AQA Additional Science C2. » There is some subject knowledge you will need to know from your Core Science course that you completed last year. » The next.

Chemistry Revision

AQA Additional Science C2

C1 Recap

» There is some subject knowledge you will need to know from your Core Science course that you completed last year.

» The next few slide will briefly outline what you will need to remember for this topic.

Atomic Structure

» All matter is made from atoms.» Atoms are made up from smaller parts

˃ Protons˃ Neutrons˃ Electrons

» Protons and neutrons are found in the nucleus.» Electrons are found in shells or energy levels around the nucleus.

Subatomic Structure

Subatomic Particle Relative Mass Charge

Proton 1 +1

Neutron 1 0

Electron 0 (very small!) -1

» So the mass of an atom comes from the sum of its protons and neutrons.

Atomic Number

» This is the number of protons inside the nucleus of an atom.

» This found on the periodic table and is the smallest number.

» This is also the same as the number of electrons in a neutral atom.

Mass Number

» This is the number of protons and neutrons in the nucleus of an atom

» The number of protons and neutrons in an atom are not always the same.

Electron Shells

» All of an atoms electrons cannot fit into one energy shell.

» An atom has multiple energy shells to hold all of these electrons.

» Each level can hold a maximum number of electrons:˃ Shell 1 – 2 electrons˃ Shell 2 – 8 electrons˃ Shell 3 – 8 electrons˃ Shell 4 – Whatever is left!

Happy Atoms

» Atoms are happiest when they have a full outer shell

» Atoms can get a full outer shell by taking, sharing or losing their electrons. When this happens, you get a chemical reaction.

Reactivity

» Elements in group 1 (alkali metals) have 1 electron in their outer shell and are very reactive.

» Elements in group 7 (halogens) have 7 electrons in their outer shell and are very reactive.

» Elements in group 8/0 (noble gases) have a full outer shell of electrons and are unreactive (inert).

Chemical Reactions

» Chemical reactions occur when elements are trying to get a full outer shell of electrons.

» When a reaction has occurred a chemical bond has been formed between two or more elements to form a compound.

» There are two types of chemical bond:˃ Ionic bond: Formed when atoms join together as a result of gaining or

losing electrons˃ Covalent bond: Formed when atoms join together as a result of

sharing electrons

Properties and Structures of Substances

Overview

» Substances that have simple molecular, giant ionic and giant covalent structures have very different properties˃ Ionic, covalent and metallic bonds are strong

+ The forces between molecules are weaker e.g. carbon dioxide and iodine

» Nanomaterials have new properties because of their very small size!

Substances

» Substances that consist of simple molecules are gases, liquids or solids that have relatively low melting points and boiling points

» Substances that consist of simple molecules do not conduct electricity because the molecules do not have an overall electric charge

Ionic Bonding

» Ionic bonds form between a metal and a non-metal.

» Metals lose electrons to gain a full outer shell.» Non-metals gain electrons to gain a full outer

shell.

» Elements in group 1 and group 7 are likely to form ionic bonds.

Ionic Bonding

X

Na Cl

Na ClX

-+

The metal loses its outer electron and gives it to the non-metal.

The metal gains a positive charge and a full outer shell.The non-metal gains a negative charge and a full outer shell.

They both become oppositely charged ions that are attracted to each other.

Structure of Ionic Compounds

» Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces in all directions between oppositely charged ions

Properties of Ionic Compounds

» Because of the regular structures (giant ionic lattices) these compounds have high melting points and high boiling points

» When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry the current

Covalent Bonding

» Covalent bonds occur between two non-metals.» It is not always possible for an electron to be

transferred from one atom to another» When this happens, electrons are shared

between atoms so that they can BOTH have a full other shell

» Covalent bonds are formed when electrons are shared between atoms

Covalent Bonding

ClCl

XX

XX

XX

XThe non-metals share a pair of electrons to gain a full outer shell.

ClCl

XX

XX

XX

XThe shared pair of electrons are drawn in between the two atoms.

Structure of Covalent Compounds

» Atoms that share electrons can form simple molecules and giant structures or macromolecules

» Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms

Simple Covalent Compounds

» Bond between atoms are strong.

» But bonds between molecules are weak which gives these compounds a low boiling point.

» They do not conduct electricity as there is not electrical charge between the atoms.

Giant Covalent Compounds

» All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high melting points

» The two main examples of these structures are graphite and diamond.

Diamond

» In DIAMOND, each Carbon atom forms four covalent bonds with other Carbon atoms in a giant covalent structure, so diamond is very hard

» This also gives it an extremely high melting point.

» Diamond does not conduct electricity.

Graphite

» In GRAPHITE, each Carbon atom bonds to three others, forming layers˃ The layers are free to slide over each other and so

graphite is soft and slippery

» Since Carbon only forms three bonds it has a spare electron which it delocalises.

» The delocalised electrons can move between the sheets of Carbon which allows graphite to conduct electricity.

Fullerenes

» Besides graphite and diamond, carbon can also form another type of giant covalent structure.

» Fullerenes (named after the scientists that discovered them) are made by conjoined hexagonal carbon rings.

Uses of Fullerenes

» Possible uses of Fullerenes in the future could be:• Drug delivery• In lubricants• As catalysts in reactions• To make carbon nanotubes to reinforce structures

Metallic Bonding

» Metals do not form ionic or covalent bonds.» Bonds between metals are called metallic

bonds.» This also includes the bonding between

different metals in alloys.

Structure of Metallic Bonding

» Metals exist as positive metal ions with delocalised electrons.

» This forms a lattice of positive metal ions surrounded by a ‘sea’ of delocalised electrons.

Properties of Metallic Bonding

» The negative electrons sandwiched between the positive ions lead to strong electrostatic forces in the metal. This means that most metals have VERY HIGH melting/boiling points.

» When a current is applied, the electrons carry the charge. This is why metals can conduct electricity.

» The layers of atoms in metals are able to slide over each other and so metals can be bent and shaped.

Alloys

» An alloy is a mixture of two or more elements, where at least one of the elements is a metal.

» Many alloys are mixtures of two or more metals.» Alloying metals can create compounds that are:

• Stronger• More flexible• Have a higher melting point• Less easily corroded• Lighter

Commonly Used Alloys

» Commonly used alloys are:• Steel - Carbon and Steel• Stainless steel – Carbon, Steel and Nickel• Brass – Copper and Zinc• Solder – Lead and Tin• White gold – Gold, Nickel and Palladium

Structure of Alloys

» In a normal metal, the atoms are arranged in a regular structure. In an alloy, the atoms of the second element fit into the spaces between the metal atoms.

» Having atoms in these spaces makes it more difficult for the layers to slide over each other. This is why most alloys are stronger than the metal alone.

Atoms of second

element

Summary of Properties

Ionic Simple(covalent)

Giant(covalent)

Metallic

Melting point

Boiling point

Electrical/heat conductor

Yes, whenmolten or insolution as allowsions to move

No, due to no overall charge

No – diamondYes – graphite due to delocalisedelectrons

Yes, due to delocalised electrons

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Polymers

» A polymer is a long chain molecule that is made up of lots of small molecules that have been chained together. The small molecules that have been linked together are called monomers.

Poly(ethene)

» Formed at very high pressures with trace amounts of oxygen. Polymer chains are branched, making them hard to pack together.

» Formed at lower pressures and using a catalyst at 50°C. Polymer chains are straighter, allowing them to pack closer together.

Heating Polymers

» How polymer chains are arranged can have a major effect on how they are affected by heat.

» There are two kinds of polymers that you are required to know about:

• Thermosoftening• Thermosetting

Thermosoftening Plastics

• The polymer chains are all coiled together• However, there are no cross bonds between the

molecules!• When the polymer is heated, the molecules unwind and

can slide past each other• The material is flexible when it is hot and can be shaped• When it cools, it hardens• Heating the polymer causes it to soften again• e.g. Polythene

Thermosetting Plastics

• The polymer chains are all coiled together• There are cross bonds between the molecules!• When the polymer is heated, the chains are

linked to each other and can’t slide past each other

• Once moulded, the plastic will not soften when heated and can’t be reshaped• e.g. Vulcanised rubber used to make tyres.

Nanoscience

» Nanoscience refers to structures that are 1-100nm in size, of the order of a few hundred atoms

» Nanoparticles show different properties to the same materials in bulk and have a high surface area to volume ratio, which may lead to the development of new:˃ Catalysts˃ Computers˃ Coatings˃ Highly selective sensors˃ Lighter construction materials

Nanoscience Examples

» Titanium oxide nanoparticles: » Self-cleaning glass is covered with a

thin titanium dioxide coating. Even on cloudy days, the layer allows ultraviolet light from the Sun to oxidize organic dirt and loosen it from the surface. Raindrops slide down normal glass, leaving dirt streaks and evaporated spots. Self-cleaning glass forces water to spread out evenly in a sheet, washing away dirt loosened by the photocatalytic action.

Nanoscience Examples

» The smell in smelly socks come from the bacterial breakdown of oils released by your skin. We can’t stop the skin releasing these chemicals but we can stop the bacteria working. Nanoparticles of silver inhibit bacterial growth and so prevent the smell.

» Silver particles are also used in sticking plasters for cuts.

Nanoscience Examples

» Testing for some chemicals needs expensive laboratories and can take hours or days to produce a result.

» We have now developed ways to condense chemical reagents onto cards. These react with the substances that would normally need a whole lab to detect. In this way simple tests can detect viruses or other chemicals in the field and guide doctors in treatment.

Nanoscience Examples

» The hair on the feet of geckos are so small that they can stick to individual molecules!

» Nanotechnologists can now coat a tape with sticky hairs of the same size. This means it sticks to anything - and needs no glue.

Quantitative Chemistry – How Much?!

Atoms

» In the periodic table atoms are represented as shown below:

MASS NUMBER = number of protons + number of neutrons

PROTON NUMBER = number of protons

Relative Masses

» The relative masses of protons, neutrons and electrons are:

» Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element

Particle Relative Mass

Proton 1

Neutron 1

Electron Very small

Relative Atomic Mass (RAM/Ar)

» The relative atomic mass (RAM/Ar) is a measure of the mass of one atom of the element

» In all chemical reactions reactants create products˃ The greater the amount of reactants used, the

greater the amount of products produced. + The total mass of reactants used is equal to the total

mass of the products made.

Relative Formula Mass (RMM/Mr)

» The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of the atoms in the numbers shown in the formula

» i.e. What is the relative formula mass of H2O?+ Relative Atomic Mass (Ar) of H = 1+ Relative Atomic Mass (Ar) of O = 16

˃ We have 2 atoms of H so 2 x 1 = 2, and we have 1 atom of O so 1 x 16 = 16

˃ 16 + 2 = 18 So, Relative formula mass (Mr) of H2O is 18

Relative Formula Mass and Moles

» The relative formula mass of a substance, in grams, is known as one mole of that substance˃ i.e. A mole of Carbon has a mass of 12g˃ i.e. A mole of Water (H2O) has a mass of 18g

Moles

» To calculate the number of moles in an amount of substance you can use the formula triangle below.

Moles RAM/RMM

Mass

Percentage Element in a Compound

» The percentage of an element in a compound can be calculated from the relative mass of the element in the formula and relative formula mass of the compound

% of element = relative mass of element x 100 relative formula mass of compound

Reacting Masses

» Calculating reacting masses is calculating the ratio of reactants and products in a reaction. This is important because industrially it would be vital to ensure that no reactant is wasted or unused otherwise the process will not be as profitable as it could be.

Percentage Yield of a Reaction

» Calculating percentage yield refers to calculating how much product is obtained. The amount that should be obtained is called the Maximum Theoretical Yield.

» We may not always obtain the full Maximum Theoretical Yield due to three main reasons:˃ Some product may be lost when separated from the reaction mixture˃ Reaction might be reversible˃ Reactants might react in ways which are different from what is

expected

» % yield = actual yield x 100 maximum possible yield

Analysing Substances

Paper Chromatography

» Paper chromatography can be used to identify additives/artificial colours in food.

» Here is the process:» Draw a pencil baseline on filter paper» Add a spot of the sample on the pencil baseline» Put the filter paper in a beaker with solvent » Allow the solvent to rise up the paper taking the dyes with

out» The dyes will separate out as they will travel at different

speeds» You can use known samples alongside an unknown sample

to identify the dyes that the unknown sample contains.

Gas Chromatography

» Pass a gas through a column packed with a solid material

» Substances are separated as they travel at different speeds.

» The time it takes to reach the detector is called retention time

» Recorder draws gas chromatograph˃ Number of peaks shows number of different compounds in substance˃ Position of peaks shows retention time of each substance

Mass Spectrometry

» You can attach the gas chromatography column to a mass spectrometer. This process is called GC-MS. The mass spectrometer can identify the substances leaving the column very accurately

» The mass spectrometer produces a mass spectrum on which the molecular ion peaks shows the relative molecular mass. A mass spectrometer can identify substances quickly, accurately and in very small amounts (very sensitive)

Rates of Reaction

Aims

» to interpret graphs showing the amount of product formed (or reactant used up) with time, in terms of the rate of the reaction

» to explain and evaluate the development, advantages and disadvantages of using catalysts in industrial processes

Measuring the Rate of Reaction

» The rate of a chemical reaction can be found by measuring the amount of a reactant used or the amount of product formed over time

Factors that Affect the Rate of Reaction

» The rate of a chemical reaction increases:˃ if the temperature increases˃ if the concentration of dissolved reactants or

the pressure of gases increases˃ if solid reactants are in smaller pieces

(greater surface area)˃ if a catalyst is used

Collision Theory

» Chemical reactions can only occur when reacting particles collide with each other and with sufficient energy. The minimum amount of energy particles must have to react is called the activation energy.

Increasing Temperature

» Increasing the temperature increases the speed of the reacting particles so that they collide more frequently and more energetically. This increases the rate of the reaction

Increasing Concentration / Pressure

» Increasing the concentration of reactants in solutions and increasing the pressure of reacting gases also increases the frequency of collisions and so increases the rate of reaction.

Increasing Surface Area

» Increasing the surface are of the reactants increases the number of collisions.

» So the number of successful collisions will increase.

Catalysts

» Catalysts change the rate of chemical reactions but are not used up during the reaction˃ Different reactions need different catalysts

» Catalysts are important in increasing the rates of chemical reactions used in industrial processes to reduce costs

Using Catalysts

» The catalyst lowers the activation energy required for a reaction to occur

» This means that there are more collisions that have the minimum activation energy to react

» This increases the rate of reaction

Advantages and Disadvantages of Catalysts

» Advantages˃ Less energy used˃ Reusable˃ Lowers production costs˃ Requires only small amounts

» Disadvantages˃ Can be toxic (for example transition metals)˃ Expensive

Exothermic and Endothermic Reactions

Exothermic Reactions

» An exothermic reaction is one that transfers energy from the reacting chemicals to the surroundings.

» More simply, it’s a reaction that gives off heat.

Examples of Exothermic Reactions

» Burning Fuels (combustion)» Neutralisation» Respiration

Endothermic Reactions

» An endothermic reaction is one that TAKES IN energy from the surroundings.

» These reactions are a lot less common than exothermic reactions

» Examples are thermal decomposition and photosynthesis.

Reversible Reactions

Reversible Reactions

» In normal chemical reactions, it is very difficult to change your reactants back to your products e.g. it is very difficult to make your cooked food back into its raw form

» However, in reversible reactions this IS possible! Reversible reactions are represented in a particular way

A + B C + D⇌

Energy Changes During Reversible Reactions

» If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction

» The same amount of energy is transferred in each case:

Using Energy Changes

» Exothermic:˃ Hand warmers: this could contain chemicals which have

reversible reactions which would make it reusable. It could also contain a non-reversible reaction such as calcium oxide and water which would be disposable. However, it would stay warm for longer.

» Endothermic:˃ Cold packs: this could contain ammonium nitrate and water.

This can be used to treat sports injuries.

Acid, Bases, Salts and Electrolysis

Acids

» Acids release a hydrogen ion into water (aqueous) solution (H+).

» Acids neutralise bases in a neutralisation reaction. » Acids corrode reactive metals. » Acids turn blue litmus to red, and universal

indicator red - yellow.» Acids taste sour. » Conduct electricity» pH <7

Bases

» Bases release a hydroxide ion into water solution. » Not all bases are soluble.» Bases neutralise acids in a neutralisation reaction. » Bases denature proteins. This accounts for the

"slippery" feeling on hands when exposed to base.» Bases turn red litmus to blue, and universal indicator

blue – purple» Bases taste bitter. » Conduct electricity in solution.» pH>7

What is the difference between a base and an alkali?

» A base is the name given for ANY chemical that can neutralise an acid.

» An alkali is a of base that can be dissolved in water to form a solution.

» Bases are usually metal oxides and metal hydroxides. Of these, hydroxides are usually soluble and can be used to make alkaline solutions.

The pH Scale

» The pH scale is a visual way of representing acidity and alkalinity.

» The scale not only identifies a substance as acid or base but also tells you how strong it is.

» pH measures the concentration of H+ ions in solution.

» The more H+ ions the stronger the acid.

» The more OH- ions the stronger the base.

Neutralisation

» Neutralisation reactions ALWAYS have the following form:

Acid + Base Salt + Water» Salt is just a general name used to describe an

ionic compound.» When an acid and a base are reacted together

the H+ ions in the acid react with the OH- ions in the alkali. This makes water i.e.

H+ + OH- H2O

Making Salts

» Salts can be made using acids by reacting them with three different substances:

1) Metals – will give you the salt you want, but some metals are too reactive and so salts can’t always be made this way.

˃ Acid + Metal Salt + Hydrogen

2) Insoluble bases – You can keep on adding the base until it stops reacting, then filter off the excess.

3) Alkalis – You can use an indicator to measure when the acid has been neutralised.

Naming Salts

Sodium Chloride

The first part of the salt name comes from the metal in the base / alkali.

E.g. Sodium hydroxide

The second part of the salt name comes from the acid.

E.g. Hydrochloric acid

Hydrochloric acid

Chloride

Nitric Acid Nitrate

Sulphuric Acid

Sulphate

State Symbols

» The state symbols in equations are:˃ (s) - solid˃ (l) – liquid˃ (g) - gas˃ (aq) – aqueous solution (dissolved in water)

Electrolysis

» Electrolysis literally means ‘splitting up using electricity’.

» In electrolysis, we use electricity to break down (decompose) a substance into simpler substances.

» In electrolysis, the electrolyte (substance being broken down) is dissolved or melted into a molten state.

» This separates the substance into positive and negative ions. The ions travel to different electrodes, separating the substance into simpler substances.

Electrodes

» The positive electrode is called the anode.» The negative electrode is called the cathode.» The electrodes are usually made out of

unreactive (inert) substances. » Substances such as graphite, which will not

react with the substances made are a common choice

At the Electrodes

» At the negative electrode, positively charged ions gain electrons (reduction) and at the positive electrode, negatively charged ions lose electrons (oxidation).

» If there is a mixture of ions, the products formed depend on the reactivity of the elements involved.

» Reactions at electrodes can be represented by half equations, for example:

Electrolysis in Detail

» Let’s consider the electrolysis of lead bromide. Lead bromide is an ionic compound and can be dissolved in water. The lead forms positive ions with a charge of +2 and the Bromine forms negative ions with a charge of -1.

Pb 2+

Pb 2+

Br -

Br -

Electrolysis in Detail

» When the electrolyte is connected to a battery, the lead ions act as the cations. They have a positive charge and are attracted to the negative cathode. The Bromine anions have a negative charge and are attracted to the positive anode.

Pb 2+

Pb 2+

Br -

Br -

- +

Electrolysis in Detail

» When the electrolyte is connected to a battery, the lead ions act as the cations. They have a positive charge and are attracted to the negative cathode. The Bromine anions have a negative charge and are attracted to the positive anode.

- +Bromine gas

Lead deposits

Half Equations - Anode

» We can represent what is happening at the electrodes using ‘half equations’. For example, from the previous example, the following reaction will be occurring at the anode:

» 2Br- Br2 + 2e-

» Two Bromine ions lose an electron each to form Bromine gas, depositing the electrons on the positive anode.

Half Equations - Cathode

» We can represent what is happening at the electrodes using ‘half equations’. For example, from the previous example, the following reaction will be occurring at the cathode:

» Pb2+ + 2e- Pb

» The lead ions gain electrons from the negative cathode and form lead deposits on the cathode.

Aluminium

» Aluminium is a very versatile metal. It is light and does not corrode or rust easily, meaning it can be used to make everything from drink cans to aeroplane parts.

» However, it is very expensive to extract, mainly because of the amount of electricity that is used to extract it.

Extracting Aluminium

» Aluminium is usually found in an ore called Bauxite.

» The Bauxite is purified to form Aluminium oxide. This is a white powder, from which Aluminium can be extracted.

Extracting Aluminium

» Aluminium oxide has a melting point of 2000°C! Melting this would be extremely expensive! Instead….

» The Aluminium oxide is dissolved in molten Cryolite. The Cryolite has a melting point of about 1000°C, reducing the cost of extracting Aluminium (slightly…)

Extracting Aluminium

» The dissolved aluminium oxide can then be separated using Electrolysis. Graphite anodes and cathodes are used to conduct electricity through the molten electrolyte.

Extracting Aluminium

» The Aluminium forms Al3+ ions in the solution. They are attracted to the negative cathode, where they form molten aluminium.

» Al3+ + 3e- Al

Extracting Aluminium

» At the positive anode, O2-

ions form Oxygen. The oxygen reacts with the graphite (a type of carbon) to form carbon dioxide. The anode must therefore be regularly replaced.

» 2O2- + 4e- O2

Electrolysis of Brine

» The electrolysis of sodium chloride solution produces hydrogen, chlorine and sodium hydroxide.

» These are important reagents for the chemical industry.

Overall Reaction:NaCl(aq) H2(g) + Cl2(g) + NaOH(aq)

At the positive electrode:2Cl- Cl2 + 2e-

At the negative electrode:2H+ + 2e- H2

Electrolysis of Brine

» Uses of chlorine...˃ Bleach which is good for disinfecting˃ Other types of disinfectant˃ Keeps tap water clean˃ Used in the productions of plastic such as PVC

» Uses of hydrogen...˃ Make margarine˃ Rocket fuel

» Uses of Sodium Hydroxide...˃ Makes detergents

sodium hydroxide

Sodium chloride solution

Electroplating

» Electroplating is covering a metal with another metal to either make it attractive or give it a layer of protection.

» The bad metal is used as a cathode and a good metal is used as an anode.

» The process of electrolysis takes place, coating the bad metal in the atoms from the good metal.

Benefits of Electroplating

» To protect the metal underneath from corroding.

» To make the object look more attractive.» To increase the hardness of a surface (scratch

resistance).» To save money – an example of this is in

jewellery, a thin surface of a more precious metal is electroplated over a cheaper metal.

» Don’t have to replace items as often.

Disadvantages of Electroplating

» Using electroplating to protect large surfaces can make things more expensive.

» The electricity required to electroplate is expensive and uses valuable resources.

Explaining Electroplating

» The metal to be plated is used as the negative electrode.

» The positive electrode is made from the plating metal.

» At the positive electrode the plating metal atoms are oxidised (electrons lost).

» At the negative electrode the metal to be plated atoms are reduced (electrons gained).

Electroplating Half Equations

» E.g. Electroplating by nickel:» Positive Nickel Electrode:Ni(s) Ni2+ + 2e-

» Negative Electrode to be Plated:Ni2+

(aq) + 2e- Ni(s)

Keep Calm and Revise!