Revision: Electrolysis (I)
11/07/2011
(A) Electrolysis of molten ionic compounds
1
The electrolysis of molten calcium bromide using graphite electrodes was carried
out as shown in Fig. 9.1
battery
bulb
anode
cathode
molten
calcium
bromide
crucible
heat
Fig. 9.1
After some time into the electrolysis, the heating was stopped and the contents of the
crucible solidified. It was observed that the bulb continued to glow.
(a) Explain why calcium bromide needs to be heated to the molten form.
[2]
(b) What would you expect to observe around the anode during electrolysis?
[1]
(c) Write an ionic equation for the reaction that would occur at the cathode.
[1]
(d) Suggest why the bulb continued to glow after the heating was stopped and the
calcium bromide solidified.
[1]
(B) Electrolysis of aqueous ionic compounds
2. (a) The diagram shows the experimental set up by a student for the electrolysis of
concentrated copper (II) chloride.
(i) Identify the gas that is produced from the reaction. [1]
(ii) Describe a test for this gas. [1]
(iii) The student was expecting to collect the gas in the test tube, but after some time, he
still did NOT see any gas collected. Explain why he did not see any gas collected. [1]
(iv) Describe what you would observe at the negative electrode. Give the ionic equation
for the reaction taking place at the negative electrode. [2]
(v) Would there be any colour changes in the electrolyte? Explain your answer. [1]
3. A constant current of 2.0 A (amperes) was passed through for nearly three hours
through concentrated aqueous copper (II) chloride, using carbon electrodes. The
diagram below shows the setup of the experiment.
electrode L
electrode K
U-tube containing
concentrated aqueous
copper (II) chloride
(a)
beaker containing
aqueous potassium
bromide
Write down the formulae of all the ions present in the concentrated
aqueous copper (II) chloride.
(b)
[2]
After 10 minutes, a gas was produced at electrode L and it turned the
colourless aqueous potassium bromide reddish brown.
(i)
Name the gas that is produced at electrode L.
(ii)
Write an ionic equation for the reaction that occurred in the beaker.[1]
(c)
The table below shows the observations for the experiment at different
stages.
Stage of experiment
Stage I
(after 1 hr)
Observations
Pink deposit at electrode K.
The solution in the beaker remained reddish brown.
The solution in the U-tube became much paler.
Stage II
(after 2 hrs)
Bubbling is observed at both electrodes.
[1]
Stage III
All bubbling had ceased and the liquid in the U-tube would no
(after 3 hrs)
(i)
longer conduct electricity.
Name the products responsible for the bubbling at the cathode and anode
in Stage II.
[2]
4. Aqueous copper(II) sulfate can be electrolysed using either carbon electrodes or
copper electrodes.
(a) Give the formulae of all the ions present in the aqueous copper(II) sulfate.
[2]
(b) When aqueous copper(II) sulfate is electrolysed using carbon electrodes, a
product is formed at the anode (positive electrode).
(i)
What is the name of this product? [1]
(ii)
Describe a test for this product.
[1]
(c) The electrodes are changed and aqueous copper(II) sulfate is electrolysed
using copper electrodes.
(i)
Explain why the anode decreases in mass. [1]
(ii)
Explain why the cathode increases in mass. [1]
(iii)
Write the half equation for the reaction at the cathode. [1]
(d) Describe what you would see when aqueous ammonia is added drop by
drop, until it is in excess, to aqueous copper(II) sulfate. [3]
(C) Industrial Application of Electrolysis
Electroplating
5.
A student decided to silver-plate a locker key using the apparatus shown below.
power supply
silver
electrode
AgNO3
locker
key
(a) State the anode and cathode [1]
(b) Write the equations for the reactions that happen in each electrode. [2]
Fuel Cell
6
In the following experimental set-up, the Hydrogen-oxygen fuel cell supplies
electricity for the electrolysis of concentrated sodium chloride solution.
(a)
Redox reaction occurs in the Hydrogen-oxygen fuel cell. At the
positive electrode (cathode), oxygen is reduced to form hydroxide ions.
At the negative electrode (anode), hydrogen is oxidised to form water.
Write the chemical reaction for
reactions occurring at the cathode and anode.
[2]
(b) What is liquid C?
(D) Simple Cell
7.
In which of the following cells will the electrons flow in the anti clockwise direction?
A
B
Fe
C
D
Ag