Equilibrium Constant

Kc and Kp

Learning Objectives

• 1. Deduce expressions for equilibrium constants in terms of concentrations, Kc, and equilibrium partial pressures, Kp, for homogeneous and heterogeneous systems.

• 2. Perform simple calculations on Kc and Kp and work out the units of the equilibrium constants.

What happens to the rate of reaction if more reactant is added to the reaction

mixture at equilibrium?

The Equilibrium Constant

: At a given temperature, no matter the starting composition of reactants and products, the same ratio of concentrations is achieved at equilibrium.

a A + b B c C + d D the equilibrium constant expression

for the general reaction is

where Kc is the equilibrium constant

and is dependent on temperature.

[ ] [ ][ ] [ ]

c d

C a b

C DK

A B

A B

Homogenous equilibrium applies to reactions in which all reacting species are in the same phase.

N2O4 (g) 2NO2 (g)

Kc = [NO2]2

[N2O4]

CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)

Kc =‘[CH3COO-][H3O+][CH3COOH]

homogeneous equilibrium• Write the equilibrium expressions for

the following reactions:

H2 (g) + I2 (g) 2 HI (g)

HF (aq) H+ (aq) + F- (aq)

2

2 2

[ ]

[ ][ ]eq

HIk

H I

[ ][ ]

[ ]eq

H Fk

HF

A B

A B

aA + bB cC + dD

Kc= [C]c[D]d

[A]a[B]b

K >> 1K << 1

Lie to the right

Favor productsLie to the

leftFavor reactants

Equilibrium Will

Recall: for the reaction

What does the Kc value mean?

QuestionsQ2. Ammonia is made in the Haber process in the industry. a) Write the balanced equation that shows how ammonia is made in the Haber process. b) Write the equilibrium expression to find KC for the above reaction.

Q2.a) Consider the following reaction: H2(g) + I2(g) 2HI(g) ∆ H= _10 KJ mol -1

At certain temperature, analysis of an equilibrium mixture of the gases yielded the following results: PH2 = 1.6x104KPa PI2 =1.6x104KPs PHI=4.0x104KPa

Calculate the Kp value for this reaction .