Edexcel A-Level Chemistry A2 16.7 A Standard Emf Eocell By Combining Two Standard Electrode Potentials Questions

Use the Data Booklet value for each half-cell, subtract in the correct direction and check a positive E°cell before writing the feasible cell reaction.

Exam points

Calculate E°cell from Data Booklet values using right minus left or equivalent.
Deduce unknown electrode potentials from measured emf and a reference half-cell.
Use positive E°cell to choose reaction direction and combine balanced half-equations.

[Maximum number: 1]

This question is about mercury, Hg, and its compounds.
Mercury is a liquid element in the same group of the Periodic Table as zinc.
The electronic configuration of mercury is $[\mathrm{Xe}] 4 \mathrm{f}^{14} 5 \mathrm{~d}^{10} 6 \mathrm{~s}^{2}$ .

Mercury(I) chloride, $\mathrm{Hg}_{2} \mathrm{Cl}_{2}$ , is also known as calomel.

A saturated calomel electrode may be used as an alternative to the standard hydrogen electrode.

Figure for Question Edexcel A-Level Chemistry A2 16.7 A Standard Emf Eocell By Combining Two Standard Electrode Potentials Questions question 1 — Edexcel A-Level Chemistry A2

The half-equation for the calomel electrode is

\mathrm{Hg}_{2} \mathrm{Cl}_{2}(\mathrm{~s})+2 \mathrm{e}^{-} \rightleftharpoons 2 \mathrm{Hg}(\mathrm{l})+2 \mathrm{Cl}^{-}(\mathrm{aq})

The standard electrode potential of the calomel electrode is $E^{\ominus}=+0.24 \mathrm{~V}$ .

A calomel electrode was used to measure the standard electrode potential of the $\mathrm{Sn}^{2+}(\mathrm{aq}) \mid \mathrm{Sn}(\mathrm{s})$ half-cell.

The reading on the voltmeter in this cell was +0.37 V .
Deduce the standard electrode potential for the $S n^{2+}(a q) \mid S n(s)$ half-cell.

An answer that makes reference to the following point:

- $(0.24-0.37=)-0.13(\mathrm{~V})$

Ignore working, even if incorrect

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Edexcel A-Level Chemistry A2 16.7 A Standard Emf Eocell By Combining Two Standard Electrode Potentials Questions question 1