Chemistry 9701 · AS & A Level · Electrochemistry

Electrochemistry — practice question

(a(i))

Write equations that show the reactions of the following oxides with water: phosphorus(V) oxide; sulfur(IV) oxide.

(a(ii))

When $\text{NO}_2$ reacts with water, nitrogen is disproportionated, with one nitrogen atom decreasing its oxidation number by 1 and another nitrogen atom increasing its oxidation number by 1. A pair of acids is formed. Suggest an equation for the reaction between $\text{NO}_2$ and water.

(a(iii))[4]

In a similar disproportionation reaction, $\text{ClO}_2$ reacts with aqueous $\text{NaOH}$ to give a solution containing two chlorine-containing sodium salts. Suggest an equation for the reaction between $\text{ClO}_2$ and aqueous $\text{NaOH}$.

(b(i))

Complete and balance the following equation to show the complete combustion of a gaseous mixture containing $2\,\text{mol}$ of $\text{CH}_4$, $1\,\text{mol}$ of $\text{C}_2\text{H}_6$ and $1\,\text{mol}$ of $\text{H}_2\text{S}$.\n\n$2\text{CH}_4 + \text{C}_2\text{H}_6 + \text{H}_2\text{S} + \underline{\hspace{1cm}} \rightarrow \text{SO}_2 + \underline{\hspace{1cm}} + \underline{\hspace{1cm}}$

(b(ii))

Explain why it is important to remove the $\text{H}_2\text{S}$ before the natural gas is burned industrially.

(b(iii))

If a natural gas sample contains $5\%$ by volume of $\text{H}_2\text{S}$, calculate the mass of ethanolamine needed to remove all the $\text{H}_2\text{S}$ from a $1000\,\text{dm}^3$ sample of gas under room conditions.\n\nThe reaction is:\n\n$\text{HOCH}_2\text{CH}_2\text{NH}_2 + \text{H}_2\text{S(g)} \rightarrow \text{HOCH}_2\text{CH}_2\text{NH}_3^{+} + \text{SH}^-$

(b(iv))

The $\text{H}_2\text{S}$ can be recovered by warming the solution to $120^{\circ}\text{C}$, when the reaction above goes in reverse. What type of reaction is occurring here?

(b(v))[8]

Use the data below to calculate $\Delta H^{\circ}$ for the reaction between $\text{H}_2\text{S}$ and $\text{SO}_2$.\n\nReactions:\n\n$\text{H}_2\text{S} + 1.5\text{O}_2 \rightarrow \text{SO}_2 + \text{H}_2\text{O}$\n\n$2\text{H}_2\text{S(g)} + \text{SO}_2\text{(g)} \rightarrow 3\text{S}\text{(g)} + 2\text{H}_2\text{O}\text{(g)}$\n\nGiven:\n$\Delta H_f^{\circ}(\text{H}_2\text{S(g)}) = -21\,\text{kJ mol}^{-1}$,\n$\Delta H_f^{\circ}(\text{SO}_2\text{(g)}) = -297\,\text{kJ mol}^{-1}$,\n$\Delta H_f^{\circ}(\text{H}_2\text{O(g)}) = -242\,\text{kJ mol}^{-1}$,\n$\Delta H_f^{\circ}(\text{S(g)}) = +11\,\text{kJ mol}^{-1}$.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: $\mathrm{P_2O_5 + 3H_2O \rightarrow 2H_3PO_4}$ or $\mathrm{P_4O_{10} + 6H_2O \rightarrow 4H_3PO_4}$

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