Exam-style Questions: Electrochemistry

1. Dichromate (VI) ions are powerful oxidising agents and are reduced to chromium III ions. This colour change was once used in 'breath test' apparatus to determine if a driver had consumed excessive alcohol.

Explain the term 'redox reaction'

(Marks available: 1)

Answer

Answer outline and marking scheme for question: 1

A redox reaction is one where electrons are transferred from one species to another.

(Marks available: 1)

2. Aluminium metal is extracted from molten bauxite (Al2O3.2H2O)
using electrolysis. Cryolite (AlF3) added to the ore in order to
lower the melting point required and thus the energy required by the process.

a) Write an half equation to show how aluminium metal is produced from
the ore.

(1 mark)

b) What mass of aluminium metal would be produced if a current of 30,000A
is applied to a cell for 1 hour.

(4 marks)

c) In the molten mixture there is a mixture of anions which mostly consists
of O2- and F-. Write an equation to show which
of these anions will be oxidised in the cell?

(2 marks)

(Marks available: 7)

Answer

Answer outline and marking scheme for question: 2

a) Al3+(l) + 3e- = Al(l)

(1 mark)

b) Charge passed; E = It

E = 30,000 x 3600 = 1.08 x 108C (1)



1 mole of electron has a charge of 96,500C


Mols electrons = (1.08 x 108) / 96,500 = 1119 mols (1)



To reduce 1 mol of Al3+, 3 mols of electrons are required



In the cell 1119 / 3 = 373 mols of Al produced (1)



Ar Al = 27.0

Mass Al produced = 373 x 27.0 = 10071g = 10.071kg of Al (1)

(4 marks)

c) 2O2-(l) - 4e- = O2(g)
(1 for correct anion, 1 for correct equation)

(2 marks)

(Marks available: 7)