Chemistry Question – Redox Reactions

20 Feb

Posted by: sean in: Chemistry Notes & Tips

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.

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4 Responses

Jennifer Wee
03|Mar|2008 1

Thank you sir for all your efforts and explanation. They are greatly appreciated! I understood Redox much better after that. Thank you Mr. Sean and I would surely recommend you to all my friends!

Reply
Jo
09|May|2008 2

hey. i recently found out about this website, and i feel that it is extremely helpful.

i was wondering, if aqueous potassium iodide by itself could be used to test for oxidising agents?

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dona
14|May|2008 3

kI(aq)changes from colourless to brown with oxidising agents

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sean
14|May|2008 4

KI can certainly be used to test for oxidizing agent on its own.

As what “dona” has stated, color will change from colourless (KI) to brown (I2).

Starch solution is used mainly like an indicator to enhance the visual confirmation.

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