IB DP Chemistry 19.1 Electrochemical cells HL Paper 2

${\text{MnO}}_4^{–}$;

${\text{2MnO}}_4^{–}\text{(aq)}+\text{16}{\text{H}}^{+}\text{(aq)}+\text{10C}{\text{l}}^{–}\text{(aq)}\to \text{2M}{\text{n}}^{2+}\text{(aq)}+\text{8}{\text{H}}_2\text{O(l)}+\text{5C}{\text{l}}_2\text{(g)}$

Accept equation with all coefficients divided by 2 (i.e.

MnO₄^– + 8H⁺ + 5Cl^– $\to$ Mn²⁺ + 4H₂O + 2$\frac{1}{2}$Cl₂).

Award [1] for correct reactants and products, [1] for correct balancing.

Ignore state symbols.

$E_{\text{cell}}^{\mathrm{\Theta }}=(1.51–1.36)=(+)0.15\text{ (V)}$;

(i)     Positive/+/anode

$\text{2C}{\text{l}}^{–}\text{(l)}\to \text{C}{\text{l}}_2\text{(g)}+\text{2}{\text{e}}^{–}$;

Negative/–/cathode

$\text{N}{\text{a}}^{+}\text{(l)}+{\text{e}}^{–}\to \text{Na(l)}$;

Penalize missing or incorrect states such as (aq) or (s) once only.

Award only [1] if electrodes not specified or if equations switched.

$\text{1C}{\text{l}}_2$ to 2Na;

(ii)     (choice of $\text{C}{\text{l}}^{–}$ or ${\text{H}}_2\text{O}/\text{O}{\text{H}}^{–}$ to be oxidized), $\text{C}{\text{l}}^{–}$ oxidized because of concentrated solution/higher concentration / OWTTE;

(choice of $\text{N}{\text{a}}^{+}$ or ${\text{H}}_{\text{2}}\text{O}/{\text{H}}^{+}$ to be reduced), ${\text{H}}_{\text{2}}\text{O/}{\text{H}}^{+}$ reduced because $\text{N}{\text{a}}^{+}$ is a (much) weaker oxidizing agent/ $\text{N}{\text{a}}^{+}$ not reduced to Na in water / ${\text{H}}^{+}$ easier to reduce than $\text{N}{\text{a}}^{+}$ / OWTTE;

positive/+/anode

$\text{2C}{\text{l}}^{–}\text{(aq)}\to \text{C}{\text{l}}_2\text{(g)}+\text{2}{\text{e}}^{–}$;

negative/–/cathode

$\text{2}{\text{H}}_2\text{O(l)}+\text{2}{\text{e}}^{–}\to {\text{H}}_2\text{(g)}+\text{2O}{\text{H}}^{–}\text{(aq) / 2}{\text{H}}^{+}\text{(aq)}+\text{2}{\text{e}}^{–}\to {\text{H}}_2\text{(g)}$;

Penalize missing or incorrect states once only.

Award only [1] out of the last two marks if electrodes not specified or if equations switched.

Positive/+/anode

$\text{2}{\text{H}}_2\text{O(l)}\to {\text{O}}_2\text{(g)}+\text{4}{\text{H}}^{+}\text{(aq)}+\text{4}{\text{e}}^{–}/{\text{H}}_2\text{O(l)}\to \frac{1}{2}{\text{O}}_2\text{(g)}+\text{2}{\text{H}}^{+}\text{(aq)}+\text{2}{\text{e}}^{–}/$

$\text{4O}{\text{H}}^{–}\text{(aq)}\to \text{2}{\text{H}}_2\text{O(l)}+{\text{O}}_2\text{(g)}+\text{4}{\text{e}}^{–}$;

Negative/–/cathode

$\text{C}{\text{u}}^{2+}\text{(aq)}+\text{2}{\text{e}}^{–}\to \text{Cu(s)}$;

Ignore state symbols.

Award only [1] if electrodes not specified or if equations switched.

Observations:

blue colour of $\text{C}{\text{u}}^{2+}\text{(aq)}$ fades;

Cu/metal deposited on negative/–/cathode/tin (jewellery);

gas produced/bubbles formed (at positive/+/anode);

pH of solution decreases/acidity increases (observed with indicator/pH paper);

Most candidates were able to place Ag, Cu and Pb in the correct order with the strongest reducing agent first although some equations given were not balanced. However, explanations were sometimes sloppy with statements such as “Pb is the strongest reducing agent because it reduces Cu and Ag” instead of $\text{C}{\text{u}}^{2+}$ and $\text{A}{\text{g}}^{+}$. Most candidates chose the correct oxidizing agent that could oxidize $\text{C}{\text{l}}^{–}$ but not ${\text{F}}^{–}$.

Electrolysis of molten sodium chloride was less well answered with incorrect use of states of matter and a lack of appreciation of their significance. In electrolysis of a concentrated solution of NaCl, many attempted to give explanations in terms of position in the electrochemical series without explaining the significance in terms of oxidising/reducing strength or the concentration of chloride ion. The half-equations taking place were not always correct. Sometimes the electrons were on the wrong side or the electrodes were reversed.

The part on electroplating was well answered although many did not pick up on the fact that, in this question, the anode is inert, leading to incorrect oxidation half-equation for the reaction at the positive anode. This question is an alert for teachers as it shows how easy candidates are taken by surprise by unexpected situations which are intrinsic to Aim 8.