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 E_cell for each stage of reduction of vanadium species to determine its extent of reduction.

E_cell 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⁺.

E_cell 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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