Answering Techniques for Redox Reaction

Often, you will see commonly asked questions that revolve around the chapter of redox reaction. The answers are somewhat standard. Therefore, I compiled a list of frequently asked questions and model answer template that you can use to answer such questions. I included essential keywords that you must memorise and write down in your answer to obtain the full credit.

Question type 1: Explaining redox.

Usually, the question will ask you why a reaction is redox.

Your answer should always answer in terms of oxidation state, instead of gain/loss in electrons, hydrogen or oxygen, unless the question state otherwise. For example, if the question state “Explain in terms of electron transfer, why %%compound name%% undergoing oxidation/reduction.” In that case, please use the question requires to answer the question directly.

To answer the question using the oxidation state, you can follow a fixed template. The phrasing of the answer is very important to show that you are strong in your concepts.

Answering why this species has undergone reduction or oxidation in terms of oxidation state

The %%name of species/chemical formula%% is oxidised/reduced because the oxidation state of %%element name/formula%% has increased/decreased from %%original oxidation state%% in %%original compound that contains the element%% to %%final oxidation state%% in %%final compound that contains the element%%.

Answering why this species has undergone reduction or oxidation in terms of gain/loss of hydrogen, oxygen or electron

%%initial name of substance%% is oxidised/reduced because it has lost/gained hydrogen/electron/oxygen to form %%final name of substance%%.

Answering why this species is an oxidising or reducing agent

Since %%initial name of substance%% is oxidized/reduced, it is a reducing/oxidising agent.

Answering why this reaction is a redox reaction

There are three parts to this question, and the total marks are usually two marks, though I have seen school exam papers awarding students up to 3 marks.

The three marking points are:

  1. Explain the compound undergoing oxidation (which and why)
  2. Explain the compound undergoing reduction (which and why)
  3. Summarise: Since both oxidation and reduction occur simultaneously in a same chemical reaction, redox reaction occur.

All 3 points must be present to get the full credit for the question. For point 1 and 2, it is the same explanation as above. Also, you must read the question clearly; if the question asks (Explain in terms of electron transfer…), then you must write your answer in terms of electron transfer.

Point 1: The %%name of species/chemical formula%% is oxidised because the oxidation state of %%element name/formula%% has increased from %%orginal oxidation state%% in %%original compound that contains the element%% to %%final oxidation state%% in %%final compound that contains the element%%.

Point 2: The %%name of species/chemical formula%% is reduced because the oxidation state of %%element name/formula%% has decreased from %%orginal oxidation state%% in %%original compound that contains the element%% to %%final oxidation state%% in %%final compound that contains the element%%.

Point 3: Summary: Since both oxidation and reduction occur simultaneously in a same chemical reaction, redox reaction occur.

*Missing any one point – 1m
*All 3 points must be present for 2m.

Explaining colour changes

Normally you need to explain colour changes for the three reducing/oxidising agents that are required for Singapore O Level syllabus. That is:

  1. Acidified aqueous potassium manganate (VII) KMnO4(aq)
  2. Acidified aqueous potassium dichromate(VI) K2Cr2O7(aq)
  3. Potassium iodide solution KI (aq)

1. When Acidified aqueous potassium manganate (VII) is reduced (reacted with reducing agent)

The solution turned from purple to colourless. * Need to mention before and after colour change. (MnO4 reduced to Mn2+)

2. When Acidified aqueous potassium dichromate(VI) is reduced (reacted with reducing agent)

The solution turned from orange to green. (Cr2O72- reduced to Cr3+)

3. When Potassium Iodide solution is oxidised (reacted with oxidising agent)

The solution turned from brown to colourless. (I2 reduced to I).

Leave a Comment