Using Equilibrium Data

• Chemical companies need to predict the effect of changes in reaction conditions on equilibrium yield

• Le Chatelier’s principle helps but is not quantitative

• So…. Equlibrium constants Kc are used

N2(g) + 3H2(g) 2NH3(g)

The Equilibrium Constant Kc

• At equilibrium the ratio of the concentrations stays constant

• If we know the concentrations of the various substances we can calculate the value of the constant

• The equilibrium constant Kc is the ratio of concentrations of the reactants and products.

The Equilibrium Constant Kc

aA + bB cC + dD

Kc = [C]c [D]d [A]a [B]b

[ ] represents the equilibrium concentration in moldm-3

a,b,c & d are the numbers of moles of substances A, B, C & D

The general equation for any homogeneous reaction at equilibrium is…

Product concentrations

Reactant concentrations

Units of Kc

• The units of Kc depend on the number of moles in the chemical equation.

• Units can be calculated by substituting the units for concentration, mol dm-3 into the equation for Kc

Kc Expressions & Units

Kc = [NH3]2 [N2] [H2]3

N2(g) + 3H2(g) 2NH3(g)

Kc = [moldm-3]2 [moldm-3] [moldm-3]3

Kc = 1 [moldm-3]2

= mol-2dm6

Kc Expressions & Units

Kc = [SO3]2 [SO2]2 [O2]

2SO2(g) + O2(g) 2SO3(g)

Kc = [moldm-3]2 [moldm-3]2 [moldm-3]

Kc = 1 moldm-3

= mol-1dm3

Kc Expressions & Units

Kc = [CH3COOCH2CH3] [H2O] [CH3COOH ] [CH3CH2OH]

CH3COOH (aq) + CH3CH2OH(aq) CH3COOCH2CH3 (ag) + H2O(l)

Kc = [moldm-3] [moldm-3] [moldm-3] [moldm-3]

Kc no units

Calculating Kc values

• Every equilibrium has its own Kc

• The value of Kc is constant for a particular reaction at a particular temperature

• Kc values for different reactions can be calculated experimentally by finding the concentrations of the substances at equilibria.

Calculating Kc values

CH3COOH (aq) + CH3CH2OH(aq)

CH3COOCH2CH3 (ag) + H2O(l)

Reagent Conc at start Conc at equilib

CH3CH2OH 1.0 0.33

CH3COOH 1.0 0.33

CH3COOCH2CH3 0 0.67

H2O 0 0.67

Kc = [CH3COOCH2CH3] [H2O] [CH3COOH ] [CH3CH2OH]

Kc = 0.67 x 0.67 0.33 x 0.33

Kc = 4.12

Using The Equilibrium Constant Kc

• Kc can be used:• To provide a quantitative measure of the extent of

a reaction• To determine the position of equilibrium• To calculate the effect of changing concentrations

• In general, if the position of the equilibrium lies well over to the product side Kc will be large

• A small equilibrium constant indicates that the position of the equilibrium lies well over to the side of the reactants and not much conversion to products takes place.

Kc Expressions

Kc = [NH3]2 [N2] [H2]3

N2(g) + 3H2(g) 2NH3(g)

Kc = 3.0 x 10-2 mol-2dm6 at 600K

Small Kc indicates low yield of NH3