AQA GCSE Chemistry Paper 2 Revision
4.4: Chemical Changes
➔ 4.4.1: The reactivity of metals
Metals react with oxygen to produce metal oxides. The reactions are oxidation reactions because the metals
gain oxygen.
Unreactive metals such as gold are found in the Earth as the metal itself but
most metals are found as compounds that require chemical reactions to extract
the metal.
Metals less reactive than carbon can be extracted from their oxides by
reduction with carbon.
Reduction involves the loss of oxygen.
Oxidation is the loss of electrons and reduction is the gain of electrons.
➔ 4.4.2: Reactions of Acids
Redox is oxidation (loss of electrons) and reduction (gain of electrons) happening simultaneously. (OILRIG)
Soluble salts can be made from acids by reacting them with solid insoluble substances, such as metals, metal
oxides, hydroxides or carbonates.
The solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the
salt.
Salt solutions can be crystallised to produce solid salts.
Acids produce hydrogen ions (H+) in aqueous solutions. Aqueous solutions of alkalis contain hydroxide ions
(OH–).
The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution, and can be measured using
universal indicator or a pH probe.
A solution with pH 7 is neutral.
Aqueous solutions of acids have pH values of less than 7 and aqueous solutions of alkalis have pH values
greater than 7.
In neutralisation reactions between an acid and an alkali, hydrogen ions react with hydroxide ions to produce
water. This reaction can be represented by the equation:
H+(aq) + OH-(aq) → H2O(g)
A strong acid is completely ionised in aqueous solution. Examples of strong acids are hydrochloric, nitric and
sulfuric acids.
A weak acid is only partially ionised in aqueous solution. Examples of weak acids are ethanoic, citric and
carbonic acids.
For a given concentration of aqueous solutions, the stronger an acid, the lower the pH. As the pH decreases by
one unit, the hydrogen ion concentration of the solution increases by a factor of 10.
, ➔ 4.4.3 Electrolysis
When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or
solution. These liquids and solutions are able to conduct electricity and are called electrolytes.
Passing an electric current through electrolytes causes the ions to move to the electrodes. Positively charged
ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode
(the anode). Ions are discharged at the electrodes producing elements. This process is called electrolysis.
When a simple ionic compound (eg lead bromide) is electrolysed in the molten state using inert electrodes, the
metal (lead) is produced at the cathode and the non-metal (bromine) is produced at the anode.
Metals can be extracted from molten compounds using electrolysis. Electrolysis is used if the metal is too
reactive to be extracted by reduction with carbon or if the metal reacts with carbon. Large amounts of energy
are used in the extraction process to melt the compounds and to produce the electrical current.
Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite using
carbon as the positive electrode (anode).
The anode (positive electrode) is made of carbon are needs to be regularly replaces as it reacts with oxygen to
produce carbon dioxide.
The ions discharged when an aqueous solution is electrolysed using inert electrodes depend on the relative
reactivity of the elements involved.
At the negative electrode (cathode), hydrogen is produced if the metal is more reactive than hydrogen.
At the positive electrode (anode), oxygen is produced unless the solution contains halide ions when the halogen
is produced.
This happens because in the aqueous solution water molecules break down producing hydrogen ions and
hydroxide ions that are discharged.
4.5: Energy Change
➔ 4.5.1: Exothermic and Endothermic Reactions
An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings
increases.
Exothermic reactions include combustion, many oxidation reactions and neutralisation. Everyday uses of
exothermic reactions include self-heating cans and hand warmers.
An endothermic reaction is one that takes in energy from the surroundings so the temperature of the
surroundings decreases.
Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium
hydrogencarbonate. Some sports injury packs are based on endothermic reactions.
The minimum amount of energy that particles must have to react is called the activation energy.
4.4: Chemical Changes
➔ 4.4.1: The reactivity of metals
Metals react with oxygen to produce metal oxides. The reactions are oxidation reactions because the metals
gain oxygen.
Unreactive metals such as gold are found in the Earth as the metal itself but
most metals are found as compounds that require chemical reactions to extract
the metal.
Metals less reactive than carbon can be extracted from their oxides by
reduction with carbon.
Reduction involves the loss of oxygen.
Oxidation is the loss of electrons and reduction is the gain of electrons.
➔ 4.4.2: Reactions of Acids
Redox is oxidation (loss of electrons) and reduction (gain of electrons) happening simultaneously. (OILRIG)
Soluble salts can be made from acids by reacting them with solid insoluble substances, such as metals, metal
oxides, hydroxides or carbonates.
The solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the
salt.
Salt solutions can be crystallised to produce solid salts.
Acids produce hydrogen ions (H+) in aqueous solutions. Aqueous solutions of alkalis contain hydroxide ions
(OH–).
The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution, and can be measured using
universal indicator or a pH probe.
A solution with pH 7 is neutral.
Aqueous solutions of acids have pH values of less than 7 and aqueous solutions of alkalis have pH values
greater than 7.
In neutralisation reactions between an acid and an alkali, hydrogen ions react with hydroxide ions to produce
water. This reaction can be represented by the equation:
H+(aq) + OH-(aq) → H2O(g)
A strong acid is completely ionised in aqueous solution. Examples of strong acids are hydrochloric, nitric and
sulfuric acids.
A weak acid is only partially ionised in aqueous solution. Examples of weak acids are ethanoic, citric and
carbonic acids.
For a given concentration of aqueous solutions, the stronger an acid, the lower the pH. As the pH decreases by
one unit, the hydrogen ion concentration of the solution increases by a factor of 10.
, ➔ 4.4.3 Electrolysis
When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or
solution. These liquids and solutions are able to conduct electricity and are called electrolytes.
Passing an electric current through electrolytes causes the ions to move to the electrodes. Positively charged
ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode
(the anode). Ions are discharged at the electrodes producing elements. This process is called electrolysis.
When a simple ionic compound (eg lead bromide) is electrolysed in the molten state using inert electrodes, the
metal (lead) is produced at the cathode and the non-metal (bromine) is produced at the anode.
Metals can be extracted from molten compounds using electrolysis. Electrolysis is used if the metal is too
reactive to be extracted by reduction with carbon or if the metal reacts with carbon. Large amounts of energy
are used in the extraction process to melt the compounds and to produce the electrical current.
Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite using
carbon as the positive electrode (anode).
The anode (positive electrode) is made of carbon are needs to be regularly replaces as it reacts with oxygen to
produce carbon dioxide.
The ions discharged when an aqueous solution is electrolysed using inert electrodes depend on the relative
reactivity of the elements involved.
At the negative electrode (cathode), hydrogen is produced if the metal is more reactive than hydrogen.
At the positive electrode (anode), oxygen is produced unless the solution contains halide ions when the halogen
is produced.
This happens because in the aqueous solution water molecules break down producing hydrogen ions and
hydroxide ions that are discharged.
4.5: Energy Change
➔ 4.5.1: Exothermic and Endothermic Reactions
An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings
increases.
Exothermic reactions include combustion, many oxidation reactions and neutralisation. Everyday uses of
exothermic reactions include self-heating cans and hand warmers.
An endothermic reaction is one that takes in energy from the surroundings so the temperature of the
surroundings decreases.
Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium
hydrogencarbonate. Some sports injury packs are based on endothermic reactions.
The minimum amount of energy that particles must have to react is called the activation energy.
