What do square brackets mean in a Kc expression?

The concentration of the species in mol dm⁻³ at equilibrium.

Write Kc for N₂(g) + 3H₂(g) ⇌ 2NH₃(g).

Kc = [NH₃]² / ([N₂][H₂]³)

When does Kc have no units?

When the numerator and denominator contain the same number of concentration terms, so the units cancel.

What is the effect of increasing a concentration on Kc?

None. The position shifts but Kc is unchanged.

What happens to Kc for an endothermic reaction when the temperature increases?

Kc increases, because the equilibrium shifts to the right.

What happens to Kc for an exothermic reaction when the temperature increases?

Kc decreases, because the equilibrium shifts to the left.

When can you use amounts in mol instead of concentrations in Kc?

When the number of moles of gas or solute is equal on both sides, so the volume cancels.

The equilibrium constant KcAQA A-Level Chemistry: Flashcards

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What is the expression for Kc for aA + bB ⇌ cC + dD?

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What is the expression for Kc for aA + bB ⇌ cC + dD?: Kc = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ)
What do square brackets mean in a Kc expression?: The concentration of the species in mol dm⁻³ at equilibrium.
Write Kc for N₂(g) + 3H₂(g) ⇌ 2NH₃(g).: Kc = [NH₃]² / ([N₂][H₂]³)
What are the units of Kc for N₂ + 3H₂ ⇌ 2NH₃?: dm⁶ mol⁻²
What are the units of Kc for N₂O₄ ⇌ 2NO₂?: mol dm⁻³
When does Kc have no units?: When the numerator and denominator contain the same number of concentration terms, so the units cancel.
What is the only factor that changes the value of Kc?: Temperature.
What is the effect of adding a catalyst on Kc?: None.
What is the effect of increasing a concentration on Kc?: None. The position shifts but Kc is unchanged.
What happens to Kc for an endothermic reaction when the temperature increases?: Kc increases, because the equilibrium shifts to the right.
What happens to Kc for an exothermic reaction when the temperature increases?: Kc decreases, because the equilibrium shifts to the left.
How do you convert an amount in mol to a concentration?: Divide by the volume in dm³.
When can you use amounts in mol instead of concentrations in Kc?: When the number of moles of gas or solute is equal on both sides, so the volume cancels.

Exam questions on The equilibrium constant Kc

Nitrogen and hydrogen are sealed in a vessel at 500 K and form ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). When equilibrium is reached, the concentrations are [N₂] = 0.40 mol dm⁻³, [H₂] = 0.20 mol dm⁻³ and [NH₃] = 0.16 mol dm⁻³.
Calculate the value of Kc for this reaction at 500 K, including its units.2 marks
Ethanoic acid and ethanol react in the presence of a small amount of sulfuric acid catalyst: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l). At room temperature the equilibrium constant Kc for this reaction is 4.0.
A mixture of 1.00 mol of ethanoic acid and 1.00 mol of ethanol is left to reach equilibrium at room temperature. Calculate the amount, in mol, of ethyl ethanoate present at equilibrium.2 marks
Dinitrogen tetraoxide, a colourless gas, is in equilibrium with nitrogen dioxide, a brown gas: N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. A sealed 2.00 dm³ vessel initially contains 0.0600 mol of N₂O₄ only. It is held at a fixed temperature until equilibrium is reached, when 0.0800 mol of NO₂ is present.
State and explain the effect of increasing the temperature on the value of Kc, and state the effect on Kc of increasing the pressure by compressing the vessel.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).