© Boardworks Ltd 20071 of 55. 2 of 55© Boardworks Ltd 2007.

© Boardworks Ltd 20071 of 55

2 of 55 © Boardworks Ltd 2007


What is a chemical reaction?

A chemical reaction is a change that takes place when one or more substances (called reactants) form one or more new substances (called products).

reactants products

For example:

carbon carbon dioxideoxygen

+

+


Reactions all around us

How many chemical reactions go on around us everyday?


How can you spot a chemical reaction?

Chemical reactions can appear very different.

As you observe a chemical reaction, you may detect:

a colour change

precipitate (solid) forming

energy being produced (fizzing, burning)

an odour being produced.


Why doesn’t the mass change?

In a chemical reaction, no atoms are made or destroyed.

The reaction just changes how the atoms are bonded together.


Does mass change during a reaction?


What is a word equation?

For example, when a piece of sulfur is burned in oxygen gas it produces a white solid called sulfur dioxide.

A word equation uses the names of the reactants and products to show what happens in a chemical reaction.

+ sulfur oxygensulfur

dioxide

The word equation for this reaction is:


What is a symbol equation?

A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction.

This equation shows that one atom of sulfur (S) reacts with one molecule of oxygen (O2) to make one molecule of sulfur dioxide (SO2).

A symbol equation must be balanced to give the correct ratio of reactants and products.

+ S O2 SO2

+


What do state symbols show?

State symbols are added to a symbol equation to show whether the reactants and products are:

With state symbols in place, this symbol equation now shows that the sulfur is a solid, the oxygen is a gas and the sulfur dioxide is also a gas.

solid – symbol is (s)

liquid – symbol is (l)

gas – symbol is (g)

dissolved in water – symbol is (aq).

+ S (s) O2 (g) SO2 (g)


Chemical reactions – true or false?



What is thermal decomposition?


Thermal decomposition – easy or hard?

The more reactive a metal, the harder it is to decompose its carbonate by heating.

incr

ease

in

rea

ctiv

ity

Predict how easy it is to decompose these carbonates:

copper carbonate

calcium carbonate

sodium carbonate

manganese carbonate

zinc carbonate

iron carbonate.

potassium

sodium

calcium

magnesium

aluminium

manganese

zinc

iron

copper

silver

gold


Thermal decomposition – activity


Thermal decomposition of carbonates

FeOFeCO3 + CO2

ironoxide

ironcarbonate + carbon

dioxide

heat

What are the word and symbol equations for the thermal decomposition of these carbonates?

When the metal carbonate is heated, it decomposes to produce a metal oxide and carbon dioxide.

copper carbonate

manganese carbonate

zinc carbonate.


When calcium carbonate is heated, it decomposes to produce calcium oxide and carbon dioxide.

This reaction is carried out in industry to make calcium oxide (quicklime) from calcium carbonate (limestone):

Calcium oxide is used to make concrete and calcium hydroxide (slaked lime).

Heating calcium carbonate

CaOCaCO3 + CO2

calciumoxide

calciumcarbonate + carbon

dioxide

heat


Baking powder and self-raising flour contain sodium hydrogencarbonate (also known as sodium bicarbonate).

What effect does this reaction have on dough as it is being baked?

When sodium hydrogencarbonate is heated, it decomposes to make sodium oxide, carbon dioxide and water.

The carbon dioxide released during the reaction creates tiny bubbles, which help the dough to rise.

Heating sodium hydrogencarbonate


What is the decomposition equation?



What are indicators?

Indicators are chemicals that change colour in the presence of an acid or an alkali.

There are many different indicators. Lots of them come from plants, like red cabbage. Different indicators turn different colours.

Universal indicator is a mixture of indicators. It is useful because it shows a range of colours from pH 1–14.

Geranium plants grown in acidic soil have red flowers, while geraniums grown in alkali soil have blue flowers.


What does the pH scale show?

The pH scale runs from 1 to 14. The numbers 1 to 6 represent acidic conditions and 8 to 14 represent alkali conditions. A pH value 7 is neutral.

1 2 3 4 5 6 7 8 9 10 11 12 13 14

What are the pH values of some everyday items?


What is the order of pH?


What are acids?

Acids are substances that:

Hydrochloric acid is a strong acid.

Ethanoic acid (vinegar) is a weak acid.

Have a pH below 7 and turn universal indicator yellow, orange or red.

Turn litmus red.

Form solutions containing hydrogen ions (H+).


What are alkalis?

Have a pH above 7 and turn universal indicator blue or purple.

Alkalis are substances that:

Sodium hydroxide is a strong alkali.

Ammonia is a weak alkali. When ammonia is dissolved in water, it forms ammonium hydroxide (NH4OH), a fertilizer.

Turn litmus blue.

Can neutralize acids

Form solutions containing hydroxide ions (OH-).


What are bases?

All alkalis are bases. Bases are substances that react with acids by absorbing hydrogen ions (H+).

Some bases are soluble in water – these are called alkalis.

All alkalis contain hydroxide ions (OH–). The more OH– ions in the solution, the stronger the alkali.

The oxides, hydroxides and carbonates of metals, such as sodium hydroxide, are bases.

Ammonia is a base that does not contain a metal.

alkalis(solublebases)

bases (react with acids)


Acids and bases – true or false?



+acid base a salt

How are salts made and named?

When an acid reacts with a base, a neutralization reaction occurs and produces a chemical called a salt.

The name of the salt depends on the names of the reactants.

The first part of the salt’s name comes from the base:

e.g. sodium hydroxide sodium…

The second part of the salt’s name comes from the acid:

e.g. sulfuric acid sulfate…

For example, if sodium hydroxide neutralizes sulfuric acid, the product is a salt called sodium sulfate.


What is the name of the salt?


What are salts used for?

Table salt is sodium chloride. This is the salt used to flavour and preserve food.

Indigestion remedies often contain magnesium salts.

Salts can also be used as coloured pigments in paints, and to help fuels burn better.

The colours of fireworks are formed when certain salts burn. Calcium chloride, for example, burns a bright red colour.


Water is formed because OH– ions from the alkali react with H+ ions from the acid to produce molecules of water (H2O).

Making salts 1: acid + alkali

When an acid reacts with an alkali, the products are a salt and water.

acid alkali salt water+ +

For example:

+ +

++

sodiumhydroxide

NaOH (aq)

sodiumchloride

NaCl (aq)

water

H2O (aq)

hydrochloricacid

HCl (aq)


When is the reaction complete?

There is no obvious sign when this reaction is complete, so an indicator is used to show when the solution is neutral.

This process is called titration.

The reaction between sodium hydroxide and hydrochloric acid produces sodium chloride, which is soluble in water.

+ +

++

sodiumhydroxide

NaOH (aq)

sodiumchloride

NaCl (aq)

water

H2O (aq)

hydrochloricacid

HCl (aq)


What is a titration?


Making salts 2: acid + metal

When a metal is added to an acid, the products are a salt and hydrogen gas.

For example:

This method of making salts only works with some metals.

hydrochloricacid

2HCl (aq)

+

+

+

+

zinc

Zn (aq)

zincchloride

ZnCl2 (aq)

hydrogen

H2 (aq)

acid metal a salt hydrogen+ +

What would happen if potassium was used? Or copper?


Which metal will react the most?


Making salts 3: acid + metal oxide

When a metal oxide is mixed with an acid, the products are a salt and water.

calciumchloride + water

calciumoxide

CaO (aq)

hydrochloricacid

2HCl (aq) CaCl2 (aq) +

+

+ H2O (aq)

For example:

acid metal oxide salt water+ +

Water forms because oxygen ions from the oxide join up with H+ ions from the acid to produce molecules of water (H2O).


Obtaining salts from metal oxides

When a metal oxide is added to an acid, it dissolves as it reacts.

How could the solid copper sulfate salt be separated from the water?

You know when you have added enough of the metal oxide because it stops dissolving.

+ +copperoxide

CuO (s)

sulfuricacid

H2SO4 (aq)

coppersulfate

CuSO4 (aq)

water

H2O (aq)


Obtaining salts from copper oxide


Making salts 4: acid + carbonate

When a carbonate is mixed with an acid, the products are a salt, carbon dioxide and water.

coppernitrate + water +

carbondioxide

coppercarbonate

CuCO3 (s)

nitricacid

2HNO3

(aq)

Cu(NO3)2

(aq)+

+

+ H2O(aq)

+ CO2

(g)

For example:

What would you expect to observe in this reaction?

acid carbonate salt water+ +carbondioxide+

Water and carbon dioxide are formed because the carbonate ions (CO3

2-) react with H+ ions from the acid.


Making salts – summary

Salts can be made by reacting acids with bases.

acid + alkali a salt + water

acid + metal a salt + hydrogen

acid + metal oxide a salt + water

acid + carbonate a salt + carbon dioxide + water

There are four ways of making salts from acids:


Matching reactants and salts


Complete the neutralization reaction



Sometimes when two solutions are mixed, they react to form an insoluble solid product.

For example, mixing solutions of lead nitrate and sodium chloride produces a yellow precipitate of lead chloride.

What is a precipitation reaction?

The insoluble solid product is called a precipitate.

You can spot a precipitate because the mixture goes cloudy.

leadnitrate

Pb(NO3)2 (aq)

sodiumchloride

2NaCl (aq)

leadchloride

PbCl2 (s)

sodiumnitrate

2NaNO3 (aq)

+

+

+

+


How can hard water be softened?

Hard water contains magnesium and calcium ions, which can clog up pipes and heating elements. These ions can be removed using precipitation reactions.

The magnesium ions are removed by this reaction:

What are the word and symbol equations for the removal of the calcium ions?

Sodium carbonate is added to the hard water to form precipitates, which can then be removed by filtration.

sodiumcarbonate

Na2CO3 (aq)

magnesiumchloride

MgCl2 (aq)

sodiumchloride

2NaCl (aq)

magnesiumcarbonate

MgCO3 (s)+ +

+ +


Many metals form hydroxide precipitates, which have characteristic colours.

Precipitates of hydoxides

For example, when sodium hydroxide solution is added to a solution of iron(III) chloride, the reaction produces a brown precipitate of iron(III) hydroxide.

+ +

FeCl3 (aq) 3NaOH (aq) Fe(OH)3 (s) ++ 3NaCl (aq)

iron(III)chloride

sodiumhydroxide

iron(III)hydroxide

sodiumchloride


Identifying precipitates


Identifying metals


Precipitation – true or false?



Glossary

acid – A substance that releases H+ ions in solution.alkali – A substance that releases OH- ions in solution.base – Any substance that reacts with an acid.decomposition – A type of reaction in which a compound

is broken down into two or more substances.neutralization – A type of reaction in which an acid reacts

with a base to produce a salt.precipitate – A solid, insoluble product of a reaction.precipitation – A type of reaction in which two aqueous

solutions react to form an insoluble product.salt – A substance formed when an acid reacts with a base.titration – A method of indicating when an undetectable

reaction, such as neutralization, is complete.


Anagrams


Completing equations


Multiple-choice quiz

© Boardworks Ltd 20071 of 55. 2 of 55© Boardworks Ltd 2007.

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