Chemical changes · GCSE Chemistry
Equilibria and Le Chatelier’s principle
GCSE Chemistry revision on reversible reactions, dynamic equilibrium in a closed system, and Le Chatelier’s principle for concentration, pressure and temperature, including why catalysts do not change yield.
Dynamic equilibrium: forward rate equals reverse rate, concentrations constant, closed system. Le Chatelier: the mixture shifts to oppose the change you made. Catalysts get you there faster but do not move the position.
The important bits
What you need to know
- 1
A reversible reaction can go forwards and backwards. The symbol is ⇌. Example: hydrated copper sulfate ⇌ anhydrous copper sulfate + water, or N₂ + 3H₂ ⇌ 2NH₃.
- 2
Dynamic equilibrium is reached in a closed system when the forward rate equals the reverse rate. Substances are still reacting, but concentrations (and colour, pressure, and so on) stay constant.
- 3
A closed system means no substances can enter or leave. An open evaporating basin cannot reach this equilibrium because a gas or vapour can escape.
- 4
Le Chatelier’s principle (Higher): if a change is made to the conditions of a system at equilibrium, the position of equilibrium shifts to oppose that change.
- 5
Concentration: adding a reactant shifts the position towards products; removing a product also shifts towards products. Adding a product shifts towards reactants.
- 6
Pressure (gases only): increasing pressure shifts equilibrium towards the side with fewer gas molecules. Decreasing pressure favours the side with more gas molecules. If both sides have the same number of gas molecules, pressure does not shift the position.
- 7
Temperature: increasing temperature favours the endothermic direction; decreasing temperature favours the exothermic direction. Rate still increases with temperature even if yield falls.
- 8
A catalyst increases the rate of both directions equally, so equilibrium is reached faster but the position (yield) does not change. Industry uses this to save time, then chooses T and P for yield and cost.
Quotations worth analysing
Short evidence. Real method.
“At equilibrium the forward and reverse reactions happen at the same rate and the concentrations of reactants and products are constant.”
Same rate and constant concentrations are both required. “The reaction stops” is the classic wrong line — it is dynamic, not static.
“If a system at equilibrium is subjected to a change, the position of equilibrium shifts to oppose the change.”
State the principle, then apply it to the specific change: concentration, pressure or temperature. An unexplained “it shifts right” does not score full marks.
“A catalyst does not affect the position of equilibrium.”
Pair it with “it increases rate” or “equilibrium is reached faster”. Haber’s iron is the usual example.
Go deeper
Closed, dynamic, constant — three words, three marks
Closed: the products cannot escape, so they are available to react backwards. Dynamic: molecules keep colliding and reacting both ways. Constant: the amounts stop changing because the two rates have matched, not because everything has been used up. A sealed tube of NO₂/N₂O₄ is the usual colour example: brown NO₂ and colourless N₂O₄, 2NO₂ ⇌ N₂O₄. Heat it and the mixture goes browner if the forward direction that makes NO₂ is endothermic. Cool it and it fades as more N₂O₄ forms. If you open the tube, you no longer have equilibrium. Students write “equilibrium means equal amounts of reactants and products”. It does not. The amounts are constant, not necessarily equal. Ammonia at Haber conditions is still a minority product.
Go deeper
Apply Le Chatelier in a full sentence
Template: I increase X. The system opposes this by doing Y. Therefore the position shifts towards Z. Example: increase pressure on N₂ + 3H₂ ⇌ 2NH₃. The system opposes the increase by reducing the number of gas molecules, so it shifts to the ammonia side. Increase temperature: the system opposes this by taking in energy, so it shifts in the endothermic direction (back to N₂ and H₂). Add extra H₂: the system opposes the extra reactant by making more product. That template stops you writing “pressure increases yield because the molecules are squashed” without naming fewer moles of gas. If the gas counts are equal, say so, and do not invent a shift.
Go deeper
Yield, rate and money are different questions
Le Chatelier predicts the direction of the shift, not how fast it happens. A low temperature may improve yield of an exothermic product and still be useless if the plant would take days to get there. A catalyst fixes the time problem without touching the yield. High pressure may help both yield and rate for Haber, but the steel vessel costs more. When a six-mark industrial question appears, sort your paragraphs: what equilibrium wants, what rate wants, what the company can afford, and what is recycled. Do not use Le Chatelier for rate-only factors such as surface area of a solid, and do not use collision theory instead of Le Chatelier when the question says “explain the effect on the yield”.
See the idea in action
For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH negative (contact process), predict the effect of higher pressure, higher temperature, and a vanadium(V) oxide catalyst on the equilibrium yield of SO₃. Pressure: 3 gas molecules → 2, so higher pressure shifts right, yield of SO₃ up. Temperature: forward exothermic, so higher temperature shifts left, yield down (but rate up). Catalyst: yield unchanged, equilibrium reached faster. Industry still uses a moderately high temperature so the rate is practical, and a catalyst, with pressure only moderately high because the yield is already good — another compromise.
Exam technique
Turn knowledge into marks
Define dynamic equilibrium in a closed system before you apply Le Chatelier. For each change, say how the system opposes it and which side is favoured. Keep yield and rate in separate sentences. Write that a catalyst does not change the position.
Common mistakes
Do not give these marks away
- 01
Saying reactions stop at equilibrium, or that equilibrium means equal amounts of reactants and products.
- 02
Using Le Chatelier to explain rate, or saying a catalyst increases yield.
- 03
Forgetting to count gas molecules on each side when predicting a pressure change.
A reaction at equilibrium is exothermic in the forward direction. What is the effect of increasing temperature?
AThe position of equilibrium shifts to the right and the yield of products increases
BThe position of equilibrium shifts to the left and the yield of products decreases
CThe position of equilibrium does not change because temperature only affects rate
DThe catalyst is destroyed so no reaction occurs
Show the answer
The position of equilibrium shifts to the left and the yield of products decreases. The system opposes the temperature rise by favouring the endothermic reverse reaction, so the yield of products falls. Rate still increases.
Quick questions
If this is the bit you searched
What is dynamic equilibrium GCSE Chemistry?
In a closed system, the forward and reverse reactions continue at the same rate, so concentrations of reactants and products stay constant. The reactions have not stopped.
What is Le Chatelier’s principle?
If you change the conditions of a system at equilibrium, the position of equilibrium shifts to oppose that change. Use it for concentration, pressure and temperature.
Does a catalyst change the position of equilibrium?
No. It speeds the forward and reverse reactions equally, so equilibrium is reached faster but the yield stays the same.
How does pressure affect a gaseous equilibrium?
Increasing pressure favours the side with fewer gas molecules. If both sides have the same number of gas molecules, pressure does not shift the position.