Ethanol can be oxidised by heating with potassium dichromate(VI) in dilute sulfuric acid
5 marks · HardA student adds ethanol to a solution of potassium dichromate(VI) in dilute sulfuric acid. She then heats the mixture in a water bath.
State the initial colour of the potassium dichromate(VI) solution before the ethanol is added and heated.
Describe the colour change observed as the reaction occurs and explain what causes this change.
Which ion is responsible for the green colour observed at the end of the reaction?
Explain why the colour change provides evidence that a redox reaction has taken place.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Orange
(b) The solution changes from orange to green due to the reduction of Cr(VI) ions (orange) to Cr3+ ions (green) during the oxidation of ethanol.
(c) Cr3+
(d) The colour change indicates that chromium ions have been reduced from Cr(VI) (orange) to Cr3+ (green), showing that a redox reaction has happened.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that the initial colour is orange.
B2 (b) · 1 mark · Describes that the colour changes from orange to green.
B3 (b) · 1 mark · Explains that the change is caused by the reduction of Cr(VI) to Cr3+ ions.
B4 (c) · 1 mark · Correctly identifies Cr3+ ion as responsible for the green colour.
B5 (d) · 1 mark · Explains that the colour change is evidence for reduction of chromium ions and thus a redox reaction.
Worked solution
(a) The initial colour of the potassium dichromate(VI) solution is orange.
(b) The colour changes from orange to green due to the reduction of Cr(VI) ions (orange) to Cr3+ ions (green) as ethanol is oxidised.
(c) The ion responsible for the green colour is Cr3+.
(d) The appearance of green colour shows that Cr(VI) ions (orange) have been reduced to Cr3+ ions, which is a redox reaction because chromium is reduced while ethanol is oxidised.