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To predict an equilibrium shift, identify the change, then apply Le Chatelier's principle to counteract it. Free notes with key terms and 5 questions answered.
Predicting Changes in Equilibrium Systems is part of Reversible reactions and dynamic equilibrium in GCSE AQA Higher Chemistry.
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To predict an equilibrium shift, identify the change, then apply Le Chatelier's principle to counteract it. Free notes with key terms and 5 questions answered.
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Use this lesson to stay inside Reversible reactions and dynamic equilibrium while connecting the detail back to the wider Topic 6: The rate and extent of chemical change topic.
Read the core explanations from Predicting Changes in Equilibrium Systems before testing yourself from memory.
Picture a busy motorway junction where cars enter and leave at exactly the same rate. The number of cars stays constant, even though individual cars are constantly moving. This is what happens in a reversible chemical reaction at equilibrium.
The key word here is 'dynamic' - reactions haven't stopped, they're balanced. For the reaction $A + B \rightleftharpoons C + D$, molecules of A and B are still forming C and D, while C and D are breaking down to reform A and B. The rates are equal, so nothing appears to change.
When you disturb an equilibrium system, it doesn't just sit there - it fights back. This is Le Chatelier's principle, and it's your key to predicting what happens when conditions change.
Think of equilibrium as stubborn - it always tries to undo whatever you do to it. Add more reactant? The equilibrium shifts right to use it up. Remove product? The equilibrium shifts right to make more. It's like a chemical see-saw that automatically rebalances itself.
Temperature changes are different because they change the actual balance point of the reaction, not just the immediate direction of shift. You need to know whether the forward reaction is exothermic (releases heat) or endothermic (absorbs heat).
Worked example: Predicting Temperature Effects
For the reaction: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ ΔH = -92 kJ/mol. Predict what happens when temperature increases.
Answer: Higher temperature shifts equilibrium left, producing less ammonia
Remember the pattern: increasing temperature favours the endothermic direction (the one that absorbs heat). Decreasing temperature favours the exothermic direction (the one that releases heat).
Pressure changes only affect reactions involving gases, and the key is counting gas molecules on each side of the equation. Concentration changes work for any reaction - the equilibrium always shifts away from what you add.
For pressure: higher pressure favours the side with fewer gas molecules (less volume). For concentration: adding a substance shifts the equilibrium away from that substance.
Here's a crucial point that catches many students: catalysts speed up both forward and reverse reactions equally. They help you reach equilibrium faster, but they don't change where the equilibrium position ends up.
Think of a catalyst as a better road between two cities. It makes the journey faster in both directions, but it doesn't change which city has more people living there at the end.
Equilibrium questions follow predictable patterns. Examiners want to see you identify the change, apply Le Chatelier's principle, and explain the shift with scientific reasoning.
The most common mistake is confusing rate with position. A catalyst makes reactions faster but doesn't change the final amounts of products and reactants. Temperature, pressure, and concentration can all change the rate, but the key GCSE idea is how they shift the equilibrium position.
Exam tip: state which change is made, then explain which direction uses up the change and why. This equilibrium topic is Higher Tier only. - Reversible reactions and dynamic equilibrium, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
At GCSE, equilibrium questions are answered qualitatively. If concentration changes, the system shifts to oppose that change. If pressure changes, the side with fewer gas molecules is favoured. Temperature is different because it changes which direction is energetically favoured, not just the immediate disturbance.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
To predict an equilibrium shift, identify the change, then apply Le Chatelier's principle: the equilibrium shifts to counteract it. Adding a substance shifts the equilibrium away from that substance, higher pressure favours the side with fewer gas molecules, and higher temperature favours the endothermic direction.
Temperature is different because it changes which direction is energetically favoured and the actual balance point of the reaction, not just the immediate disturbance. Increasing temperature favours the endothermic direction, and decreasing temperature favours the exothermic direction.
Rate is how fast the reactions happen and equilibrium is reached, while equilibrium position is the final amounts of products and reactants. A catalyst changes the rate but not the position; confusing rate with position is the most common mistake.
For an exothermic forward reaction, heat is a product, for example N₂ + 3H₂ ⇌ 2NH₃ + heat when ΔH = -92 kJ/mol. Increasing temperature then acts like adding heat, so the equilibrium shifts left to remove the extra heat.
A catalyst speeds up both the forward and reverse reactions equally. It helps a system reach equilibrium faster, but it doesn't change where the equilibrium position ends up, like a better road between two cities that makes the journey faster in both directions.