Chemistry 9701 · AS & A Level · Nitrogen and sulfur

Nitrogen and sulfur — practice question

Sulfuric acid is made by the Contact process. One step in this process converts sulfur dioxide into sulfur trioxide in the presence of the heterogeneous catalyst vanadium(V) oxide, $\text{V}_2\text{O}_5$. The equation is: $2\text{SO}_2\text{(g)} + \text{O}_2\text{(g)} \rightleftharpoons 2\text{SO}_3\text{(g)} \quad \Delta H = -196\,\text{kJ mol}^{-1}$
(a(i))[2]

State the effect of a catalyst on a reaction. Explain how a catalyst produces this effect.

(a(ii))[1]

State what the term heterogeneous means when applied to catalysts.

(b)[2]

The following bond energies are provided: $\text{S} = \text{O}$ (in $\text{SO}_2$): $534\,\text{kJ mol}^{-1}$ $\text{O} = \text{O}$: $496\,\text{kJ mol}^{-1}$ Using these data, together with the enthalpy change for the conversion of sulfur dioxide into sulfur trioxide, calculate the $\text{S} = \text{O}$ bond energy in $\text{SO}_3$.

(c)[3]

A reaction pathway diagram for both the catalysed and uncatalysed reactions between $\text{SO}_2$ and $\text{O}_2$ is shown. The letters A-E stand for energy changes. Complete the table by saying which letter, A-E, shows: • the energy change for the production of $\text{SO}_3$ • the activation energy for the production of $\text{SO}_3$ without a catalyst • the activation energy for the first step in the decomposition of $\text{SO}_3$ in the presence of a catalyst

(d(i))[3]

State and explain what happens to the rate of production of $\text{SO}_3$ when temperature is increased.

(d(ii))[3]

State and explain the effect of increasing temperature on the yield of $\text{SO}_3$.

(e)[1]

The $\text{SO}_3$ made is changed into sulfuric acid in two stages. In the first stage, the $\text{SO}_3$ is reacted with concentrated sulfuric acid to make oleum, $\text{H}_2\text{S}_2\text{O}_7$. The oleum is then reacted with water to produce sulfuric acid. Suggest an equation for the reaction of oleum, $\text{H}_2\text{S}_2\text{O}_7$, with water to form sulfuric acid.

(f(i))[1]

Complete the ‘dot-and-cross’ diagram so that it shows the bonding in one $\text{SO}_2$ molecule. Show only outer electrons.

(f(ii))[2]

State what is meant by the term strong Brønsted-Lowry acid.

(f(iii))[2]

Write an equation showing the acid-base behaviour of sulfuric acid in water. Include state symbols.

Worked solution & mark scheme

This 20-mark question has a full step-by-step worked solution and mark scheme. One marking point: A catalyst is a substance that increases the rate of a reaction

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