1. Peter Atkins • Julio de Paula Atkins’ Physical Chemistry
Eighth Edition
Chapter 7 – Lecture 2
Chemical Equilibrium
Copyright © 2006 by Peter Atkins and Julio de Paula

2. How equilibria respond to pressure
Defined at a single standard pressure, 1 bar
Therefore K is independent of pressure:
However, equilibrium composition is
NOT necessarily pressure-independent!
e.g., 3 H2 (g) + N2 (g) ⇌ 2 NH3 (g)
Recall two ways to pressurize a gas:

3. Fig 4.11
Two methods of
applying pressure
to a condensed phase

4. Fig 7.6 Compression of a reaction at equilibrium
Consider: A ⇌ 2B
Le Chatelier’s principle:
If an external stress is applied to a system at equilibrium, the system will adjust itself in order to minimize the stress A B
∴ as system is compressed,
A ← 2B and K remains constant

5. Fig 7.7 Pressure dependence of degree of dissociation
Consider: A ⇌ 2B
Pure B
values of K
Pure A

6. Example:
3 N2 (g) + H2 (g) ⇌ 2 HN3 (g)
Predict the effect on K of a ten-fold increase in pressure
so to preserve the initial value of K,
LeChaletier’s principle says that K must increase by 100-fold

7. How equilibria respond to temperature
From Le Chatelier’s principle for a system at equilibrium:
System will shift in endothermic direction as T
System will shift in exothermic direction as T
Summary:
Exothermic reactions: Increased T favors reactants
Endothermic reactions: Increased T favors products

8. How equilibria respond to temperature
The Gibbs-Helmholtz equation (3.53):
Since Eqn 7.17 is:
The van’t Hoff equation:

9. Fig 7.8 Effect of temperature on a chemical equilibrium A ⇌ B
in terms of the Boltzmann distribution
Endothermic
Exothermic
B increases at
expense of A
A increases at
expense of B

10. Fig 7.9 Plot of ‒ ln K versus 1/T
Variation of K with temperature for:
Ag2CO3 (s) ⇌ Ag2O (s) + CO2 (g)

11. How equilibria respond to temperature
The value of K at different temperatures
To find K2 at T2, given K1 at T1, integrate:
Assuming ΔHro is T-independent