1. Topic 7 - Equilibrium
7 – Dynamic Equilibrium 1

2. Le Chatelier’s Principle
Objectives:
Understand the impact of Le Chatelier’s Principle
Use Le Chatelier’s Principle to explain the effect of changes on a system at equilibrium
Complete some short experiments to observe the effect of Le Chatelier’s Principle

3. Recap
The sequence of diagrams represents the system as time passes for a gas phase reaction in which reactant X is converted to product Y.
Which statement is correct?
At t = 5 days the rate of the
forward reaction is greater than the rate of the backward reaction.
At t = 7 seconds the reaction has reached completion.
At t = 10 minutes the system has reached a state of equilibrium.
At t = 5 days the rate of the forward reaction is less than the rate of the backward reaction.

4. Le Chatelier’s Principle
This describes how a dynamic chemical equilibrium responds to disturbances
‘A system at equilibrium responds to disturbances in such a way as to minimize that disturbance’
This applies to changes in:
Concentration
Pressure
Temperature
REACTANTS
PRODUCTS

5. Changes in Concentration
Decrease in [Reactant] or increase in [Product]
Equilibrium shifts to the left
This has the effect of increasing [Reactant] and decreasing [Product]
REACTANTS
PRODUCTS
Equilibrium constant is not affected
Ethanol Ethanoic Acid ⇌ Ethyl ethanoate water
CH3CH2OH(aq) CH3CO2H(aq) ⇌ CH3CH2OOCCH3(aq) H2O(l)

6. Changes in Concentration
Increase in [Reactant] or decrease in [Product]
Equilibrium shifts to the right
This has the effect of decreasing [Reactant] and increasing [Product]
REACTANTS
PRODUCTS
Equilibrium constant is not affected
Ethanol Ethanoic Acid ⇌ Ethyl ethanoate water
CH3CH2OH(aq) CH3CO2H(aq) ⇌ CH3CH2OOCCH3(aq) H2O(l)

7. Equilibrium constant is not affected
Ethanol Ethanoic Acid ⇌ Ethyl ethanoate water
CH3CH2OH(aq) CH3CO2H(aq) ⇌ CH3CH2OOCCH3(aq) H2O(l)

8. 2N2(g) + 3H2(g) ⇌ 2NH3(g) H = -91.8 kJ mol-1
Changes in Pressure
Increasing Pressure:
Shifts equilibrium to the side with fewest gas molecules
This has the effect of reducing the pressure increase
Decreasing Pressure:
Shifts equilibrium to the side with more gas molecules
This has the effect of increasing the pressure
Equilibrium constant is not affected
2N2(g) + 3H2(g) ⇌ 2NH3(g) H = kJ mol-1

9. Changes in Temperature
Exothermic Reaction:
Increasing temperature shifts equilibrium to left
The left is the ‘cold’ side of the reaction, so temperature decreases
Decreasing temperature shifts equilibrium to the right
This is the ‘hot’ side of the reaction, so temperature increases
Endothermic Reaction:
Increasing temperature shifts equilibrium to the right
This is the ‘cold’ side of the reaction, so temperature decreases
Decreasing temperature shifts equilibrium to the left
Equilibrium constant is affected
Exothermic Reaction: Higher Temp  Lower Kc; Lower Temp  Higher Kc
Endothermic Reaction: Higher Temp  Higher Kc; Lower Temp  Lower Kc
2N2(g) + 3H2(g) ⇌ 2NH3(g) H = kJ mol-1

10. Problem
The diagrams below represent equilibrium mixtures for the reaction Y + X2  XY + X at 350 K and 550 K respectively. Deduce and explain whether the reaction is exothermic or endothermic.
350K K

11. Recap
Le Chatelier’s Principle explains how equilibrium systems respond to changes
Any disturbance to a system will be minimised but not eliminated.