I noticed that students have problem to score full marks for the following question on redox reactions. This is a question that is somehow repeating in GCE O Level examinations as well as Preliminary Examinations in several local secondary schools.
Question:
(a) Define oxidation in terms of electron transfer. [1 mark]
(b) Name a substance which is an oxidizing agent in aqueous solution. Explain how aqueous potassium iodide can be used to confirm that this named substance is an oxidizing agent. [3 marks]
Suggested Answer to get Full Marks:
(a) Oxidation is the transfer of electron from one particle to another. The particle that looses the electron is said to be oxidized.
(b) Chlorine is an oxidizing agent (1 mark). Aqueous chlorine is added to aqueous potassium iodide with starch added. A dark blue solution is seen, indicating the presence of I2 (1 mark). So I- ion must have been oxidized to I2 (1 mark).
Note: I- is oxidized to I2 which can be tested with starch solution. Presence of starch will turn I2 solution to dark blue.