002 mol of H2SO4(aq)

Since we now know the number of moles (0.004 mol) and the volume (25 ml, which is the same as 0.025 l) of the sodium hydroxide solution used, we can calculate the concentration of the sodium hydroxide solution used in the titration.

For the sodium hydroxide solution - NaOH(aq)

Concentration of sodium hydroxide =

number of moles (mol)   /   volume used (l)0.004 mol   /   0.025 l0.16 mol/l

The concentration of sodium hydroxide used is 0.16 mol/l

 NEW WORDS AND THEIR MEANINGS

NEUTRALISATION - A chemical reaction in which an acid (or an alkali) is made neutral.

NEUTRALISER - A substance that can neutralise an acid or an alkali.

SALT - A chemical that is always made during a neutralisation reaction.

VOLUMETRIC TITRATION - A reaction in which accurate volumes used and concentration are noted in order to calculate the concentration of another solution

BURETTE - A piece of glassware with a tap used to slowly and accurately add solution during to another. When the reaction is complete, the volume of solution added can be accurately determined.

PIPETTE - A piece of glassware used to accurately measure out a set volume of solution to be used in a titration.

INDICATOR - A chemical added to a reaction which changes colour when the reaction is complete.

PRECIPITATION - A chemical reaction in which an insoluble solid is produced in a liquid.

PRECIPITATE - The name given to the solid produced in a precipitation reaction.

BASE - A substance that can neutralise an acid.

 Topic 10 Making Electricity

GENERAL

Chemical reactions can produce electricity.

A cell (often referred to as a battery) contains chemicals which react to make electricity.

The energy change is:

Chemical Energy Electrical Energy

In dry cells, the chemicals are used up and the cells then have to be replaced.

In a simple dry cell, the chemicals are shown in the diagram below.

The paste containing ammonium chloride is the electrolyte needed to complete the circuit ( See definition in Topic 7).

 Rechargeable cells.

In rechargeable cells the chemicals are not used up and can be regenerated by recharging the cell.

 Energy Changes in Cells

Using the cell (Discharging):

Chemical Energy Electrical Energy

Charging the cell:

Electrical Energy Chemical Energy

 Lead-sulphuric acid battery (Car battery)

This is charged using electricity and a brown coating appears on the positive lead plate.

When the charger is removed and replaced by a bulb, the bulb lights.

Gradually the bulb dims as the chemical energy in the brown coating is used up and the cell is discharged.

The cell now has to be recharged.

 Making electricity using two different metals

Electricity is produced when two different metals are dipped in an electrolyte and connected together with a wire.

There is a flow of electrons in the wire from one metal to the other, and ions move through the electrolyte.

(In topic 7 we learned that electricity is a flow of moving charged particles which can either be electrons or ions)

The data book (Page 7) gives a list of metals in a table called the electrochemical series.

When two different metals are joined together as shown in the above cell, electrons always flow through the wire from the metal higher in the electrochemical series to the metal lower in the series e.g. from Zinc to Copper.

If a voltmeter is used in the wire between the two metals, a different voltage is obtained when different metals are used.

The voltage values can be used to put the metals into the order shown in the electrochemical series

e.g. a larger voltage is obtained when magnesium is connected to copper than when zinc is connected to copper, and magnesium is placed above zinc in the electrochemical series.

.

 Displacement Reactions

When grey magnesium metal is added to blue copper(II)sulphate solution, brown copper metal is made and the solution becomes colourless.

In this reaction magnesium has pushed copper out of copper(II) sulphate solution as shown in the following equation.

Mg + CuSO4 MgSO4 + Cu

blue colourless

brown

A displacement reaction will happen when a metal higher in the electrochemical series is added to a solution containing a metal lower in the electrochemical seriesThe metal which is lower in the series is displaced or pushed out of its compound which is in solution.

CREDIT

Displacement

You are expected to decide if displacement will occur between a metal and a solution of another metal using the electrochemical series and also predict what changes would happen (mainly colour changes).

Remember that most metals are grey/silver except copper (red-brown) and gold, while most solutions are colourless except solutions containing transition metals such as copper (blue), and nickel (green), though solutions containing zinc are colourless.

When copper metal is added to colourless silver(I) nitrate solution, the colour changes will be the loss of the brown copper, the appearance of silver metal and the solution turning blue as the copper dissolves.

Cu + 2AgNO3 Cu(NO3)2 + 2Ag

brown

colourless blue silve

r

Copper has displaced silver from a compound containing silver ions.

Cu + 2Ag+NO3- Cu2+(NO3-)2 + 2Ag

When the nitrate spectator ions are removed, the reaction is between copper atoms and silver(I) ions.

Cu + 2Ag+ Cu2+ + 2Ag

 Position of hydrogen in the electrochemical series

Acid + Metal Salt + Hydrogen In this reaction, hydrogen is displaced from an acid by a metal.

As with other displacements, only metals above hydrogen can cause this to happen.e.g. magnesium will react but copper will not.

As lead will displace hydrogen from an acid, hydrogen is placed between lead and copper in the electrochemical series.

 The reaction of magnesium with hydrochloric acid is shown below:

Mg + 2HCl MgCl2 + H2 Mg + 2H+Cl- Mg2+(Cl-)2 + H2

Eliminating spectator ions gives us

Mg + 2H+ Mg2+ + H2

This shows the reaction is between magnesium atoms and hydrogen ions.

There are equations involving magnesium and hydrogen in the data book (page 7)

2H+ + 2e- H2 Mg2+ + 2e- Mg

Turning the second equation around shows the correct equations for this reaction

2H+ + 2e- H2 Mg Mg2+ + 2e-

As each of these equations show half of the chemical reaction, they are known as half reaction equations. You can quickly test your knowledge of the above information.

GENERAL

Electricity from different metals in solutions of their own ions

The ammeter will show a flow of electrons through the wire between the two metals always from the metal higher in the electrochemical series to the one lower in the series.

If the ion bridge is removed, the current stops flowing through the ammeter.

This happens because the ion bridge is needed to complete the circuit.

CREDIT

By separating the reaction in this way, the cell is divided into two half cells.

In the cell above the two reactions are:

Zn Zn2+ + 2e- Cu2+ + 2e- Cu

Note that these two equations are in the data book and the equation for the metal higher in the list has been reversed.

They explain how electrons made at zinc travel through the wire to copper where they join onto copper(II) ions to make copper metal.

This is similar to the displacement reaction that occurs when zinc metal is added to a solution which contains copper(II) ions e.g. copper(II) sulphate.

The ion bridge completes the circuit by allowing ions to move through it.

 Electricity from cells where at least one of the half cells does not involve a metal

The electrochemical series in the data book has some reactions that involve non-metals e.g.

I2 + 2e- 2I-

A cell can be set up by using a carbon rod dipping into a solution of iodine dissolved in potassium iodide solution (which contains iodide ions) as one half cell and connecting this to another half cell as shown below:

The diagram shows that electrons flow from zinc metal to the carbon rod and the reactions are shown below

Zn Zn2+ + 2e- I2 + 2e- 2I-

These equations are in the data book.  

GENERAL

Battery or Mains?

Batteries are expensive to buy but make the appliance portable.

Battery-operated appliances are safer but use finite resources such as metals in the battery.

Rechargeable cells use less of the finite resources, but still use fossil fuels to make the electricity they need.

Rechargeable cells contain toxic chemicals and should be disposed of carefully.

Mains appliances are cheaper to run, but can be dangerous. Mains appliances are not as portable, and still use up fossil fuels.

CREDIT

Oxidation

In a displacement reaction, the metal that causes displacement changes to a compound. This is classed as oxidation, e.g. when magnesium displaces zinc from zinc sulphate, magnesium sulphate and zinc are formed.

In this reaction, the magnesium is oxidised.

Oxidation also describes the reaction where a substance loses electrons e.g.

Cu Cu2+ + 2e-

In this reaction copper undergoes oxidation.

 Reduction

In a displacement reaction, the metal that is displaced changes from part of a compound into atoms of metal.

This is classed as reduction.

e.g. when copper is displaced by iron from copper(II) sulphate, iron sulphate and copper are formed.

In this reaction, the copper(II) sulphate is reduced.

Reduction also describes the reaction where a substance gains electrons e.g.

Cu2+ + 2e- Cu

In this reaction copper ions undergo reduction.

 Redox Reactions

In a displacement reaction, the metal is oxidised (loses electrons) while the ions of the other metal are reduced (gain electrons).

As both oxidation and reduction occur together, the complete reaction is called a REDOX reaction.

NEW WORDS AND THEIR MEANINGS

BATTERY - a device containing chemicals that react to produce electricity.

CELL - the correct term for devices called batteries.

ELECTROLYTE - a substance that contains ions which can move (either molten ionic compounds or ionic compounds in solution or a watery paste as in a dry cell).

RECHARGEABLE CELL - a cell in which electricity can regenerate chemicals to allow the cell to produce electricity many times.

VOLTAGE - a measure of the push of electrons between two reactions.

ELECTROCHEMICAL SERIES - a list of reactions in the data book (page 7) which can be used to determine which reaction of a pair is better at pushing electrons onto the other reaction.

DISPLACEMENT REACTION - where one metal higher in the electrochemical series displaces (pushes out) another metal from a solution of the other metal.

ION BRIDGE - a link containing ions which completes a circuit.

 CREDIT

ION BRIDGE - a link containing ions which completes a circuit by allowing ions to travel through it.

OXIDATION - the reaction of a metal element to form a compound.

OXIDATION - the loss of electrons by a reactant in any reaction.

REDUCTION - the reaction of a compound to form a metal.

REDUCTION - the gain of electrons by a reactant in any reaction.

REDOX REACTION - a reaction in which reduction and oxidation occur together.

OILRIG - This will help you remember oxidation and reduction.

OIL stands for Oxidation Is the Loss (of electrons).

RIG stands for Reduction Is the Gain (of electrons).

Topic 11 - Metals

 GENERAL

Most elements in the periodic table are metals

 Properties of Metals

Density - page 2 in the data book gives values in g/cm3, and values for most metals are higher than for non-metals.

Density is high because the atoms are packed closely together.

e.g. dense metals are used in a deep-sea diver's boots, while aluminium is used to make aircraft because it has a low density for a metal.

Thermal Conductivity - metals all conduct heat well because of the close contact of the atoms.

e.g metals are used in cooking utensils and radiators.

Electrical Conductivity - metals all conduct electricity when solid and when molten because electrons can travel easily through the structure (Refresh your memory on this by looking at Topic 7 notes).

e.g. metals are used for electrical wiring and elements in fires.

Malleability - metals can be beaten into shape e.g. the bodies of cars are pressed into shape.     

Strength - most metals are strong because of the metallic bond which holds the atoms together.

e.g. The Forth Rail Bridge is made from steel.

Metals need to be recycled because they will not last forever.

How long a metal can last can be found from the previous bar graphs.Metals are not finite.

Large quantities of metals are thrown away and the need for recycling is shown below:

 Reactions of Metals

a) With water Metals that react with water at measurable rates are few.The word equation is:

Metal + Water Metal hydroxide + Hydrogen

Potassium reacts vigorously, sodium very quickly, calcium quickly and magnesium slowly.

Potassium + Water Potassium hydroxide + HydrogenK + H2O KOH + H2

 Sodium + Water Sodium hydroxide + Hydrogen

Na + H2O NaOH + H2  

Calcium + Water Calcium hydroxide + HydrogenCa + H2O Ca(OH)2 + H2

 Magnesium + Water Magnesium hydroxide + Hydrogen

Mg + H2O Mg(OH)2 + H2

The order of metals reacting with water (most reactive first) is :

Potassium, sodium, calcium and magnesium.

b) Metal reacting with Acid The word equation for this reaction is

Metal + Acid Salt + Hydrogen Magnesium + Hydrochloric acid Magnesium chloride + Hydrogen

Mg + HCl MgCl2 + H2

This reaction was met in Topic 9.

The order of reaction can be obtained by observation of the rate at which gas is given off.

(At credit level in Topic 10, it was learned that all metals above hydrogen in the electrochemical series would react with an acid and displace hydrogen).

The order of metals reacting with acid (most reactive first) is

Magnesium, aluminium, zinc, iron, tin and lead.

Copper, mercury, silver and gold do not react, while potassium, sodium and calcium are too reactive to add to acid.  

c) Metals reacting with oxygen Oxygen can be made by heating potassium permanganate in a test tube and allowing the gas to pass through the preheated metal as shown.

A glow spreads through the metal (exothermic reaction), and the speed is related to the relative activity of the metal.

Metal + Oxygen Metal oxide Magnesium + Oxygen Magnesium oxide

Mg + O2 MgO

The order of metals reacting with oxygen (most reactive first) is

Magnesium, aluminium, zinc, iron, tin, lead, copper and mercury.

Silver and gold do not react, while potassium, sodium and calcium are too reactive to react with oxygen in this way.

These reactions give an indication of the reactivity of the metal and are summarised below:

 Metal Ores

Ores are naturally-occuring compounds of metals from which metals can be extracted.The three main types of ore are metal carbonates, metal oxide and metal sulphides.

Common name Chemical name Metal present Haematite Iron oxide Iron

Bauxite Aluminium oxide Aluminium Galena Lead sulphide Lead

Cinnabar Mercury sulphide Mercury Malachite Copper(II) carbonate Copper

 Elements on earth

Metals such as gold and silver occur uncombined on earth because they are unreactive and because of this these elements were among the first to be discovered.

Other metals, such as those in the table above are found in compounds and have to be extracted.

 Extraction of metals from ores

The demand for metals is high and methods are now available to extract all metals from their ores.Methods using carbon (coke) are cheaper and have been used longer than methods which use electricity.

 Methods of extraction

a) Heating metal oxides Silver oxide Silver + Oxygen

Ag2O Ag + O2

Few metals can be obtained in this way

b) Heating metal oxides with carbon The main reaction is :

Metal oxide + Carbon Metal + Carbon dioxide  Iron oxide + Carbon Iron + Carbon dioxide

Fe2O3 + C Fe + CO2

This method is used to extract metals below aluminium in the reactivity series.

c) Using electricity Electricity can be used to split ionic compounds into their elements in a process called electrolysis (Refresh your memory in Topic 7).The method is used to extract reactive metals above zinc in the reactivity series.

A large electric current is passed through the molten compound, and metal appears at the negative electrode.

Aluminium oxide Aluminium + Oxygen

d) Heating with carbon monoxide Iron is extracted from its ore in the blast furnace by heating with carbon (coke) in the presence of air.

At the bottom of the furnace the reaction makes carbon dioxide (Zone 1)

C + O2 CO2

Higher up, the carbon dioxide reacts with carbon to make carbon monoxide (Zone 2)

CO2 + C CO

Further up the carbon monoxide reacts with iron oxide to make iron and carbon dioxide. (Zone 3)

The formation of a metal from a compound is known as reduction.

Fe2O3 + CO Fe + CO2

CREDIT

Extracting metals from ores

This is an example of reduction (refresh your memory in Topic 10)

The method used to extract a metal depends on the reactivity of the metal.

The more reactive the metal, the more difficult it is to extract. The less reactive the metal, the easier it is to extract.

Electricity is used to extract the most reactive metals such as potassium, sodium, calcium, magnesium and aluminium.

Al2O3 2Al + 3O2

Oxides of metals from zinc to mercury are obtained by heating with carbon.

2PbO + C 2Pb + CO2

Silver and gold are found uncombined on earth and do not need to be extracted.

GENERAL

Alloys

The properties of metals can be extended or altered by mixing them with other metals or with non-metals.

Iron can be changed into stainless steel by mixing it with small amounts of chromium. This stops the metal rusting.

The table below shows some common alloys

Alloy Main Metal

Other Elements present Uses Reason

Stainless steel Iron Chromium, Nickel Sinks, Cutlery Non-rusting, strong

Mild steel Iron Carbon Girders, Car bodies Strong, rust resistant

Gold Gold Copper Rings, Electrical contacts

Good conductor, unreactive

Solder Lead (50%) Tin (50%) Joining metals,

electrical contacts Low melting point, good

conductor

Brass Copper Zinc Machine bearings, ornaments Hard wearing, attractive

CREDIT

Percentage Composition

Calcium carbonate has the formula CaCO3 and contains 1 atom of calcium, 1 atom of carbon and 3 atoms of oxygen in the formula unit.

The relative atomic masses of Ca, C and O are respectively 40, 12 and 16 (Page 4 in the data book).

This gives a formula mass of 40 + 12 + (16 x 3) = 100

% of Calcium = Mass of calcium in the formula unit x 100Formula mass

 = 40 x 100

100 

= 40%

For carbon and oxygen, the percentages are respectively 12% and 48%.

 Empirical Formulae

Ethane has the molecular formula C2H6 but its empirical formula is CH3. Empirical formula is the simplest representation of the formula, but still

shows the correct ratio of the elements present.

For water the molecular formula and the empirical formula are both H2O.

 Empirical formula by calculation

A compound contains 12.7 g of copper and 1.6 g of oxygen.

What is its empirical formula?

Element Copper Oxygen

Mass /g 12.7 1.6

Relative atomic mass 63.5 16

Moles 12.7/63.5 1.6/16

Value 0.2 0.1

Ratio 2 1

Formula = Cu2O

The same method can be used if the quantities in the question are given as percentages.

If 100 g of the compound is considered, the percentage values become masses in g.

New terms and their meanings

DENSITY - the mass of a metal divided by the space it takes up. A metal is dense because the atoms are packed closely together and take up little space.

MALLEABILITY - the ability of a metal to be beaten into shape.

CONDUCTIVITY - the ability of a substance to let heat or electricity through it.

ORE - a naturally-occuring compound of metals.

ELECTROLYSIS - splitting a substance into its elements using electricity.

REDUCTION - a reaction is which a metal is obtained from a compound.

ALLOY - a mixture of metals or of metals with non-metals.

EMPIRICAL FORMULA - the simplest formula of a compound.

Topic 12 - Corrosion

 GENERAL

Corrosion

Corrosion is the changing of the surface of the metal from an element into a compound.

This natural change of metals into compounds is very costly.

 Speed of Corrosion

Most metals corrode, but the speed at which they corrode is related to the chemical activity series (Refresh your memory from Topic 11)

e.g. potassium corrodes very quickly, copper corrodes very slowly, while gold does not seem to corrode.

 Rusting

Rusting is the special name given to the corrosion of iron

As iron, in the form of steel, is the most commonly used metal in the world, the corrosion of iron is important.

 The Cause of Rusting

The previous experiment will show that oxygen and water are both needed for rusting to occur.

Further proof that oxygen is needed is seen in the following experiment.

As about 80% of the air in the cylinder is left after rusting - it means that oxygen is used up during rusting and water rises to take its place.

 Detecting Rusting

The typical brown colour of rust is the end result of rusting. The first stage of the rusting process can be detected by ferroxyl indicator. As a blue colour is only made with iron(II) chloride solution, and with a

rusting nail, it proves that iron(II) ions (Fe2+) are made during rusting.

 Test for iron(II) ions

Iron(II) ions (Fe2+) give a blue colour with ferroxyl indicator. Fe2+ are produced when iron atoms lose 2 electrons.

CREDIT

Corrosion is an example of oxidation because it involves a loss of electrons.

Fe Fe2+ + 2e-

The rusting process continues when iron(II) ions lose another electron to form iron(III) ions.

Fe2+ Fe3+ + e-

The iron(III) ions can be shown using a colourless solution of ammonium thiocyanate. The solution will turn blood-red.

Test for iron(III) ions

Iron(III) ions give a blood-red colour with ammonium thiocyanate solution.

 GENERAL

Corrosion requires an electrolyte such as dissolved salt or acid rain. Acid can speed up corrosion in two ways:

(a) by acting as an electrolyte

(b) by reacting with the metal.

Electrolytes increase the speed of rusting, and cars rust faster in winter when salt is spread on the roads.

 CREDIT

An electrolyte is needed for rusting to occur. This can even be dissolved carbon dioxide (see Topic 8).

The electrolyte helps to carry ions away from the rusting iron and this speeds up the oxidation (corrosion).

 The role of oxygen and water in rusting

Oxygen and water accept the electrons lost by the iron.

2H2O + O2 + 4e- 4OH-

Ferroxyl indicator also shows that hydroxide ions are made during rusting by giving a pink colour.

Test for Hydroxide ions

Ferroxyl indicator turns pink when hydroxide ions are present (in greater numbers than in water).

CREDIT

Rusting and Redox

As iron atoms lose electrons in rusting and oxygen/water molecules gain these electrons, rusting is described as a Redox reaction. (see Topic 10)

 GENERAL

Using a battery to prevent the rusting of iron

Examine the animation below

The battery causes the nail connected to the positive terminal to rust rapidly, but the nail connected to the negative terminal does not rust.

Why does the nail connected to the negative terminal not rust?

Iron has to lose electrons in order to rust. The negative terminal of the battery is pushing electrons onto this nail and

this prevents this nail from losing any electrons. This nail cannot rust. Ferroxyl indicator turns pink because hydroxide ions are made here. Electrons flowing to the nail stop rusting.

Why does the nail connected to the positive terminal rust rapidly?

The positive terminal of the battery is removing electrons from the nail connected to it. This nail rusts rapidly, changing into iron(II) ions which turn ferroxyl indicator blue.

Electrons flowing from the nail increase rusting.

CREDIT

If a nail and a carbon rod are connected by a wire and dipped into salt solution containing ferroxyl indicator, a blue colour quickly appears around the nail, and a pink colour around the carbon rod.

This means that electrons are flowing from the nail to the carbon.

When electrons flow from iron, it rusts.

GENERAL

Connecting different metals to iron

In Topic 10 it was found that metals higher in the electrochemical series could push electrons onto metals lower in the electrochemical series.

Magnesium stops iron rusting, while copper makes iron rust quicker.

Metals that push electrons onto iron stop rusting, but metals that let electrons flow from iron increase the speed of rusting.

CREDIT

Ferroxyl indicator shows what happens when metals are connected to iron. Remember that a blue colour shows that iron(II) ions are made, while a pink colour shows that hydroxide ions are made.

GENERAL

Preventing Corrosion

A. Physical Protection

This is where a metal is given a coating to stop it coming in contact with air and water and thus prevents corrosion.

Methods available for physical protection

painting e.g. the Forth Rail Bridge. greasing or oiling - protects moving parts of machinery. coating with plastic - dish drainers have a metal core and a plastic coating. coating with other metals such as tin, zinc, silver, gold.

Tin-plating - metals can be coated with other metals which are less likely to corrode. Food cans are steel cans dipped into molten tin giving a layer of tin.

Electroplating - e.g. chromium-plating of car bumpers and the silver-plating of cutlery are done using this process to give an attractive appearance which provides protection against corrosion. (Some details of this process were given in Topic 7 when metals are deposited on the negative electode during electrolysis of a salt solution.)

Galvanising - galvanised iron in made by dipping iron into molten zinc which coats the iron with zinc. It is used to protect dustbins, car exhausts and special nails.

B. Chemical Protection

Tin-plating works well provided the layer of tin remains unbroken. If the tin layer to scratched, the iron corrodes quickly because electrons travel to tin from iron.

Zinc-plating works well if the layer of zinc remains unbroken and also when scratched because then the zinc corrodes quickly and electrons are pushed onto the iron. This is an example of sacrificial protection - where a more reactive metal is allowed to corrode in order to protect a less reactive metal.

NEW WORDS AND THEIR MEANINGS

CORROSION - the changing of the surface of a metal

RUSTING - the special name for the corrosion of iron

FERROXYL INDICATOR - turns blue in the presence of iron(II) ions

FERROXYL INDICATOR - turns pink in the presence of hydroxide ions

PHYSICAL PROTECTION - stopping corrosion by keeping out air and/or water

CHEMICAL PROTECTION - using more reactive metals to push electrons onto iron

GALVANISING - coating iron with a layer of zinc

TIN PLATING - coating iron with a layer of tin

ELECTROPLATING - coating a metal with a layer of another metal using electricity

SACRIFICIAL PROTECTION - where a more reactive metal sacrifices itself to protect the less reactive metal

Topic 13 - Plastics

 GENERAL

Origin of plastics

Crude oil is used to make plastics and synthetic fibres.

Long chain hydrocarbons can be cracked or broken into a mixture of short chain hydrocarbons, some of which are saturated and others unsaturated. (See Topic 6 revision)

 Synthetic fibres

Many items of clothing contain materials such as polyester, polyamide, terylene, rayon, dralon etc.The items previously mentioned are all man-made fibres, and are called synthetic fibres.

 Properties of plastics in relation to their uses

Plastics Uses PropertiesPoly(ethene) Plastic bags Strong, light, waterproof

Polyamide Blouses, tights Hard wearing

Polyester Shirts, blouses, quilts Wears well, helps keep shape of clothing, warm

Bakelite Pot handles, electrical sockets

Does not melt or conduct heat or electricity

Rubber Soles of shoes, tyres Flexible, waterproofDralon Furniture covers Hard wearing, stain resistant

 Use of fibres

Labels on clothes often give information on their content

 Natural and man-made fibres

Fibre

Natural / Man-made Origin

Cotton Natural Cotton plantPolyamide Synthetic Crude oil

Polyester Synthetic Crude oilSilk Natural Silk worm

Viscose Synthetic Cotton plant after treatment

Wool Natural Sheep

 Advantages and disadvantages of plastics

Plastic Advantages DisadvantagesPoly(ethene) Cheap, waterproof Does not rot awayMelamine/Formica Heat resistant Not as attractive as wood

Polyurethane foam Makes cheap seating Gives toxic fumes when

burned

Polystyrene Makes shaped, colourful TV cases Not as attractive as wood

 Problems when plastics burn

Some plastics burn or smoulder and give off toxic fumes. They can produce thick black smoke, make toxic fumes and use up large amounts of oxygen.

CREDIT

As plastics contain carbon, they all produce carbon dioxide, carbon monoxide (as air runs out), and smoke.

Plastic Elements present Toxic fumesPoly(ethene) Carbon and hydrogen Carbon monoxide

P.V.C. Carbon, hydrogen and chlorine

Hydrogen chloride (acidic)

Polyurethane foam

Carbon, hydrogen and nitrogen

Hydrogen cyanide (toxic)

Polystyrene Carbon and hydrogen Dense, black smoke

GENERAL

Biodegradable Plastics

Plastic packaging does not rot away and causes major litter problems. Many plastics are non-biodegradable, meaning they will not rot away in nature.

Plastics are now being introduced that are biodegradable e.g. Biopol (biodegradable polymer). Paper, originally used for packaging, did rot away under the action of microbes, and is described as biodegradable.

 Thermoplastic and thermosetting polymers

Thermoplastic plastics are those which can be resoftened on heating e.g. poly(ethene), poly(amide).

Thermosetting plastics are those which cannot be resoftened on heating e.g. bakelite, melamine.

Both type of plastics consist of long, tangled chains but in thermosetting polymers, there are links between the chains which gives a much more rigid structure.

 Plastics are polymers

A polymer is a very big molecule made from many small molecules (called monomers) which repeat through the structure.

The process in which the monomers join to make a polymer is called polymerisation.

 Structure of the monomers

Many plastics or polymers are made from unsaturated monomers obtained by cracking (See Topic 6 revision)

The simplest monomer is ethene, C2H4

Ethene monomers join together to give a polymer called poly(ethene), which is often called polythene.

This process is called polymerisation

CREDIT

How do monomers join to make a polymer?

A chemical is added which breaks the double bond between the two carbon atoms of ethene to make a very reactive unit.

Reactive units join together, end to end and a big molecule or polymer is made.

This molecule consists of many small units joined together and repeating along the length of the polymer.

The process is called addition polymerisation because the monomer units join together to give one product, by a series of reactions in which the double bond breaks.

The reaction between bromine and an alkene in Topic 6 was called addition because one product was made when the double bond broke.

GENERAL

Many plastics are made from alkenes, or from unsaturated molecules made from alkenes.

Modern Name of Monomer

Original Name of Monomer

Modern Polymer Name

Original Polymer Name

Ethene Ethene Poly(ethene) Polythene

Chloroethene Vinyl chloride Poly(chloroethene) Polyvinyl chloride (P.V.C.)

Propene Propene Poly(propene) PolypropenePhenylethene Styrene Poly(phenylethene) Polystyrene

 NEW WORDS AND THEIR MEANINGS

PLASTICS - a wide variety of large molecules made from products from crude oil distillation

SYNTHETIC - man-made

FIBRE - a large molecule which is made into long threads

NATURAL - occurring in nature

BIODEGRADABLE - broken down into smaller pieces by living organisms

TOXIC - harmful

THERMOPLASTIC - a plastic that can be resoftened by heating e.g. poly(ethene)

THERMOSETTING - a plastic that cannot be resoftened by heating e.g. bakelite

MONOMER - small units that join together to give a very big molecule

POLYMERS - very big units made when many monomer molecules join together

POLYMERISATION - the process in which monomers join to give a polymer

CRACKING - a reaction in which long-chain hydrocarbons are converted into unsaturated hydrocarbons

ADDITION POLYMERISATION - the making of a polymer by a series of addition reactions

Topic 14 - Fertilisers

 GENERAL

Population and Food Needs

The ever increasing world population means more food is needed and fertilisers are used to grow plants efficiently.

Organic fertilisers such as animal manure can be used but additional chemical fertilisers are also needed.

Growing plants need nutrients - soluble compounds containing nitrogen, phosphorus and potassium which are absorbed by roots.

This is why ammonium salts (such as NH4Cl), potassium salts and nitrates (such as KNO3) and phosphates (such as Na3PO4) are important fertilisers as they contain the essential elements that plants need and they are soluble in water.

Fertilisers containing Nitrogen (N), Phosphorus (P) and Potassium (K) are known as NPK fertilisers.

When crops are harvested, these nutrients are removed from the soil in the plant and need to be replaced.

CREDIT

Effects of shortages of nutrients on plants

Nutrient Role in Plant growth Effect of Shortage

Nitrogen Needed to make protein and chlorophyll

Stunted growth, pale leaves, weak roots

Phosphorus

Needed for strong roots, stem and seeds and for energy transfer Slow growth of plants, poor fruit

Potassium Needed to make protein and sugars Plants stunted with poor resistance to frost, drought and disease

 Percentage composition

In Topic 11 you learned to calculate the % of elements in a compound. Ammonium nitrate, NH4NO3 has a formula mass of 80.

Mass of Nitrogen present in the formula mass = 14 x 2 = 28

Percentage of Nitrogen = (28/80) x 100 = 35%

 GENERAL

The Nitrogen Cycle

The element nitrogen is recycled in nature as shown in the diagram below.

CREDIT

Clover, pea and bean plants change atmospheric nitrogen into soluble nitrogen compounds by 'fixing' nitrogen.

This happens because swellings (or nodules) on the roots contain nitrifying bacteria which add nitrogen to the soil, increasing fertility.

It is cheaper than chemical fertilisers and without pollution problems. Some nitrifying bacteria are free-living.

 GENERAL

Problems caused by the use of fertilisers

Fertilisers need to be soluble to get into plants.

Rain can wash fertilisers into rivers and lochs and cause harmful algae and bacteria to grow rapidly.

These remove oxygen from the water and kill fish and plants.

 GENERAL

Converting unreactive nitrogen into ammonia - the Haber Process

Nitrogen (from the fractional distillation of liquid air) and Hydrogen (from natural gas) are mixed together at a moderately high temperature (400oC) and high pressure with an iron catalyst and nitrogen hydride (ammonia, NH3) is made.

N2(g) + H2(g) NH3(g) (Unbalanced)

 CREDIT

The previous reaction is reversible i.e. it can go in both directions.If too high a temperature is used, the ammonia changes back to nitrogen. This is why a moderately high, but not too high a temperature is used. It also explains why not all of the nitrogen and hydrogen are changed into ammonia.

 CREDIT

Making ammonia in the Laboratory

Heating any ammonium compound with alkali produces ammonia

NH4Cl + NaOH NH3 + NaCl + H2O

This reaction can be used to prove that a compound is an ammonium compound

 GENERAL

Properties of Ammonia

1. Ammonia is a colourless gas which turns moist pH paper blue/purple i.e. it is an alkali.

NH3(g) + H2O(l) NH4OH(aq)

2. Ammonia has a characteristic smell. 3. Ammonia is very soluble in water as shown by the fountain experiment.

4. Ammonia reacts with acids to make salts. These salts can be used as fertilisers as they contain nitrogen.

NH3 + HNO3 NH4NO3

 Making Nitric Acid

Nitric acid is needed to make some fertilisers. It can be made as follows:

Nitrogen dioxide, a brown gas, can be made when air (21% oxygen and 78% nitrogen) is sparked for about 20 minutes.

N2 + O2 NO2 (Unbalanced)

CREDIT

N2 + 2O2 2NO2 (Balanced)

GENERAL

Nitric acid is made if the brown gas formed shaken with water and oxygen.

2H2O + 4NO2 + O2 4HNO3 (Balanced)

The spark plug causes the same reaction in a petrol engine, as does lightning. (See Topic 5)

Time and cost make this method unsuitable to make large quantities of nitric acid

 Nitrogen is unreactive

Many elements burn in air to form element oxides, but nitrogen is very unreactive. In Topic 4 you discovered that there is a triple covalent bond holding nitrogen atoms tightly in the diatomic nitrogen molecule. It is hard to break this bond and nitrogen does not burn in oxygen.

The spark plug and lightning provide more than enough Activation energy (Topic 2) to cause this reaction to happen.

 The Catalytic Oxidation of Ammonia (Ostwald Process)

This is an economic route to making nitrogen dioxide and then nitric acid. It can be done in the laboratory.

The reaction is an example of oxidation as ammonia reacts with oxygen on the platinum catalyst.

When the reaction begins, a colourless gas is formed and collects in the flask. This gas is called nitrogen monoxide, NO.

NH3(g) + O2(g) NO(g) + H2O(g)

Nitrogen monoxide reacts with air (oxygen) to form brown nitrogen dioxide.

NO(g) + O2(g) NO2(g)

This brown acidic gas dissolves in water with oxygen to make nitric acid.

2H2O + 4NO2 + O2 4HNO3 (Balanced)

CREDIT

In the Ostwald process the catalyst continues to glow once the reaction starts, without further heating. This happens because the oxidation of ammonia is an exothermic reaction.

The Haber process and the Ostwald process provide an economic route from nitrogen to nitric acid.

 NEW TERMS AND THEIR MEANINGS

NUTRIENTS - soluble compounds containing nitrogen, phosphorus and potassium which are absorbed by roots.

NPK FERTILISERS - those containing Nitrogen, Phosphorus and Potassium

NITRIFYING BACTERIA - bacteria living in swellings on the roots of pea and bean plants that can take nitrogen out of the air and give this to plants.

FIXING NITROGEN - taking nitrogen out of the air and changing it into useful compounds for plants e.g. nitrates.

DENITRIFYING BACTERIA - taking nitrates out of soil and changing it nitrogen.

FOUNTAIN EXPERIMENT - an experiment which shows the high solubility of a gas in water.

HABER PROCESS - the joining of nitrogen and hydrogen gases to make ammonia gas using an iron catalyst.

ACTIVATION ENERGY - the energy needed to start a reaction.

OSTWALD PROCESS - the oxidation of ammonia with oxygen to make nitrogen dioxide using a platinum catalyst.

EXOTHERMIC REACTION - a reaction which gives out energy.

Topic 15 - Carbohydrates and related Substances

 GENERAL

Plants make carbohydrates by photosynthesis and release oxygen.

Plants use sunlight, carbon dioxide and water to make a family of foods called carbohydrates in a process called photosynthesis.

   The process is endothermic as the energy of the sun is needed. Glucose is a carbohydrate made by photosynthesis

 The importance of chlorophyll

Chlorophyll, a green substance in leaves, absorbs the energy from sunlight to allow photosynthesis to take place.

sunlight + carbon dioxide + water carbohydrate + oxygen 6CO2 + 6H2O C6H12O6 + 6O2

 The importance of Carbohydrates

Carbohydrates are an important food for animals. Respiration is the process which releases energy in plants and animals Respiration is important for all living things because it provides energy

when glucose is burned or broken down in the body.

Glucose + oxygen carbon dioxide + water + energy

C6H12O6 + 6O2 ----------> 6CO2 + 6H2O

 Burning a Carbohydrate

Carbohydrates release energy when burned, and make carbon dioxide and water.

The release of energy can be seen in the custard-powder tin experiment.

 Using this energy

Animals use this energy for many things such as movement, and warmth to keep our body temperature at 37°C.

CREDIT

Elements in a Carbohydrate

Burning a carbohydrate makes carbon dioxide and water. This proves that a carbohydrate contains carbon and hydrogen.

GENERAL

Plants and the regulation of oxygen and carbon dioxide levels in the atmosphere

Respiration by living things and combustion of fossils fuels use oxygen and release carbon dioxide but the level of oxygen remains at 21% and carbon dioxide at 0.03%.

Photosynthesis balances this by using up carbon dioxide and releasing oxygen.

There is a lot of concern that giant rain-forests are being chopped down because these forests are seen as the oxygen factories of the Earth.

 Examples of Carbohydrates

Carbohydrate FormulaGlucose C6H12O6

Fructose C6H12O6

Maltose C12H22O11

Sucrose C12H22O11

Starch (C6H10O5)n

CREDIT

Glucose/fructose and maltose/sucrose are Isomers

The atoms in glucose (C6H12O6) and fructose (C6H12O6) are joined together differently and the two forms are called isomers. (No need to learn these structures above)

The atoms in maltose (C12H22O11) and sucrose (C12H22O11) are joined together differently and the two forms are called isomers.

GENERAL

Carbohydrates contain carbon, hydrogen and oxygen

The previous table shows that carbohydrates all contain carbon, hydrogen and oxygen.

In every carbohydrate, the ratio of hydrogen:oxygen is always 2:1 - the same as in water ('hydrate' refers to water).

 Properties of carbohydrates

The table below shows the properties of various carbohydrates some of which apply at General level only.

The iodine test is performed by adding brown iodine solution to the substance being tested. A positive test is the appearance of a blue-black colour.

The Benedict's (or Fehling's) test is performed by warming the carbohydrate solution with blue Benedict's (or Fehling's) solution. A positive test is the change in colour from blue to red-brown.

Carbohydrate

Appearance

Solubility in water

Type of solution

Iodine test

Benedict's test

Glucose White solid Soluble True * No change Blue to brown

Fructose White solid Soluble True No change Blue to brown

Maltose White solid Soluble True No change Blue to

brownSucrose White solid Soluble True No change No change

Starch White solid Does not dissolve well Colloid # Turns blue-

black No change

Starch can easily be identified from the other carbohydrates as it turns iodine solution to a blue black colour.

 Distinguishing glucose and sucrose

Glucose can be distinguished from sucrose by the Benedict's test.

CREDIT

Distinguishing glucose, maltose and fructose from sucrose

Only sucrose does not give a positive result with Benedict's solution.

GENERAL

Distinguishing a true (*) and a colloidal (#) solution

Light rays travel through a true solution unseen, but can be seen and are scattered when they pass through a colloid.

To show that starch is made in leaves by photosynthesis

1. Put plant in dark for 2 days to remove all food from the leaf. 2. Put plant in the light for one day to let it make food. 3. Remove one leaf and boil in water to kill it. 4. Put in hot alcohol to remove chlorophyll - the leaf will be white. 5. Wash in water to remove alcohol and add iodine solution to it. 6. The leaf will be blue-black proving that starch had been made.

GENERAL

Starch is a polymer of glucose molecules

Glucose molecules can join together in living things to make a big molecule called starch, a polymer.

The process is called polymerisation.

CREDIT

Water is lost when glucose molecules join together

One water molecule is lost from each glucose molecule when they join together.

This is why the formula of glucose is represented as C6H12O6, while the formula of starch is (C6H10O5)n.

The joining of glucose molecules to make starch is called condensation polymerisation

When molecules join together and water molecules are eliminated when they join, the reaction is called condensation.

If many glucose molecules join together and a polymer is made, the reaction is called condensation polymerisation.

GENERAL

Digestion of Starch

Digestion is the chemical breakdown of food in the body into smaller molecules (glucose) which pass through the gut wall and are absorbed by the blood.

Digestion is helped by acid or an enzyme Digestion of starch in food begins in the mouth where the starch meets

saliva which contains the enzyme amylase. Digestion continues in the stomach where there is acid.

CREDIT

When starch is changed into glucose, water molecules are involved in splitting the starch polymer and the reaction is called hydrolysis.

(C6H10O5)n + nH2O nC6H12O6 Polysaccharide molecule + water molecules monosaccharide molecules

Sucrose can also be split up (hydrolysed) into simpler sugars such as glucose using an enzyme or acid.

C12H22O11 + H2O C6H12O6 + C6H12O6 disaccharide molecule + water two monosaccharide molecules

 Monosaccharides and Disaccharides

Monosaccharides all have six carbon atoms in each molecule and the formula C6H12O6.

Glucose and fructose are monosaccharides.

Disaccharides have twelve carbon atoms and the formula C12H22O11

Disaccharides are formed when two monosaccharides join together.

e.g. Sucrose is made when a glucose and fructose molecule join.

C6H12O6 + C6H12O6 C12H22O11 + H2O Glucose + fructose sucrose + water

e.g. Maltose is made when two glucose molecules join.

The making of a disaccharide from two monosaccharides is an example of condensation.

GENERAL

Alcohol can be made from sugars such as glucose or starch by fermentation

Enzymes present in yeast help the reaction but can only produce an alcohol content of about 15%.

The yeast dies when the alcohol content reaches about 15%,

Glucose alcohol + carbon dioxide + energy

 Sources of sugars for fermentation

Alcoholic drink

Alcohol content Source of Carbohydrate

Beer 4% (varies) Hops, barleyCider 4% (varies) Apples

Wine about 10% Grapes, other fruit and vegetables

Whisky 40% BarleyVodka 37.5% PotatoesBrandy 40% Grapes

Distillation is used to increase alcohol concentration and drinks made by this method are called spirits.

Alcohol boils at 79°C and water boils at 100°C.In a distillery, the water/alcohol mixture is heated to 80°C and the alcohol collected by distillation.

 Alcohol is a member of the Alkanol family

The alcohol present in alcoholic drinks is called ethanol, C2H5OH.

Alkanols are similar in structure to the alkanes but have an OH group instead of one of the hydrogen atom.

 NEW WORDS AND THEIR MEANINGS

PHOTOSYNTHESIS - a process in plants in which carbon dioxide and water are changed into carbohydrates and oxygen with the help of sunlight and chlorophyll.

CARBOHYDRATE - a compound containing carbon, hydrogen and oxygen in which the ratio of hydrogen:oxygen is the same as in water.

ENDOTHERMIC - a reaction in which energy is absorbed (or taken in).

CHLOROPHYLL - a green chemical in the leaves of plants which is able to trap the energy of the sun and use this energy to make carbohydrates.

RESPIRATION - a process in living things where oxygen is used to break up food and produce water, carbon dioxide and energy

ISOMERS - compounds with the same molecular formula but with a different structural formula.

TRUE SOLUTION - where the solute breaks up into small pieces which can fit into the spaces between the water molecules. Such solutions are 'clear',

COLLOID - where a solute is too large to fit into the spaces between water molecules. Such a solution is not clear.

MONSACCHARIDE - single sugar molecules such as glucose or fructose (C6H12O6)

DISACCHARIDE - two sugar molecules joined together as in sucrose or maltose (C12H22O11)

CONDENSATION - the joining of sugar molecules with the elimination of water molecules.

POLYMERISATION - the joining of many sugar molecules together e.g. when glucose is changed into starch.

DIGESTION - a process in living things where large molecules are broken into smaller molecules with the help of enzymes.

ENZYMES - biological catalysts which are used to help reactions such as photosynthesis and digestion in living things.

HYDROLYSIS - the breakdown of molecules by splitting them with water e.g. sucrose into molecules of glucose and fructose.

FERMENTATION - a reaction between sugars and enzymes in yeast which produces alcohol and carbon dioxide.

ALKANOL - a compound made by fermentation e.g. ethanol.

ETHANOL - a member of the alkanol family of formula C2H5OH