Edexcel A-Level Chemistry A2 16.4 The Features of the Standard Hydrogen Electrode and Understand Why a Reference Electrode Is Necessary Questions

A full SHE diagram needs platinised platinum, H2 at 100 kPa, 1 mol dm-3 acid, 298 K, a salt bridge and a linked reference half-cell.

Exam points

Label Pt or platinum black, H2 at 100 kPa, 1 mol dm-3 H+ and 298 K in diagrams.
Connect the SHE to another half-cell with a salt bridge and high-resistance voltmeter.
Explain why platinum black increases the rate of the H2 and H+ equilibrium.

[Maximum number: 5]

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 $[\mathrm{Xe}] 4 \mathrm{f}^{14} 5 \mathrm{~d}^{10} 6 \mathrm{~s}^{2}$ .

(a)

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 (a) — 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 $E^{\ominus}=+0.24 \mathrm{~V}$ .

[ 5 ]

(i)

Suggest why KCl crystals are needed in the outer tube of the electrode.

[ 1 ]

An answer that makes reference to the following point:

- (to provide a) constant concentration (of $\mathrm{Cl}^{-}$ )

Allow KCl for $\mathrm{Cl}^{-}$ throughout

Allow to keep the solution / $\mathrm{Cl}^{-}$ saturated

Allow to replace $\mathrm{Cl}^{-}$

Ignore just to provide $\mathrm{Cl}^{-}$ Ignore stated concentrations

Do not award salt bridge / to complete the circuit

Do not award catalyst

(1)

Expert

(ii)

Add labels to complete the diagram of a standard hydrogen electrode. Include details of any essential conditions.
(3)

Figure for Question (ii) — Edexcel A-Level Chemistry A2

[ 3 ]

(iii)

Suggest one advantage of using a calomel electrode, in place of a standard hydrogen electrode, when measuring a cell potential.

[ 1 ]

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Edexcel A-Level Chemistry A2 16.4 The Features of the Standard Hydrogen Electrode and Understand Why a Reference Electrode Is Necessary Questions question 1

Question (a)

Question (i)

Question (ii)

Question (iii)