Chemistry 9701 · AS & A Level · Nitrogen and sulfur

Nitrogen and sulfur — practice question

The Contact process, which is used to produce sulfuric acid, was first patented in the 19th century and remains in use now. The main stage in the overall process is the reversible change of sulfur dioxide into sulfur trioxide in the presence of a vanadium(V) oxide catalyst. $\text{2SO}_2(g) + \text{O}_2(g) \rightleftharpoons \text{2SO}_3(g) \qquad \Delta H = -196\,\text{kJ mol}^{-1}$
(a)[2]

A source of the sulfur dioxide for this reaction is obtained by heating the sulfide ore iron pyrites, $\text{FeS}_2$, in air. Iron(III) oxide is formed too. Write an equation for this reaction.

(b(i))[1]

Explain why sulfur trioxide is not first combined directly with water.

(b(ii))[2]

Write equations for the two stages involved in converting sulfur trioxide into sulfuric acid.

(c(i))[2]

Sulfur dioxide and sulfur trioxide each contain only $\text{S}=\text{O}$ double bonds. Draw labelled diagrams to indicate the shapes of these two molecules. $\text{SO}_2$ $\text{SO}_3$

(c(ii))[2]

For your diagrams in (i), state the shape and estimate the bond angles. $\text{SO}_2$ shape $\text{SO}_3$ shape $\text{SO}_2$ bond angle $\text{SO}_3$ bond angle

(d(i))[4]

Using Le Chatelier’s Principle and reaction kinetics, state and explain one advantage and one disadvantage of choosing a higher temperature.

(d(ii))[1]

State the equilibrium constant expression, $K_p$, for the formation of sulfur trioxide from sulfur dioxide.

(d(iii))[5]

2.00 moles of sulfur dioxide and 2.00 moles of oxygen were added to a flask and allowed to reach equilibrium. When equilibrium was reached, the pressure in the flask was $2.00 \times 10^5\,\text{Pa}$ and the mixture contained 1.80 moles of sulfur trioxide. Calculate $K_p$. Include the units.

Worked solution & mark scheme

This 19-mark question has a full step-by-step worked solution and mark scheme. One marking point: Correct balanced equation: $\mathrm{4FeS_2 + 11O_2 \rightarrow 2Fe_2O_3 + 8SO_2}$

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