Taylor Notes Equilibria and Kc
Le Chatelier’s Principle
The Features of a Dynamic Equilibrium
The Rate of the Forwards Reaction equals the Rate of the Backwards Reaction
The Concentration of all Reactants and Products Remains Constant
Definition of Chatelier’s Principle
Le Chatelier’s principle states that when a reaction at equilibrium is subjected to a change in condition (temperature,
pressure, or concentration), the equilibrium shifts to reduce the effect of the change
The Effect of Changing Conditions
Use the following example to explain the effect of changes in conditions on the position of equilibrium
A (g) + B (g) ⇌ C (g) + 2D (g) ΔH = -175 kJ mol
Increasing Temperature
The Equilibrium shifts to the left hand side to oppose the temperature increase as the backwards reaction is
endothermic and so more A and B are produced
Increasing Pressure
The Equilibrium shifts to the left-hand side to oppose the pressure increase as the left hand side has the fewest
number of gaseous moles and so more A and B are produced
Increasing Concentration
The Equilibrium shifts to the righthand side to oppose the concentration increase and so more A and B are produced
Adding a Catalyst
The Equilibrium stays in the same position, but the rate of the forwards and backwards reaction are increased
equally
Equilibrium Example
N2O4 is a colourless gas and NO2 is a brown gas N2O4 (g) ⇌ 2NO2 (g) ΔH = +58 kJ mol-1
Explain What Happens to the Position of Equilibrium if the Pressure is Decreased?
The Equilibrium shifts to the right-hand side to oppose the decrease in pressure because the largest number of
gaseous moles is on the right-hand side
Explain What Happens to the Position of Equilibrium if the Temperature is Decreased?
The Equilibrium shifts to the left-hand side to oppose the decrease in temperature because the backwards reaction is
exothermic and so the equilibrium shifts in this direction
Equilibrium Example 2
Ethanol is manufactured by reacting ethene with steam
CH2CH2 (g) + H2O (g) ⇌ CH3CH2OH (g) ΔH = -45 kJ mol-1
Explain which Conditions Should be Chosen for the Process
Temperature: High or Low
Explanation: To shift the Equilibrium to the right-hand side to oppose the temperature decrease and to therefore
increase the yield of ethanol
Pressure: High or Low
Le Chatelier’s Principle
The Features of a Dynamic Equilibrium
The Rate of the Forwards Reaction equals the Rate of the Backwards Reaction
The Concentration of all Reactants and Products Remains Constant
Definition of Chatelier’s Principle
Le Chatelier’s principle states that when a reaction at equilibrium is subjected to a change in condition (temperature,
pressure, or concentration), the equilibrium shifts to reduce the effect of the change
The Effect of Changing Conditions
Use the following example to explain the effect of changes in conditions on the position of equilibrium
A (g) + B (g) ⇌ C (g) + 2D (g) ΔH = -175 kJ mol
Increasing Temperature
The Equilibrium shifts to the left hand side to oppose the temperature increase as the backwards reaction is
endothermic and so more A and B are produced
Increasing Pressure
The Equilibrium shifts to the left-hand side to oppose the pressure increase as the left hand side has the fewest
number of gaseous moles and so more A and B are produced
Increasing Concentration
The Equilibrium shifts to the righthand side to oppose the concentration increase and so more A and B are produced
Adding a Catalyst
The Equilibrium stays in the same position, but the rate of the forwards and backwards reaction are increased
equally
Equilibrium Example
N2O4 is a colourless gas and NO2 is a brown gas N2O4 (g) ⇌ 2NO2 (g) ΔH = +58 kJ mol-1
Explain What Happens to the Position of Equilibrium if the Pressure is Decreased?
The Equilibrium shifts to the right-hand side to oppose the decrease in pressure because the largest number of
gaseous moles is on the right-hand side
Explain What Happens to the Position of Equilibrium if the Temperature is Decreased?
The Equilibrium shifts to the left-hand side to oppose the decrease in temperature because the backwards reaction is
exothermic and so the equilibrium shifts in this direction
Equilibrium Example 2
Ethanol is manufactured by reacting ethene with steam
CH2CH2 (g) + H2O (g) ⇌ CH3CH2OH (g) ΔH = -45 kJ mol-1
Explain which Conditions Should be Chosen for the Process
Temperature: High or Low
Explanation: To shift the Equilibrium to the right-hand side to oppose the temperature decrease and to therefore
increase the yield of ethanol
Pressure: High or Low
