Edexcel IGCSE Chemistry · Spec 3.19C-3.22C
Dynamic Equilibrium & Le Chatelier's Principle
Dynamic equilibrium and how Le Chatelier's principle predicts the effect of changing conditions.
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Dynamic Equilibrium & Le Chatelier's Principle
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Explained
Dynamic equilibrium, and predicting which way it shifts
A reversible reaction can run in both directions. In a closed system, where nothing enters or leaves, it eventually reaches equilibrium.
At that point the forward and backward reactions are both still happening, at exactly the same rate. Nothing looks like it is changing, because the concentrations stay constant, but the reactions have not stopped. That is what the word dynamic is doing in the name.
Le Chatelier's principle
When a condition of a system at equilibrium is changed, the position of equilibrium shifts in the direction that opposes the change.
Read that as the system pushing back. Heat it and it shifts in the direction that absorbs heat. Squeeze it and it shifts in the direction that takes up less space. Add more of something and it shifts in the direction that uses that something up.
Temperature
Raising the temperature shifts the equilibrium in the endothermic direction, because that direction absorbs energy and so opposes the rise.
So if the forward reaction is endothermic, heating increases the yield of products. If the forward reaction is exothermic, heating decreases it, and cooling increases it. Check which way the energy goes before you decide.
Pressure
Pressure only matters when gases are involved, and only when the two sides have different numbers of gas molecules.
Increasing the pressure shifts the equilibrium towards the side with fewer gas molecules, because fewer molecules occupy less space and so reduce the pressure. Count the molecules on each side of the equation first; if the numbers are equal, pressure has no effect on the position at all.
Concentration
Adding more reactant shifts the equilibrium towards the products, because the system responds by using the extra reactant up. Removing a product does the same thing, which is why industrial processes often take the product away continuously.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme for a two mark question on raising the temperature wants two specific things: that the yield increases and the reaction shifts to the right hand side, and that this is because the forward reaction is endothermic. The second mark depends on the first.
Then comes the instruction worth remembering: ignore references to Le Chatelier. Naming the principle earns nothing. Applying it, by saying which way the equilibrium moves and why, earns everything.
The same mark scheme elsewhere asks what kind of reaction is shown by the double arrow. It credits reversible, and allows goes both ways or can go forwards and backwards at the same time, but instructs the examiner to ignore dynamic equilibrium. The arrow shows the reaction is reversible; equilibrium is a state it may later reach.
The examiner report on that paper adds that a few candidates lost the mark for writing reverse reaction, which is not the same as a reversible reaction. One word, one letter, one mark.
The industrial compromise
The Haber process makes ammonia from nitrogen and hydrogen, and the forward reaction is exothermic with fewer gas molecules on the product side.
Le Chatelier says a low temperature and a high pressure would give the best yield. In practice a moderate temperature of about 450 degrees Celsius is used, because at a low temperature the reaction would be far too slow to be useful, and a pressure of about 200 atmospheres rather than more, because very high pressures need expensive and dangerous equipment.
These conditions are a compromise between yield, rate and cost, and saying so is usually the final mark on this question. Equilibrium tells you what is possible; economics tells you what is actually done.
Spec 3.19C-3.22C
What you need to know
- Describe dynamic equilibrium
- State Le Chatelier's principle
- Predict the effect of changing conditions
Active recall
Quick check
Answer each question before opening the answer.
What is dynamic equilibrium?
In a closed system, the forward and backward reactions happen at the same rate, so the concentrations stay constant.
How does increasing pressure affect a gas equilibrium?
It shifts the equilibrium towards the side with fewer gas molecules.
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