Redox (Cambridge (CIE) O Level Chemistry): Exam Questions

A student adds aqueous sodium hydroxide to a test-tube containing aqueous iron(II) sulfate.

A green precipitate forms.

The student leaves the test-tube open to the air. After one hour, the precipitate has turned brown near the surface of the liquid. The rest of the precipitate is still green.

Explain, in terms of oxidation number, what has happened to the iron near the surface of the liquid.

Name the oxidising agent in this change.

Explain why the colour change is seen only near the surface of the liquid.

The student repeats the experiment, but this time pours a layer of oil onto the surface of the liquid before leaving the test-tube for one hour.

Predict what the student sees after one hour.

Explain your prediction.

Suggest two changes the student could make so that the comparison between the two test-tubes is more reliable.

A student records the result of the first experiment as "the precipitate goes a different colour".

Explain why this would not gain a mark.

A student is given two colourless solutions, S and T.

The student tests a portion of each solution with a few drops of acidified aqueous potassium manganate(VII), and a separate portion of each with aqueous potassium iodide.

The observations are shown.

State a conclusion about solution S.

Give the observation that supports it.

State a conclusion about solution T.

Give the observation that supports it.

Name the substance that causes the brown colour when solution T is added to aqueous potassium iodide.

A student writes that in this reaction "the potassium iodide is oxidised".

Explain why this answer is not precise enough.

Explain why the student cannot reach a definite conclusion about what solution S is from the potassium iodide test alone.

The student is told that one of the two solutions is aqueous potassium iodide.

Identify which solution is the aqueous potassium iodide.

Explain your reasoning.

A student warms solid sodium sulfite with dilute hydrochloric acid in a boiling tube.

A gas is produced.

The student holds a piece of filter paper, damp with acidified aqueous potassium manganate(VII), in the mouth of the boiling tube.

State what the student sees in the boiling tube when the dilute hydrochloric acid is added to the sodium sulfite.

State the change the student sees on the filter paper.

The gas produced is sulfur dioxide.

State whether the sulfur dioxide is acting as an oxidising agent or as a reducing agent in its reaction with the acidified potassium manganate(VII). Justify your answer.

Explain why the filter paper must be damp, and why it is held in the mouth of the boiling tube rather than dipped into the reaction mixture.

State the oxidation number of sulfur in the sulfite ion, SO₃^2-.

A student is given aqueous potassium iodide and three separate colourless solutions, X, Y and Z.

One or more of the three solutions may contain an oxidising agent.

Plan an experiment to find out which of the three solutions contain an oxidising agent.

Your plan must use aqueous potassium iodide and common laboratory apparatus. No other chemicals should be used.

Your plan should include:

• the apparatus you would use

• what you would do

• the observations you would make

• how you would use your observations to reach a conclusion