Electrolysis

Question 1

(1) Which of these ions will be attracted to the cathode during the electrolysis of
sodium chloride solution?

Answer 1

H+ and Na+ ions

Question 2

(2) 2Cl–→Cl2+2e

Explain how the half equation shows that chloride ions are oxidised

Answer 2

electron(s) (1)
• (have been) lost/removed
(1) conditional on
electrons

Question 3

(1) Suggest why the solution remaining at the end of the electrolysis is alkaline.

Answer 3

it contains (excess)
{hydroxide/OH¯
} ions (1)
• {sodium/Na+} ions and
{hydroxide/ OH¯
} ions
remain (1)
• it is sodium
hydroxide/NaOH (1)
• {hydrogen/H+} ions have
been removed (at the
cathode) (1)

Question 4

(1) The electrolysis of sodium chloride solution does not produce metallic sodium.
State what change you would make to the electrolyte to obtain metallic sodium.

Answer 4

use {molten/liquid} {sodium
chloride /electrolyte} / melt
{it/sodium chloride/electrolyte}

Question 5

(2) When copper sulfate solution is electrolysed using inert electrodes, oxygen is
formed at the positively charged anode.
Explain how the oxygen is formed from ions in the solution.

Answer 5

{hydroxide/OH-
} ions
(from water) (1)
Marking point 2
• (ions) lose electrons /are
oxidised (1)

Question 6

(1) Some metal objects are electroplated to improve their appearance.
Give another reason why some metal objects are electroplated.

Answer 6

improves resistance to corrosion

Question 7

(3) The electrolysis was carried out for a period of time.
The electrodes were removed, washed, dried and their masses redetermined.
The table shows the masses of the electrodes before and after electrolysis.
mass of electrode
before electrolysis / g
mass of electrode
after electrolysis / g change in mass
mass of impure
copper anode 40.0 35.0 5.0 g decrease
mass of pure
copper cathode 10.0 14.8 4.8 g increase

Explain these results.

Answer 7

anode dissolves / half equation
given / atoms lose electrons
/copper becomes ions/copper
(atoms) oxidised (1)
 copper ions moving from
anode to cathode (1)
 copper plated on cathode /
half equation given / ions gain
electrons/copper ions
reduced/copper ions form
atoms (1)
 {difference/0.2g} is impurities
(1)
 would expect same decrease
in mass (as increase) (1)

Question 8

(2) In an electrolysis experiment, oxide ions, O2–, form oxygen gas, O2
.
Write the balanced half equation for the reaction.

Answer 8

2O2-  O2 + 4e(-) (2)

Question 9

(6) Sodium chloride is an ionic compound.
It contains sodium ions, Na+, and chloride ions, Cl–
.
When molten sodium chloride is electrolysed, sodium metal and chlorine gas are
formed.
Describe how the sodium ions and chloride ions in solid sodium chloride are
converted into sodium and chlorine by electrolysis.

Answer 9

sodium chloride is melted and ions are free to move
at cathode
 {sodium / positive} ions move to cathode
 (sodium) metal forms at cathode
 sodium ions gain electrons ...
 to form atoms
 this is reduction
 Na+ + e-
  Na
at anode
 {chloride / negative} ions move to anode
 (chlorine) gas forms at anode
 chloride ions lose electrons ...
 to form atoms
 this is oxidation
 two chlorine atoms combine to form a chlorine molecule /
share electrons
 Cl-
  Cl + e-
 / 2Cl-
  Cl2 + 2e-

 2 Cl  Cl2

Question 10

(2) HCl(aq) + NaOH(aq) ĺ NaCl(……..) + H2O(……..)The reaction above is a neutralisation reaction.
Write the ionic equation for the reaction.

Answer 10

H+ + OH- (1)  H2O (1)

Question 11

(1) Give the name of the electrode which is made of pure copper

Answer 11

cathode

Question 12

(2) Describe how each electrode will have changed at the end of the electrolysis
process.

Answer 12

ANODE/IMPURE COPPER/ +VE
ELECTRODE
 becomes smaller/ loses mass /
loses copper / copper atoms
form ions / copper is oxidised /
copper (ions) enter solution (1)
CATHODE/ PURE COPPER/ -VE
ELECTRODE
 {red-brown/copper} deposit/
becomes larger / gains mass /
gains copper / copper ions are
reduced / copper (ions) from
solution add to electrode (1)

Question 13

(1) Cu2+(aq) + 2e–
= Cu(s)
This half equation shows the process of

Answer 13

reduction

Question 14

(2) Copper sulfate solution is the electrolyte used in this electrolysis process.
Explain how copper sulfate solution conducts electricity.

Answer 14

ions / cations / copper ions / anions /
sulfate ions (1)
(are free to) move (in solution) (1)

Question 15

(6) Sodium sulfate solution contains sodium ions, Na+, sulfate ions, SO4
2–,
hydrogen ions, H+, and hydroxide ions, OH–
.
Hydrogen is produced at one electrode and oxygen is produced at the other
electrode.
Explain how these products are formed from the ions in the electrolysis
process, indicating how you would identify the products.
You may give ionic equations in your answer.

Answer 15

electrolysis process
 ions move when current passed
 negative ions move to anode
 overall decomposition of water
 2H2O → 2H2+O2
Anode/ positive electrode
 sulphate ions move to anode
 hydroxide ions move to anode
 hydroxide ions lose electrons/oxidation
 hydroxide ions form oxygen
 half equation: 4OH- → O2 + 2H2O + 4e-
 half volume gas at this electrode
 oxygen test: glowing splint in gas relights
cathode/ negative electrode
 sodium ions move to cathode
 hydrogen ions move to cathode
 hydrogen ions gain electrons / reduction
 hydrogen ions form hydrogen
 half equation: 2H+ + 2e- → H2
 double volume gas at this electrode
 hydrogen test: lit splint in gas burns/ pops
 hence double volume of hydrogen gas