Achievement Test 1 (Question 1 & 2) – Chemistry Form 5 Chapter 1 (Redox Equilibrium)

Question 1:
Identify whether a redox reaction occurs for each reaction below.


Explain your answer based on the change in oxidation number.

Answer:
Reaction I is a redox reaction.
– The oxidation number of iron before the reaction is +2 and after the reaction is 0.

– The oxidation number of magnesium before the reaction is 0 and after the reaction is +2.

– Magnesium undergoes an oxidation reaction because the oxidation number of magnesium increases from 0 to +2.

– Iron(II) sulphate undergoes a reduction reaction because the oxidation number of iron decreases from +2 to 0.


Reaction II is not a redox reaction.


– The oxidation number of all elements: silver, nitrogen, oxygen, sodium and chlorine, remain unchanged before and after the reaction.

Question 2:
The overall ionic equation below shows the reaction between iodine, I₂ and sulphur dioxide, SO₂.

$$\mathrm{I}_2(\mathrm{aq})+\mathrm{SO}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{I}^{-}(\mathrm{aq})+\mathrm{SO}_4^{2-}(\mathrm{aq})+4 \mathrm{H}^{+}(\mathrm{aq})$$
Based on the equation above:
(a) state the change in oxidation number for iodine and sulphur.
(b) name the oxidising agent and reducing agent in this reaction.
(c) write the reduction half equation.

Answer:
(a) The oxidation number of iodine decreases from 0 to -1.

The oxidation number of sulphur in sulphur dioxide increases from +4 to +6.

(b) Oxidising agent : Iodine.
Reducing agent : Sulphur dioxide.

(c) I₂ + 2e⁻ → 2I⁻