University learnersPerturb and predictExample · 4 steps · 9 min

Shift the Equilibrium

Calculate Q after a disturbance, predict the direction of the shift, solve for the new equilibrium and find out what really changes K.

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Your example

A ⇌ P in a 1-L vessel at 25 °C

State	A (mol)	P (mol)	Q = P/A
Equilibrium	1.00	1.00	1.00 = K
+0.50 mol A	1.50	1.00	?

Reaction A ⇌ P in a 1-litre vessel at a constant 25 °C. At equilibrium A = 1.00 mol and P = 1.00 mol, so K = 1.00. Then 0.50 mol A is added: A = 1.50 mol, P = 1.00 mol.

Step 1 of 4

Right after adding 0.50 mol A, what is Q, and how does it compare with K = 1.00?

Q = 1.50, larger than K.Q ≈ 0.67, smaller than K.Q = 1.00, still equal to K.

About this activity

What you practise: Comparing Q with K, predicting the direction of a shift, calculating a new equilibrium and separating concentration changes from temperature changes.

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Right after adding 0.50 mol A, what is Q, and how does it compare with K = 1.00? Reaction A ⇌ P in a 1-litre vessel at a constant 25 °C. At equilibrium A = 1.00 mol and P = 1.00 mol, so K = 1.00. Then 0.50 mol A is added: A = 1.50 mol, P = 1.00 mol. Q = P/A = 1.00/1.50 ≈ 0.67. The added A sits in the denominator, so Q drops below K.