Le Chatelier's Principle Explained for OCR A-Level Chemistry (With Worked Examples)
- Oct 28, 2017
- 4 min read
Updated: 14 hours ago
By Paul Morgan
One of the most heavily tested topics in OCR A-Level Chemistry is Le Chatelier's Principle. Students often memorise the rules but struggle to apply them in exam questions.
The problem isn't remembering what happens when temperature, pressure or concentration changes. The difficulty is understanding why equilibrium shifts in a particular direction.
In this guide you'll learn:
• What Le Chatelier's Principle actually means
• How equilibrium responds to changes in concentration
• The effect of pressure on gaseous equilibria
• Why temperature behaves differently from concentration and pressure
• Common OCR exam mistakes
• Worked OCR-style examples
• Examination tips to help you avoid losing easy marks
What Is Le Chatelier's Principle?
Le Chatelier's Principle states:
"If a system at dynamic equilibrium is disturbed, the equilibrium position shifts to oppose the change."
The important phrase is oppose the change.
The equilibrium does not stop.
Instead, the forward and reverse reactions continue until a new equilibrium is established.
What Is Dynamic Equilibrium?
Before applying Le Chatelier's Principle, it's important to understand dynamic equilibrium.
Dynamic equilibrium only exists when:
• The reaction is reversible.
• The reaction takes place in a closed system.
• The rate of the forward reaction equals the rate of the reverse reaction.
At equilibrium:
• Concentrations remain constant.
• The reactions continue.
• Particles are still reacting.
The reactions haven't stopped—they are simply occurring at the same rate.
The Haber Process Example
The Haber Process provides the perfect example.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
The forward reaction is exothermic.
This means heat is released when ammonia forms.
We'll use this reaction throughout the guide.
Effect of Changing Concentration
Suppose more nitrogen is added.
The system now contains excess nitrogen.
According to Le Chatelier's Principle, the equilibrium shifts to remove some of the extra nitrogen.
Therefore, the equilibrium moves to the right.
More ammonia is produced.
Removing Nitrogen
If nitrogen is removed instead, the equilibrium shifts left to replace the missing nitrogen.
The equilibrium always acts to reduce the disturbance.
OCR Exam Tip
Never write:
"The equilibrium moves because more collisions occur."
Instead explain:
"The equilibrium shifts to oppose the increase in concentration by using up the added reactant."
Effect of Pressure
Pressure only affects equilibria involving gases.
It only matters if the number of gas molecules differs on each side of the equation.
For the Haber Process:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Left-hand side:
4 moles of gas
Right-hand side:
2 moles of gas
Increasing Pressure
Increasing pressure favours the side with fewer gas molecules.
Therefore, equilibrium shifts to the right.
More ammonia forms.
Decreasing Pressure
Lower pressure favours the side with more gas molecules.
The equilibrium shifts to the left.
Less ammonia is produced.
Effect of Temperature
Temperature affects equilibrium differently from concentration and pressure because it changes the energy of the reacting particles.
A useful way to think about temperature is to treat heat as if it were another reactant or product in the balanced equation.
For the Haber Process:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + Heat
The forward reaction is exothermic, so heat is released as ammonia forms.
Increasing Temperature
When the temperature is increased, it is like adding extra heat to the products.
According to Le Chatelier's Principle, the equilibrium shifts to remove some of the added heat.
The only way it can do this is by favouring the endothermic (reverse) reaction.
As a result:
• The equilibrium shifts to the left.
• Less ammonia (NH₃) is produced.
• More nitrogen (N₂) and hydrogen (H₂) are formed.
Decreasing Temperature
When the temperature is decreased, heat is removed from the system.
The equilibrium shifts to replace the lost heat by favouring the exothermic (forward) reaction.
As a result:
• The equilibrium shifts to the right.
• More ammonia (NH₃) is produced.
• The yield of ammonia increases.
OCR Exam Tip
Many students try to remember whether equilibrium moves left or right.
A much safer method is to ask yourself:
"Which direction absorbs the change in temperature?"
• Increasing temperature always favours the endothermic reaction.
• Decreasing temperature always favours the exothermic reaction.
This rule works for every equilibrium question in OCR A-Level Chemistry.
Common OCR Mistake
Students often think increasing temperature always speeds up the forward reaction.
Although both the forward and reverse reactions become faster, the endothermic reaction speeds up proportionally more, causing the equilibrium position to change.
Always identify whether the forward reaction is exothermic or endothermic before deciding which way the equilibrium shifts.
Continue Your OCR Chemistry Revision
You may also find these guides helpful:
Free OCR A-Level Chemistry Guides
If you're studying OCR A-Level Chemistry and want to avoid the most common mistakes that hold students back, download my free guides:
Year 12 Students:
4 Mistakes That Cause Strong GCSE Students To Struggle In Year 12 Chemistry
Year 13 Students:
4 Mistakes Keeping Capable OCR Chemistry Students Stuck At Grade B Or Below
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