Table 8.1 shows some aqueous solutions electrolysed using inert electrodes. Some products are missing. Table 8.1 Aqueous electrolyte Product at cathode Prod...
Question 1 Report
Table 8.1 shows some aqueous solutions electrolysed using inert electrodes. Some products are missing.
| Aqueous electrolyte | Product at cathode | Product at anode |
|---|---|---|
| dilute sulfuric acid | ||
| concentrated hydrochloric acid | hydrogen | |
| copper(II) sulfate | oxygen |
(a) Complete the four empty boxes in Table 8.1. [4]
(b) State the rule that decides whether a metal or hydrogen is formed at the cathode. [2]
(c) Explain why chlorine is formed at the anode with concentrated hydrochloric acid, but oxygen is formed with dilute sulfuric acid. [2]
(d) State one observation that shows chlorine is being formed at the anode. [1]
(e) Name one material used to make an inert electrode. [1]
Answer Details
This question tests the electrolysis of aqueous solutions with inert electrodes, where water competes with the dissolved ions. At the cathode either the metal or hydrogen is discharged; at the anode either a halogen (if concentrated halide) or oxygen is discharged.
(a) The completed table is:
| Aqueous electrolyte | Product at cathode | Product at anode |
|---|---|---|
| dilute sulfuric acid | hydrogen [1] | oxygen [1] |
| concentrated hydrochloric acid | hydrogen | chlorine [1] |
| copper(II) sulfate | copper [1] | oxygen |
(b) The rule at the cathode: if the metal is more reactive than hydrogen, hydrogen is formed at the cathode [1]; if the metal is less reactive than hydrogen (for example copper), the metal is formed [1].
(c) At the anode: a concentrated halide solution releases the halogen (chlorine) [1], whereas a dilute solution or a non-halide (like sulfate) releases oxygen from the water/hydroxide ions [1].
(d) An observation for chlorine: a pale green/yellow gas is seen that bleaches damp litmus paper [1].
(e) A material for an inert electrode: carbon (graphite) or platinum [1].
Exam tip: sulfuric acid electrolysis is really the electrolysis of water (hydrogen and oxygen in a 2:1 volume ratio); the sulfate ion is too stable to be discharged, so oxygen comes off instead.
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