What happens to K if concentration is changed?

Nothing: K is unchanged, although the position of equilibrium may shift.

What happens to K when a catalyst is added?

Nothing: a catalyst increases both rates equally.

Why does the position of equilibrium move when the temperature changes?

Because the value of K changes, so the ratio of products to reactants at equilibrium must change to match.

What happens to the position of equilibrium when an exothermic reaction is heated?

It shifts to the left, towards the reactants.

Why does pressure not affect the position when there are equal moles of gas on both sides?

Every partial pressure is multiplied by the same factor, so the ratio in the K expression is unchanged.

A mixture is compressed. The ratio in the Kp expression is now larger than Kp. Which way does the position shift?

Towards the reactants, to reduce the ratio until it equals Kp.

What does a catalyst change about an equilibrium?

Only the time taken to reach it; not the position or K.

Effect of conditions on KEdexcel A-Level Chemistry: Flashcards

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Which condition changes the value of K?

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Which condition changes the value of K?: Only temperature.
What happens to K if concentration is changed?: Nothing: K is unchanged, although the position of equilibrium may shift.
What happens to K if pressure is changed?: Nothing: K is unchanged.
What happens to K when a catalyst is added?: Nothing: a catalyst increases both rates equally.
How does K change when an exothermic reaction is heated?: K decreases.
How does K change when an endothermic reaction is heated?: K increases.
How does K change when an exothermic reaction is cooled?: K increases.
Why does the position of equilibrium move when the temperature changes?: Because the value of K changes, so the ratio of products to reactants at equilibrium must change to match.
What happens to the position of equilibrium when an exothermic reaction is heated?: It shifts to the left, towards the reactants.
Why does pressure not affect the position when there are equal moles of gas on both sides?: Every partial pressure is multiplied by the same factor, so the ratio in the K expression is unchanged.
A mixture is compressed. The ratio in the Kp expression is now larger than Kp. Which way does the position shift?: Towards the reactants, to reduce the ratio until it equals Kp.
What does a catalyst change about an equilibrium?: Only the time taken to reach it; not the position or K.

Exam questions on Effect of conditions on K

The Haber process makes ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. A chemical engineer studies how the equilibrium constant Kp, and the composition of the equilibrium mixture, change when the conditions are altered.
The iron catalyst is added to the reactor. Explain why this does not change the value of Kp.2 marks
Hydrogen is made on a large scale by steam reforming of methane: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g), ΔH = +206 kJ mol⁻¹. The reformers run at about 1100 K.
Explain why the percentage of hydrogen in the equilibrium mixture is higher at 1100 K than at 900 K.2 marks
In hydrogen plants carbon monoxide reacts with steam in the water–gas shift reaction: CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g), ΔH = −41 kJ mol⁻¹. A first reactor runs at about 650 K and a second reactor at about 470 K.
Explain the effect on Kp, and on the position of equilibrium and the yield of hydrogen, of lowering the temperature from 650 K to 470 K.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).