The contact process is used industrially to produce concentrated liquid sulfuric acid. A simplified production line carries out the following sequential steps at 298 K and 1 bar.
Step
Chemical transformation
ΔH (kJ mol⁻¹, 298 K)
1
S(s) + O₂(g) → SO₂(g)
–296.8
2
2 SO₂(g) + O₂(g) → 2 SO₃(g)
–197.8
3
SO₃(g) + H₂O(l) → H₂SO₄(l)
–130.2
4
H₂SO₄(l) → H₂SO₄(aq, 98 wt %)
–9.2
Additional data: molar mass of H₂SO₄ = 98.09 g mol⁻¹.
Assume complete conversion in every step and ignore heat losses to the environment.
(a) Write the overall balanced chemical equation for the production of 1 mol of 98 wt % aqueous H₂SO₄ from elemental sulfur, liquid water, and gaseous O₂.
(b) Determine the overall enthalpy change, ΔH_overall (in kJ mol⁻¹ H₂SO₄ produced), for the sequence of steps shown in the table.
(c) Calculate the enthalpy change per kilogram of product acid (in kJ kg⁻¹). Give your answer to three significant figures.
(d) Discuss how the sign and magnitude of ΔH_overall would influence the design of heat-exchange equipment in a large-scale plant that operates this process continuously.