Electrolysis: Answers

Why can’t covalent compounds be broken down by electrolysis?

Covalent compounds do not contain positive and negative ions. Since they only contain neutral atoms, they cannot act as electrolytes and will not be attracted to the electrodes.

What are the positive and negative electrodes called?

The positive electrode is called the anode and the negative electrode is called the cathode. Remember that anions (-) will move towards the anode and cations (+) will move towards the cathode.

What does oxidation and reduction mean in terms of electron transfer?

Oxidation occurs when an atom loses electrons and reduction occurs when an atom gains electrons. Use the word ‘OILRIG’ to help you remember.

What would be produced from electrolysis of the following solutions:

  • Copper chloride solution

    To answer these questions we need to remember the rules for the formation of different products at the cathode and anode. At the cathode hydrogen is produced unless the cations are less reactive than hydrogen. Copper, silver, gold and platinum are the only elements less reactive than hydrogen. This means that if we have a copper chloride solution, copper will be produced at the cathode.

    At the anode, oxygen will be produced unless the anions belong to group 7 of the Periodic Table (we call these elements halogens). Chlorine is indeed one of the group 7 elements so it is produced at the anode.

  • Copper sulfate solution

    Copper is produced at the cathode (it is less reactive than hydrogen).

    Oxygen is produced at the anode (sulfate ions are not halogens).

  • Sulfuric acid solution

    Hydrogen is produced at the cathode (the cation in this case is hydrogen so it is the only positive ion we have in our solution).

    Oxygen is produced at the anode (sulfate ions are not halogens).

Write half equations for the electrolysis of zinc bromide solution and molten lead bromide.

Zinc bromide solution contains the following ions: Zn2+, Br-, H+ and OH-. Of the positive ions, hydrogen gas will be formed at the cathode (because zinc is not below hydrogen in the reactivity series. Of the negative ions, bromine gas will form at the anode because it is in group 7 of the Periodic Table. Therefore the two half equations are:
2H+ + 2e- --> H2
2Br- --> Br2 + 2e-

Molten lead bromide only contains the positive lead ions and negative bromide ions (remember there is no water in molten liquids so no H+ or OH- ions. The two half equations are:
Pb2+ + 2e- --> Pb
2Br- --> Br2 + 2e-