Edexcel A-Level Chemistry A2 16 Redox Equilibria Questions

Topic 16 questions ask you to move between oxidation-number reasoning, standard half-cell conditions, labelled cell diagrams and E°cell calculations.

Exam points

Use oxidation numbers to justify whether a stated reaction is redox or not.
Draw standard cells with electrodes, 1 mol dm-3 ions, salt bridge and voltmeter.
Calculate E°cell from half-cell data and use the sign to decide reaction direction.

Covered syllabus points

16.1—The terms ‘oxidation’ and ‘reduction’ in terms of electron transfer and changes in oxidation number, applied to s-, p-16.2—What is meant by the term ‘standard electrode potential’, Eo 16.3—The standard electrode potential, Eo, is measured in conditions of: i 298 K temperature ii 100 kPa pressure of gases iii 16.4—The features of the standard hydrogen electrode and understand why a reference electrode is necessary 16.5—Different methods are used to measure standard electrode potentials of: i metals or non-metals in contact with their ions 16.6—CORE PRACTICAL 12 Investigating some electrochemical cells 16.7—A standard emf, Eocell , by combining two standard electrode potentials 16.8—Write cell diagrams using the conventional representation of half-cells 16.9—The importance of the conditions when measuring an electrode potential, E 16.10—Standard electrode potentials to predict the thermodynamic feasibility of a reaction 16.11—Eocell is directly proportional to the total entropy change and to lnK for a reaction 16.12—The limitations of predictions made using standard electrode potentials, in terms of kinetic stability of systems 16.13—Standard electrode potentials are sometimes referred to as standard reduction potentials and can be listed as 16.14—How standard electrode potentials can be used to predict the thermodynamic feasibility of disproportionation reactions 16.15—Carry out both structured and unstructured titration calculations involving redox reactions 16.16—Discuss the uncertainty of measurements and their implications for the validity of the final results 16.17—CORE PRACTICALS 13a and 13b Carry out redox titrations with both: i iron(II) ions and potassium manganate(VII) ii sodium 16.18—Fuel cells use the energy released on the reaction of a fuel with oxygen to generate a voltage Knowledge that methanol 16.19—The electrode reactions that occur in a hydrogen-oxygen fuel cell Knowledge of hydrogen-oxygen fuel cells with both acidic

[Maximum number: 9]

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 reacts with nitric acid to form an aqueous solution of $\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}$ and nitrogen monoxide gas.

The unbalanced equation is shown.

\mathrm{Hg}(\mathrm{l})+\mathrm{HNO}_{3}(\mathrm{aq}) \rightarrow \mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{NO}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})

[ 2 ]

(i)

Explain, using oxidation numbers, why this is a redox reaction.
(2)

[ 2 ]

An explanation that makes reference to the following
points:

Allow oxidation numbers from annotation to the equation
Ignore any reference to electron loss/gain

(2)

Expert

- Hg/mercury oxidised and
from 0 (in Hg ) to +2 (in $\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}$ )

(1)

Do not award reference to oxidation of any other element

- N / nitrogen is reduced
and
from +5 (in $\mathrm{HNO}_{3}$ ) to +2 (in NO)

(1)

Do not award $\mathrm{HNO}_{3}$ is reduced
Do not award reference to reduction of any other element

If no other mark awarded,
Hg/mercury oxidised and N/nitrogen reduced
OR
Hg / mercury from 0 to +2 and N / nitrogen from +5 to +2
scores (1)

(b)

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

[ 7 ]

(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)

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.

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

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.

[ 1 ]

An answer that makes reference to the following point:

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

Ignore working, even if incorrect

(1)

Expert

(iii)

Write the overall equation for the cell reaction.

[ 1 ]

An answer that makes reference to the following point:
$\text { - } \mathrm{Hg}_{2} \mathrm{Cl}_{2}+\mathrm{Sn} \rightarrow 2 \mathrm{Hg}+\mathrm{Sn}^{2+}+2 \mathrm{Cl}^{-}$

Allow $\mathrm{Hg}_{2} \mathrm{Cl}_{2}+\mathrm{Sn} \rightarrow 2 \mathrm{Hg}+\mathrm{SnCl}_{2}$ Allow multiples

Allow ⇌ for →

Ignore state symbols even if incorrect Ignore half-equations even if incorrect Ignore use of cell diagrams

Do not award uncancelled electrons

Do not award $2 \mathrm{Hg}^{+}\left(+2 \mathrm{Cl}^{-}\right)$ for $\mathrm{Hg}_{2} \mathrm{Cl}_{2}$

If answer to (d)(ii) is $+0.61(\mathrm{~V}) /+0.37(\mathrm{~V}) /$ greater than $+0.24(\mathrm{~V})$ , equation must be reversed:

$2 \mathrm{Hg}+\mathrm{Sn}^{2+}+2 \mathrm{Cl}^{-} \rightarrow \mathrm{Hg}_{2} \mathrm{Cl}_{2}+\mathrm{Sn}$ OR

$2 \mathrm{Hg}+\mathrm{SnCl}_{2} \rightarrow \mathrm{Hg}_{2} \mathrm{Cl}_{2}+\mathrm{Sn}$

(1)

Expert

(iv)

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

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

[ 3 ]

(v)

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

[ 1 ]

[Maximum number: 5]

An electrochemical cell is made from the electrode systems shown by these half-equations.

\begin{aligned} \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{e}^{-} & \rightleftharpoons \mathrm{Ag}(\mathrm{~s}) & E^{\ominus}=+0.80 \mathrm{~V} \\ \mathrm{VO}_{2}^{+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{e}^{-} & \rightleftharpoons \mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) & E^{\ominus}=+1.00 \mathrm{~V} \end{aligned}

The apparatus used to measure the value for $E_{\text {cell }}^{\ominus}$ under standard conditions is shown.

Figure for Question 2 — Edexcel A-Level Chemistry A2

(a)

Which silver compound could be used as the electrolyte in the left-hand half-cell?

silver nitrate

silver hydroxide

silver chloride

silver carbonate

[ 1 ]

(b)

The electrolyte in the right-hand half-cell is prepared using equal volumes of

$1 \mathrm{~mol} \mathrm{dm}^{-3}$ acidified $\mathrm{VO}_{2}^{+}(\mathrm{aq})$ and $1 \mathrm{~mol} \mathrm{dm}^{-3}$ acidified $\mathrm{VO}^{2+}(\mathrm{aq})$

$2 \mathrm{~mol} \mathrm{dm}^{-3}$ acidified $\mathrm{VO}_{2}^{+}(\mathrm{aq})$ and $2 \mathrm{~mol} \mathrm{dm}^{-3}$ acidified $\mathrm{VO}^{2+}(\mathrm{aq})$

$1 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{VO}_{2}^{+}(\mathrm{aq})$ and $1 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{HCl}(\mathrm{aq})$

$1 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{VO}^{2+}(\mathrm{aq})$ and $1 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{HCl}(\mathrm{aq})$

[ 1 ]

(c)

Which is the equation for the overall cell reaction under standard conditions?

$\mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{VO}_{2}^{+}(\mathrm{aq})+\mathrm{Ag}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq})$

$\mathrm{VO}_{2}^{+}(\mathrm{aq})+\mathrm{Ag}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

$\mathrm{VO}^{2+}(\mathrm{aq})+3 \mathrm{Ag}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{VO}_{2}^{+}(\mathrm{aq})+3 \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

$\mathrm{VO}_{2}^{+}(\mathrm{aq})+3 \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{VO}^{2+}(\mathrm{aq})+3 \mathrm{Ag}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq})$

[ 1 ]

(d)

Which is the value of $E_{\text {cell }}^{\ominus}$ in volts?

-1.80

-0.20

+0.20

+1.80

[ 1 ]

(e)

Which is the cell diagram for this cell, using the conventional representation of half-cells?

$\mathrm{Ag}(\mathrm{s})\left|\mathrm{Ag}^{+}(\mathrm{aq})\right|\left|\left[\mathrm{VO}_{2}^{+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})\right]\right|\left[\mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\right] \mid \mathrm{Pt}(\mathrm{s})$

$\mathrm{Ag}(\mathrm{s})\left|\mathrm{Ag}^{+}(\mathrm{aq})\right|\left|\left[\mathrm{VO}_{2}^{+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})\right],\left[\mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\right]\right| \mathrm{Pt}(\mathrm{s})$

$\mathrm{Ag}(\mathrm{s})\left|\mathrm{Ag}^{+}(\mathrm{aq})\right|\left|\left[\mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\right]\right|\left[\mathrm{VO}_{2}^{+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})\right] \mid \mathrm{Pt}(\mathrm{s})$

$\mathrm{Ag}(\mathrm{s})\left|\mathrm{Ag}^{+}(\mathrm{aq})\right|\left|\left[\mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\right],\left[\mathrm{VO}_{2}^{+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})\right]\right| \mathrm{Pt}(\mathrm{s})$

[ 1 ]

All question bank results loaded

Covered syllabus points

Question 1

Question (a)

Question (i)

Question (b)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question (v)

Question 2

Question (a)

Question (b)

Question (c)

Question (d)

Question (e)