Chemistry 9701/43 — October/November 2010

Cambridge A-Level · Paper 4 Structured Questions · worked solutions for every part, with the mark scheme

questions

100

marks

105

minutes

Topics Nitrogen Compounds · Hydrocarbons · Electrochemistry · Equilibria · Carboxylic Acids and Derivatives · Polymerisation · +7 more

Q1Group 2ElectrochemistryChemical EnergeticsEquilibriaFree sample

(a)

(i)

Write equations to illustrate the reactions of the following oxides with water.

phosphorus(V) oxide

sulfur(IV) oxide

(ii)

When $\text{NO}_2$ reacts with water, nitrogen undergoes a disproportionation reaction in which one nitrogen atom decreases its oxidation number by 1 and another nitrogen atom increases its oxidation number by 1. A mixture of two acids results.

Suggest an equation for the reaction between $\text{NO}_2$ and water.

(iii)

In a similar disproportionation reaction, $\text{ClO}_2$ reacts with aqueous $\text{NaOH}$ to produce a solution containing two chlorine-containing sodium salts.

Suggest an equation for the reaction between $\text{ClO}_2$ and aqueous $\text{NaOH}$ .

(b)

The major source of sulfur for the manufacture of sulfuric acid by the Contact process is the de-sulfurisation of 'sour' natural gas. Many natural gas wells produce a mixture of volatile hydrocarbons (mainly $\text{CH}_4$ and $\text{C}_2\text{H}_6$ ) together with up to 25% hydrogen sulfide, $\text{H}_2\text{S}$ .

(i)

Complete and balance the following equation showing the complete combustion of a gaseous mixture consisting of $2 \text{ mol}$ of $\text{CH}_4$ , $1 \text{ mol}$ of $\text{C}_2\text{H}_6$ and $1 \text{ mol}$ of $\text{H}_2\text{S}$ .

$2\text{CH}_4 + \text{C}_2\text{H}_6 + \text{H}_2\text{S} + \underline{\hspace{2em}} \rightarrow \text{SO}_2 + \underline{\hspace{2em}} + \underline{\hspace{2em}}$

(ii)

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

(iii)

The $\text{H}_2\text{S}$ is removed by passing the 'sour' natural gas through a solvent containing ethanolamine. The following reaction takes place.

\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}^-

If a sample of natural gas contains 5% by volume of $\text{H}_2\text{S}$ , calculate the mass of ethanolamine required to remove all the $\text{H}_2\text{S}$ from a $1000\text{ dm}^3$ sample of gas, measured under room conditions.

(iv)

The $\text{H}_2\text{S}$ can be recovered by warming the solution to $120\text{ }^\circ\text{C}$ , when the above reaction is reversed. The ethanolamine can then be recycled.

What type of reaction is occurring here?

(v)

The recovered $\text{H}_2\text{S}$ is converted to sulfur by the following two reactions.

I Part of the $\text{H}_2\text{S}$ is burned in air.

\text{H}_2\text{S} + 1.5\text{O}_2 \rightarrow \text{SO}_2 + \text{H}_2\text{O}

II The gas stream resulting from reaction I is then blended with the remaining $\text{H}_2\text{S}$ and fed into an iron oxide catalyst bed, where sulfur and water are produced according to the following equation.

2\text{H}_2\text{S(g)} + \text{SO}_2\text{(g)} \rightarrow 3\text{S(g)} + 2\text{H}_2\text{O(g)}

Use the following data to calculate $\Delta H^\ominus$ for the reaction between $\text{H}_2\text{S}$ and $\text{SO}_2$ .

compound	$\Delta H_f^\ominus / \text{kJ mol}^{-1}$
$\text{H}_2\text{S(g)}$	$-21$
$\text{SO}_2\text{(g)}$	$-297$
$\text{H}_2\text{O(g)}$	$-242$
$\text{S(g)}$	$+11$

\Delta H^\ominus = \text{........................................ kJ mol}^{-1}

The rest of this paper

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