Sulfur trioxide, SO₃(g)

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

Sulfur trioxide, SO₃(g), can be produced from elemental sulfur by two different routes at

298\,\text{K}

, as summarized below.

Step	Chemical equation	ΔH° (kJ)
1	S(s) + O₂(g) → SO₂(g)	−297
2	SO₂(g) + $\frac{1}{2}$ O₂(g) → SO₃(g)	−105
3	S(s) + $1\frac{1}{2}$ O₂(g) → SO₃(g)	−396

Assume that each enthalpy value refers to exactly the stoichiometric amounts shown. (a) Determine the standard enthalpy change, in

\,\text{kJ}\,\text{mol}^{-1}

of SO₃ produced, for the two-step sequence that combines steps 1 and 2. (b) Compare your result from part (a) with the enthalpy change for the direct route given in step 3. Identify which route releases more heat per mole of SO₃ formed and calculate the numerical difference in

\,\text{kJ}\,\text{mol}^{-1}

. (c) Explain why the enthalpy change for the direct route is not obtained by simply adding the numerical values of steps 1 and 2 without first considering stoichiometric consistency.

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