A student added aqueous iron(II) sulfate drop by drop to a portion of acidified potassium manganate(VII) solution in a test tube, as shown in Fig. 1.1. The ...
Question 1 Report
A student added aqueous iron(II) sulfate drop by drop to a portion of acidified potassium manganate(VII) solution in a test tube, as shown in Fig. 1.1. The purple colour of the solution was watched carefully during the experiment.
Fig. 1.1
(a) Name a piece of apparatus the student could use to add the iron(II) sulfate solution drop by drop. [1]
(b) Describe the colour change seen in the test tube as the iron(II) sulfate is added. [1]
(c) Complete the table to show whether each substance is oxidised or reduced in this reaction.
| substance | oxidised or reduced |
|---|---|
| iron(II) ions | |
| manganate(VII) ions |
[2]
(d) State why dilute sulfuric acid is added to the potassium manganate(VII) before the test. [1]
Answer Details
This question is about a redox titration-style observation. Acidified potassium manganate(VII) is a purple oxidising agent; iron(II) is a reducing agent. Watching the purple colour disappear tells you the reaction is happening.
(a) A suitable piece of apparatus to add the solution drop by drop is a (teat / dropping) pipette, and a burette is also accepted [1]. Both let you add small, controlled volumes one drop at a time.
(b) The colour change seen is purple (pink) to colourless; the solution decolourises [1]. The purple manganate(VII) ion is reduced to the almost colourless \(Mn^{2+}\) ion.
(c) Oxidised or reduced [2]:
| substance | oxidised or reduced |
|---|---|
| iron(II) ions | oxidised [1] |
| manganate(VII) ions | reduced [1] |
Iron(II) loses an electron (\(Fe^{2+} \rightarrow Fe^{3+} + e^{-}\)), which is oxidation, while manganese goes from the \(+7\) state in \(MnO_4^{-}\) to \(+2\) in \(Mn^{2+}\), a gain of electrons, which is reduction. The overall equation is \(MnO_4^{-} + 8H^{+} + 5Fe^{2+} \rightarrow Mn^{2+} + 4H_2O + 5Fe^{3+}\).
(d) Dilute sulfuric acid is added to provide the acidic conditions (\(H^{+}\) ions) needed for the manganate(VII) to be reduced and for the reaction to take place [1]. Without acid the reaction is incomplete and gives a brown manganese(IV) oxide instead of a clean colourless end-point.
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