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

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:

Fig 4.11 Two methods of applying pressure to a condensed phase

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

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

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

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

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

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

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

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