Acidified potassium manganate(VII) is a common oxidising agent used in the laboratory. A student adds three different substances, in turn, to separate sampl...
Question 1 Report
Acidified potassium manganate(VII) is a common oxidising agent used in the laboratory. A student adds three different substances, in turn, to separate samples of acidified potassium manganate(VII) and records what is seen. Table 4.1 shows the results.
| substance added to acidified potassium manganate(VII) | observation |
|---|---|
| aqueous sulfur dioxide (a reducing agent) | ............................ |
| distilled water | solution stays purple |
| aqueous iron(II) sulfate (a reducing agent) | ............................ |
(a) State the colour of acidified potassium manganate(VII) before any substance is added. [1]
(b) Complete the two blank observations in Table 4.1. [2]
(c) State what the colour change tells you about sulfur dioxide. [1]
(d) Explain, in terms of electrons, what a reducing agent does. [1]
(e) When it acts as an oxidising agent, potassium manganate(VII) is itself reduced. State, in terms of electrons, what "reduced" means. [1]
(f) Define the term oxidising agent in terms of electrons. [1]
(g) Suggest why distilled water produces no colour change. [1]
(h) Give the overall colour change seen when a reducing agent is added to acidified potassium manganate(VII). [1]
Answer Details
This question tests the language of redox in terms of electron transfer, using acidified potassium manganate(VII) as a coloured oxidising agent. The manganate(VII) ion, MnO4-, contains manganese in its highest oxidation state and is intensely purple. When it acts as an oxidising agent it takes electrons from a reducing agent and is itself reduced to the manganese(II) ion, Mn2+, which is so pale that the solution appears colourless. Watching the purple disappear is therefore a direct test for a reducing agent.
(a) Before anything is added the solution is purple [1], the colour of the MnO4- ion.
(b) Both reducing agents remove the purple colour, so the completed observations are:
| substance added | observation |
|---|---|
| aqueous sulfur dioxide | purple colour fades to colourless [1] |
| distilled water | solution stays purple |
| aqueous iron(II) sulfate | purple colour fades to colourless [1] |
(c) Because the sulfur dioxide reduces the manganate(VII), the colour change tells you that sulfur dioxide is a reducing agent [1].
(d) A reducing agent donates (gives/loses) electrons to another substance [1]. In doing so it reduces the other substance while being oxidised itself.
(e) To be reduced means to gain electrons [1]. Here Mn goes from the +7 state in MnO4- to the +2 state in Mn2+, gaining electrons.
(f) An oxidising agent is a substance that accepts (gains) electrons from another substance [1]; it oxidises the other substance and is reduced itself.
(g) Distilled water produces no change because water is neither an oxidising nor a reducing agent, so no electrons are transferred to or from the manganate(VII) [1]. This is the control that shows the colour change is caused by the added substance, not by dilution.
(h) The overall colour change when any reducing agent is added is purple to colourless [1].
Remember the memory aid OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). The reducing agent is oxidised; the oxidising agent is reduced.
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