# Explain why the equilibrium constant expression for the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) contains concentration terms raised to different powers, and state what happens to the numerical value of K if the equation is written in reverse as 2NH₃(g) ⇌ N₂(g) + 3H₂(g).

> IB DP Chemistry Standard Level (2023 syllabus) — R2.3 How far? The extent of chemical change · Explain · 4 marks

## Mark scheme (4 marks)

1. The equilibrium constant expression is K = [NH₃]² / ([N₂][H₂]³), where each concentration is raised to the power equal to its stoichiometric coefficient in the balanced equation.
2. Products appear in the numerator and reactants in the denominator of the K expression.
3. When the equation is reversed, the new equilibrium constant K' is the reciprocal of the original K (i.e. K' = 1/K).
4. This is because reversing the equation swaps products and reactants, so the numerator and denominator of the K expression are interchanged.

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