Edexcel GCSE Chemistry: Chemical Changes

Molten lead(II) bromide ($PbBr_2$PbBr2​) and concentrated aqueous sodium chloride (brine, $NaCl$NaCl) can both be electrolysed using inert electrodes.

• Electrolysis of molten lead(II) bromide produces lead at one electrode and bromine at the other.
• Electrolysis of aqueous sodium chloride produces hydrogen at one electrode and chlorine at the other — not sodium.

Compare and explain the products formed in these two electrolysis experiments. Explain why a metal is produced from the molten compound but hydrogen (not the metal) is produced from the aqueous solution. (6 marks)

Molten aluminium oxide is electrolysed to extract aluminium. During this process:

• At the cathode, aluminium ions ($Al^{3+}$Al3+) are converted to aluminium atoms.
• At the anode, oxide ions ($O^{2-}$O2−) are converted to oxygen molecules.

(a) Write the half equation for the reaction at the cathode. (2 marks)

(b) State whether the cathode reaction is oxidation or reduction, and explain your answer in terms of electrons. (2 marks)

A student wants to make pure, dry crystals of the soluble salt copper(II) sulfate by reacting an excess of copper(II) oxide (an insoluble base) with dilute sulfuric acid.

The equation is: $CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O$CuO+H2​SO4​→CuSO4​+H2​O

Describe the method the student should use to make pure, dry crystals of copper(II) sulfate from these reactants. (3 marks)

Hydrochloric acid is described as a strong acid. Ethanoic acid (the acid in vinegar) is described as a weak acid. Both can have the same concentration.

(a) Explain the difference between a strong acid and a weak acid in terms of ionisation. (2 marks)

(b) Two solutions, one of hydrochloric acid and one of ethanoic acid, have the same concentration. State which has the lower pH, and explain why. (1 mark)

When dilute hydrochloric acid is added to magnesium ribbon, the magnesium fizzes and a gas is given off:

$Mg + 2HCl \rightarrow MgCl_2 + H_2$Mg+2HCl→MgCl2​+H2​

(a) Name the gas given off. (1 mark)

(b) Describe a simple test, and the positive result, that would confirm the identity of this gas. (1 mark)

An alkaline solution has a pH greater than 7.

State the colour that universal indicator turns in a solution of pH 13. (1 mark)