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Transition elements

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Question 1

This question is about some reactions of d-block elements and their ions.

Table 1 shows standard electrode potentials which will be needed within this question.

Half-reactionEθ/VMg2+(aq)+2e−⇌Mg(s)−2.37V3+(aq)+e−⇌V2+(aq)−0.26Co2+(aq)+2e−⇌Co(s)−0.28VO2+(aq)+2H+(aq)+e−⇌V3+(aq)+H2O(l)+0.34Br2(aq)+2e−⇌2Br−(aq)+1.09Fe3+(aq)+e−⇌Fe2+(aq)+0.77S2O82−(aq)+2e−⇌2SO42−(aq)+2.01\begin{array}{|l|c|} \hline \text{Half-reaction} & E^\theta / \text{V} \\ \hline \text{Mg}^{2+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Mg}(\text{s}) & -2.37 \\ \text{V}^{3+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{V}^{2+}(\text{aq}) & -0.26 \\ \text{Co}^{2+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Co}(\text{s}) & -0.28 \\ \text{VO}^{2+}(\text{aq}) + 2\text{H}^+(\text{aq}) + \text{e}^- \rightleftharpoons \text{V}^{3+}(\text{aq}) + \text{H}_2\text{O}(\text{l}) & +0.34 \\ \text{Br}_2(\text{aq}) + 2\text{e}^- \rightleftharpoons 2\text{Br}^-(\text{aq}) & +1.09 \\ \text{Fe}^{3+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{Fe}^{2+}(\text{aq}) & +0.77 \\ \text{S}_2\text{O}_8^{2-}(\text{aq}) + 2\text{e}^- \rightleftharpoons 2\text{SO}_4^{2-}(\text{aq}) & +2.01 \\ \hline \end{array}Half-reactionMg2+(aq)+2e−⇌Mg(s)V3+(aq)+e−⇌V2+(aq)Co2+(aq)+2e−⇌Co(s)VO2+(aq)+2H+(aq)+e−⇌V3+(aq)+H2​O(l)Br2​(aq)+2e−⇌2Br−(aq)Fe3+(aq)+e−⇌Fe2+(aq)S2​O82−​(aq)+2e−⇌2SO42−​(aq)​Eθ/V−2.37−0.26−0.28+0.34+1.09+0.77+2.01​​
a.

Complete the electron configuration of:

  • a Co\text{Co}Co atom: 1s2 …1\text{s}^2 \ \dots1s2 …
  • a Co2+\text{Co}^{2+}Co2+ ion: 1s2 …1\text{s}^2 \ \dots1s2 …
[2]
b.

A standard cell is set up in the laboratory with the cell reaction shown below:

Co(s)+Br2(aq)→Co2+(aq)+2Br−(aq) \text{Co}(\text{s}) + \text{Br}_2(\text{aq}) \rightarrow \text{Co}^{2+}(\text{aq}) + 2\text{Br}^-(\text{aq}) Co(s)+Br2​(aq)→Co2+(aq)+2Br−(aq)

Describe how this cell could be set up to measure its standard cell potential. Include details of the electrodes, solutions, salt bridge, and standard conditions required.

[3]
c.

Predict the standard cell potential of this cell.

[1]
d.

Use the information in Table 1 to help answer both parts of this question.

Write the overall equation for the oxidation of Fe2+\text{Fe}^{2+}Fe2+ by S2O82−\text{S}_2\text{O}_8^{2-}S2​O82−​.

[1]
e.

Magnesium reacts with acidified VO2+\text{VO}^{2+}VO2+ ions to form V2+\text{V}^{2+}V2+ ions in two stages. Explain why this happens in terms of electrode potentials and equilibria. Include overall equations for the reactions which occur.

[3]
f.

Three different reactions of transition metal compounds are described below.

  • Reaction 1: Aqueous cobalt(II) chloride reacts with excess concentrated hydrochloric acid in a ligand substitution reaction. A deep-blue solution is formed containing a tetrahedral complex ion, C.
  • Reaction 2: Green potassium manganate(VI) reacts with dilute sulfuric acid in a disproportionation reaction to form a purple solution, D, and a dark-brown precipitate, E.
  • Reaction 3: Cobalt(II) carbonate reacts with warm dilute nitric acid in a neutralisation reaction to form a pink solution. Unreacted cobalt(II) carbonate is filtered off, and the remaining solution is evaporated to allow a hydrated salt, F, to crystallise. The hydrated salt, F, has the percentage composition by mass: Co\text{Co}Co, 20.25%; N\text{N}N, 9.62%; H\text{H}H, 4.16%; O\text{O}O, 65.97%.

Identify C–F by formulae or structures, as appropriate. Include equations, any changes in oxidation number, and working.

[5]

Transition elements Questions

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