Studyledge

Electrolysis: Question 1

Syllabus 4.1

Multiple choice Core 1 mark

A technician melts solid zinc bromide, ZnBr2\text{ZnBr}_2, until it is completely liquid, then passes a direct current through the molten compound using two inert graphite electrodes.

Which row correctly gives the products formed at the cathode and at the anode?

Choose an answer to check it, then compare with the worked solution below.
Show worked solution Hide worked solution

Worked solution

Step 1: Identify the ions present in the molten compound

Molten zinc bromide contains only two types of mobile ion, since there is no water present to supply any others:

  • Zn2+\text{Zn}^{2+} (positive zinc ions)
  • Br−\text{Br}^- (negative bromide ions)

Step 2: Apply the rule for electrode charge and ion attraction

Opposite charges attract, so:

  • The cathode is the negative electrode. It attracts the positive Zn2+\text{Zn}^{2+} ions, which each gain two electrons (reduction) and are discharged as zinc metal.
  • The anode is the positive electrode. It attracts the negative Br−\text{Br}^- ions, which each lose an electron (oxidation) and are discharged as bromine.

Step 3: Match to the general rule

This matches the standard rule for electrolysing a molten binary compound: the metal forms at the cathode, and the non-metal forms at the anode.

Why the other options are wrong

  • Option B swaps the electrodes: it would require the positive zinc ions to travel to the positive anode, which opposite-charge attraction rules out.
  • Option C introduces hydrogen and oxygen, which can only come from water, but this is a molten compound with no water present.
  • Option D wrongly gives oxygen instead of bromine at the anode; there are no oxygen-containing ions in molten zinc bromide.

Final answers

  • Cathode: zinc. Anode: bromine, option A\boxed{\text{option A}}.