Electrochemistry: Practice Questions

What is the oxidation number of sulfur in the thiosulfate ion, \( \text{S}_2\text{O}_3^{2-} \)?

In the unbalanced equation \( a\text{Cr}_2\text{O}_7^{2-} + b\text{H}^+ + c\text{I}^- \rightarrow d\text{Cr}^{3+} + e\text{I}_2 + f\text{H}_2\text{O} \), what are the values of the coefficients a and c when the equation is balanced using the change in oxidation number method?

A solution containing \( 0.10 \text{ mol} \) of \( \text{VO}_2^+ \) ions is reduced by exactly \( 0.20 \text{ mol} \) of electrons. What is the oxidation state of vanadium in the product of this reduction?

In the following redox reaction, which species acts as the reducing agent?
\( \text{Zn(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Zn}^{2+}\text{(aq)} + \text{Cu(s)} \)

When the redox equation \( a\text{MnO}_4^- + b\text{H}_2\text{O}_2 + c\text{H}^+ \rightarrow d\text{Mn}^{2+} + e\text{O}_2 + f\text{H}_2\text{O} \) is balanced using the lowest possible integer coefficients, what is the value of the ratio \( \frac{b}{a} \)?

Determine the oxidation number of S in the following species:
(i) \(\text{S}_2\text{O}_3^{2-}\)
(ii) \(\text{SO}_4^{2-}\)
(iii) \(\text{H}_2\text{S}\)

Consider the reaction: \(3\text{Br}_2 + 6\text{OH}^- \rightarrow 5\text{Br}^- + \text{BrO}_3^- + 3\text{H}_2\text{O}\). Identify the oxidising agent and the reducing agent, justifying your choice using the change in oxidation numbers of bromine.

Explain, in terms of oxidation numbers, why the reaction between iron(II) ions and acidified potassium manganate(VII) is classified as a redox process. Use the changes in oxidation states of Fe and Mn in your answer.

The oxidation of ammonia is represented by the following equation:
\(4\text{NH}_3 + 5\text{O}_2 \rightarrow 4\text{NO} + 6\text{H}_2\text{O}\)
(a) Calculate the oxidation number of nitrogen in \(\text{NH}_3\) and in \(\text{NO}\).
(b) Identify the species being oxidised and the species being reduced, giving reasons in terms of oxidation number changes.
(c) Define disproportionation and explain why the reaction above is NOT a disproportionation reaction.

Consider the redox reaction between acidified potassium manganate(VII) and iron(II) ions:
\(\text{MnO}_4^- + 8\text{H}^+ + 5\text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + 5\text{Fe}^{3+} + 4\text{H}_2\text{O}\)
(a) Define oxidising agent in terms of electron transfer.
(b) Deduce the change in oxidation number for manganese and iron in this reaction.
(c) If \(25.0\text{ cm}^3\) of \(0.020\text{ mol dm}^{-3}\) \(\text{KMnO}_4\) reacts exactly with \(25.0\text{ cm}^3\) of an \(\text{FeSO}_4\) solution, calculate the concentration of the \(\text{Fe}^{2+}\) ions in \(\text{mol dm}^{-3}\).