Chapter 15 Chemical Equilibrium

CHAPTER 15
Chemical Equilibrium
Slides by: Dr. Tetana

Adapted from: Prof. Humphries
Coordinator:
Zikhona.Tetana@wits.ac.za
1
Chemical Equilibrium
Chapter 15
The Concept of Equilibrium
A chemical equilibrium refers to the balance of two

competitive reactions ( and ) in a reversible
reaction.
In this dynamic process, the forward and reverse
reactions proceed at the same rate.
Let’s consider the catalytic methanation reaction:
CO (g) + 3H2 (g) CH4 (g) + H2O (g)
Start with 1.0 mol CO and 3.0 mol H2 in a 10.0 L vessel at

1200 K.
CO (g) + 3H2 (g) CH4 (g) + H2O (g)
Applying stoichiometry to an equilibrium mixture:
When heated, PCl5 forms PCl3 and Cl2 as follows:

PCl5(g) PCl3(g) + Cl2(g)
When 1.00 mol PCl5 in a 1.00 L container is allowed to

come to equilibrium, the mixture is found to contain
0.135 mol PCl3.
How many moles of each substance are present?
This problem, may conveniently be solved with the aid
of the following table:
The Equilibrium Constant
Consider the general reaction :
aA + bB cC + dD
[C]c [D]d
Kc =
[A]a [B]b
The Law of Mass Action:

Kc is constant for a particular reaction at a given
temperature.
Understanding Equilibrium Constants
Consider the following industrial process:
CO(g) + Cl2(g) COCl2(g)

Phosgene
Kc = [COCl2] = 4.56 x 109 at 100 oC

[CO][Cl2]
Large K: Equilibrium lies to the right (favours products)

Small K: Equilibrium lies to the left (favours reactants)
What about the reverse reaction?
COCl2(g) CO(g) + Cl2(g)

Heterogeneous Equilibria
• Homogeneous equilibrium: involves reactants and
products in a single phase only.
• Heterogeneous equilibrium: involves reactants and
products in more that one phase.
• When writing the equilibrium-constant expression
for heterogeneous equilibria, the concentration
terms for pure solids and liquids are omitted.
[H2]4
3Fe (s) + 4H 2O (g) Fe3O4 (s) + 4H2 (g) Kc =
[H2O]4
Calculating Equilibrium Constants
Haber mixed nitrogen and hydrogen and allowed it to
react at 500 K until the mixture reached equilibrium
with the product, ammonia. When he analysed the
equilibrium mixture, he found it to consist of:
0.796 M NH3, 0.305 M N2 and 0.324 M H2
What is the equilibrium constant for the reaction?
Applications of Equilibrium Constants
Carbon dioxide decomposes at elevated temperatures

to carbon monoxide and oxygen:
2CO2(g) 2CO(g) + O2(g)
At 3000 K, 2.00 mol CO2 is placed into a 1.00 L container

and allowed to come to equilibrium. At equilibrium,
0.90 mol CO2 remains. What is the Kc at this
temperature?
2CO2(g) 2CO(g) + O2(g)
The reaction quotient (Q):
Predicting the direction of reactions
The reaction quotient (Qc) is an expression that has the

same form as the equilibrium constant expression but
whose concentration values are not necessarily those at
equilibrium.
Qc > Kc : the reaction achieves equilibrium by

forming more reactants (will go left)
Qc < Kc : the reaction achieves equilibrium by
forming more products (will go right)
Qc = Kc : the reaction is at equilibrium
Given the following reaction, where Kc = 3.59 at 900C
CH4 (g) + 2H2S (g)  CS2 (g) + 4H2 (g)
Determine if the following reaction mixture; 1.07 M CH4, 1.20 M
H2S, 0.90 M CS2 and 1.78 M H2 is at equilibrium. If not in which
direction must the reaction proceed to reach equilibrium?
Changing Reaction Conditions
By changing the reaction conditions, you can increase
or decrease the yield of product.
Three ways to alter the equilibrium composition of a

gaseous reaction mixture:
• Changing the concentrations by removing products

or adding reactants to the reaction vessel.
• Changing the partial pressure of gaseous reactants
and products by changing the volume.
• Changing the temperature.
Le Chatelier’s Principle
When a system in chemical equilibrium is disturbed

(by a change of temperature, pressure, or a
concentration) the system shifts its equilibrium
position so as to counteract the affect of this
disturbance.
If the pressure is increased by decreasing the
volume of a reaction mixture, the reaction shifts in
the direction of fewer moles of gas.
Lets consider the same reaction in the Fischer-Tropsch

process:
Sasol: the largest scale implementation of
Fischer–Tropsch technology
8CO(g) + 17H2(g) C8H18(g) + 8H2O(g)
25 molecules gas 9 molecules gas

Would you expect more or less of the product octane,
C8H18, as the pressure increases?
Effect of Temperature Change
For an exothermic reaction (H negative), the amount
of product is decreased at equilibrium by an increase in
temperature.
A+B C + D + heat
For an endothermic reaction (H positive), the

amounts of products are increased by a increase in
temperature.
A + B + heat C+D
The effect of a catalyst
A catalyst has no effect on the equilibrium
composition of a reaction mixture. A catalyst merely
speeds up the reaction to achieve equilibrium.
A variety of catalysts can be used for the Fischer–
Tropsch process, the most common are
the transition metals cobalt, iron, and ruthenium
High-temperature Fischer–Tropsch is operated at

temperatures of 330 - 350 °C and uses an iron-based
catalyst.
Low-Temperature operations often use a cobalt-based
catalyst.

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Chapter 15 Chemical Equilibrium

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CHAPTER 15

Slides by: Dr. Tetana

A chemical equilibrium refers to the balance of two

CO (g) + 3H2 (g) CH4 (g) + H2O (g)

Start with 1.0 mol CO and 3.0 mol H2 in a 10.0 L vessel at

When heated, PCl5 forms PCl3 and Cl2 as follows:

When 1.00 mol PCl5 in a 1.00 L container is allowed to

Consider the general reaction :

The Law of Mass Action:

Consider the following industrial process:

CO(g) + Cl2(g) COCl2(g)

Kc = [COCl2] = 4.56 x 109 at 100 oC

Large K: Equilibrium lies to the right (favours products)

COCl2(g) CO(g) + Cl2(g)

Carbon dioxide decomposes at elevated temperatures

At 3000 K, 2.00 mol CO2 is placed into a 1.00 L container

The reaction quotient (Qc) is an expression that has the

Qc > Kc : the reaction achieves equilibrium by

Three ways to alter the equilibrium composition of a

• Changing the concentrations by removing products

When a system in chemical equilibrium is disturbed

Lets consider the same reaction in the Fischer-Tropsch

8CO(g) + 17H2(g) C8H18(g) + 8H2O(g)

25 molecules gas 9 molecules gas

For an endothermic reaction (H positive), the

High-temperature Fischer–Tropsch is operated at

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