# Sulfur dioxide reacts with oxygen in a closed vessel to form sulfur trioxide. The equation for the reaction is: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The reaction reaches equilibrium at a constant temperature. Explain how the expression for Kp is written for this equilibrium, and describe what happens to the value of Kp if the pressure of the system is increased at constant temperature.

> AQA A-Level Chemistry (7405) — 3.1.10 Equilibrium constant Kp (A-Level only) · Explain · 5 marks

> The industrial production of sulfuric acid relies on the equilibrium: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The position of this equilibrium and the equilibrium constant are both important to chemists.

## Mark scheme (5 marks)

1. Kp is written in terms of partial pressures of products over reactants (products in numerator, reactants in denominator)
2. The partial pressures are raised to the power of their stoichiometric coefficients, i.e. (p SO₃)² in the numerator and (p SO₂)² × (p O₂) in the denominator
3. Kp has units (accept: Pa⁻¹ or atm⁻¹ depending on unit used, or statement that units depend on the expression / overall change in moles of gas)
4. Increasing pressure does NOT change the value of Kp
5. Kp only changes if temperature changes / Kp is only affected by temperature

## Key terms

- [Kp](https://www.gradenine.co.uk/glossary/kp)

## Related

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