2021 P1 Q29 - Determine Redox Reaction between Zn and VO2+
Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 29.
We need to determine the redox reaction when excess zinc is added to acidified VO2+ ions.
In the options vanadium can be reduced to different oxidation states.
Let us find all the relevant half equations in the data booklet.
Notice zinc metal can be oxidised to Zn2+.
VO2+ (+5 oxidation state) can be reduced to VO2+ (+4 oxidation state).
VO2+ can then be further reduced to V3+, V3+ can be reduced to V2+, and finally V2+ reduced to vanadium metal.
Therefore we need to calculate the Ecell for each stage of reduction of vanadium species to determine its extent of reduction.
Ecell for redox reactions between Zn and VO2+ (+5 oxidation state), VO2+ (+4 oxidation state) and V3+ are all positive. This means all these reactions are feasible and VO2+ can be reduced all the way to V2+.
Ecell for redox reaction between Zn and V2+ is negative. This means Zn cannot reduce V2+ to vanadium metal.
Therefore the final oxidation state of vanadium should be V2+ and answer to this question is option C.
Topic: Electrochemistry, Physical Chemistry, A Level Chemistry, Singapore
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