Equilibrium: predicting shifts and the Haber compromise explained
Start here
Imagine a shop with a door that lets people in and out. At first people only come in. Later, as many leave as arrive, and the number inside stays the same even though the door never stops. A reversible reaction behaves in the same way.
This lesson turns that picture into rules you can use. You will predict what happens when you change the conditions, read a graph that shows equilibrium, and explain the conditions in a real factory.
Course note: reversible reactions and dynamic equilibrium are for both tiers. Predicting shifts with Le Chatelier is Higher tier. The Haber process is part of one Combined Science course only, so check yours.
Reversible reactions and dynamic equilibrium
In a reversible reaction the products can react to give the reactants back. The equation uses ⇌. Heating blue hydrated copper sulfate drives off the water and leaves a white powder. Adding water turns it blue again and it gets warm. If the forward change is endothermic, the backward change is exothermic, and the energy is the same size.
In a closed system, where nothing can enter or leave, the forward and backward reactions reach the same rate. This is dynamic equilibrium. The concentrations stay constant. They are not usually equal. Both reactions are still going on.
The position of equilibrium describes how much product there is. If it is to the right there is more product. If it is to the left there is more reactant.
Reading an equilibrium graph
A graph of concentration against time starts with only reactants. The reactant line falls and the product line rises. Both lines become flat at the same moment. The flat part is equilibrium: the rates are equal, so the concentrations stop changing.
Now disturb the system. In the graph, more reactant is added at the dashed line. The reactant line jumps up. Then it falls as the forward reaction speeds up, and the product line climbs. After a while both lines go flat again. This is a new equilibrium with more product than before.
Always say what you can see. A jump in one line shows something was added or removed. The system then moves to oppose the change, and settles down again.
Graph of concentration against time for a reversible reaction. The reactant line falls and the product line rises until both are flat at equilibrium. More reactant is then added, shown by a jump in the reactant line. The reactant line falls again and the product line rises to a new, higher equilibrium.
Concentration and pressure
Le Chatelier's principle says that if you change the conditions of a system at equilibrium, the position shifts to oppose the change. Add more reactant and the system uses some up, so the position shifts right. Remove a product and it makes more, so the position also shifts right.
Pressure only matters when there are gases, and only if the two sides have different numbers of gas molecules. Higher pressure shifts the position to the side with fewer gas molecules. Lower pressure favours the side with more.
Count molecules by the balancing numbers. In N₂ + 3H₂ ⇌ 2NH₃ there are 4 gas molecules on the left and 2 on the right, so high pressure gives more ammonia. In H₂ + I₂ ⇌ 2HI there are 2 on each side, so pressure makes no difference to the position.
Temperature and catalysts
Raising the temperature shifts the position in the endothermic direction, because that direction takes in the extra energy. Lowering the temperature shifts it in the exothermic direction, which gives energy out.
N₂ + 3H₂ ⇌ 2NH₃ is exothermic from left to right. A higher temperature shifts the position left, so less ammonia forms. A lower temperature gives more ammonia. With N₂O₄ ⇌ 2NO₂, which is endothermic to the right, heating makes more brown NO₂ gas.
A catalyst speeds up the forward and backward reactions by the same amount. Equilibrium is reached sooner, but the position and the yield do not change. Keep two ideas apart: temperature changes both the rate and the position, but a catalyst changes only the rate.
The Haber process compromise
The Haber process makes ammonia from nitrogen, taken from the air, and hydrogen, usually made from natural gas. The reaction is N₂ + 3H₂ ⇌ 2NH₃ and the forward reaction is exothermic.
High pressure favours ammonia, because there are fewer gas molecules on the right, and it also speeds up the reaction. But very high pressure needs thick, strong pipes and a lot of energy, which is expensive and less safe. So about 200 atmospheres is used.
A low temperature favours ammonia because the forward reaction is exothermic. But at a low temperature the reaction is far too slow. So a compromise of about 450 °C is used with an iron catalyst, which makes the rate acceptable. Only part of the gas turns into ammonia on each pass, so the ammonia is cooled and liquefied and the unreacted nitrogen and hydrogen are recycled.
A prediction checklist
Use this table as a checklist for N₂ + 3H₂ ⇌ 2NH₃, which is exothermic in the forward direction. Read across each row: the change, which way the position shifts, and what happens to the rate.
Notice the rate column. Almost every change makes the reaction faster, so rate and yield can disagree. A higher temperature is the clearest case: faster, but the position shifts left and the yield falls.
A good habit is to answer in two parts: first the position and yield, then the rate. That way a question that asks only about yield does not get a muddled answer.
Exam technique and recap
For "predict" and "explain" questions, use a short chain. State the change. State which way the position shifts. Say why, using oppose, fewer molecules or endothermic direction. Then say what happens to the yield. If the question mentions rate, treat it separately, because rate and yield can move in opposite directions.
For a 6 mark industrial answer, give each condition with two points: its effect on yield and its effect on rate or cost, and then say it is a compromise. Mention that the catalyst does not change the yield and that unreacted gases are recycled.
Recap. Dynamic equilibrium means equal rates, not stopped. More reactant shifts right. Higher pressure favours fewer gas molecules. Higher temperature favours the endothermic direction. A catalyst changes only the rate. The Haber process uses about 450 °C, 200 atmospheres and an iron catalyst.
Key points
Dynamic equilibrium: the forward and backward rates are equal, in a closed system, so concentrations stay constant.
More reactant, or less product, shifts the position right. Higher pressure favours fewer gas molecules.
Higher temperature favours the endothermic direction. A catalyst changes only the rate.
Haber process: N₂ + 3H₂ ⇌ 2NH₃, about 450 °C, 200 atmospheres, iron catalyst, unreacted gases recycled.
Worked examples
Example 1
A reaction is exothermic in the forward direction. Describe the energy change in the backward direction.
The forward and backward reactions have opposite energy changes.
The forward reaction is exothermic, so the backward reaction is endothermic.
The amount of energy is the same in both directions.
Answer: It is endothermic, with the same size of energy change.
Common slips
Slip: Both directions are exothermic. If both gave out energy, energy would be made from nothing. The reverse takes it in.
Slip: The backward reaction is endothermic, with a bigger energy change. The size of the energy change is the same in both directions.
Slip: The backward reaction has no energy change. Every reaction has an energy change. For the reverse it is the opposite sign.
Example 2
2SO₂ + O₂ ⇌ 2SO₃ is exothermic in the forward direction. Predict and explain the effect of increasing the pressure on the yield of SO₃.
Count the gas molecules: 2 + 1 = 3 on the left and 2 on the right.
Higher pressure shifts the position towards the side with fewer gas molecules, which is the right.
So the yield of SO₃ increases.
Answer: The yield of SO₃ increases, because there are fewer gas molecules on the right so the position shifts right.
Common slips
Slip: There are 2 molecules on the left because there are 2 SO₂. Add the O₂ too: 2 + 1 = 3 molecules on the left.
Slip: Higher pressure favours the side with more molecules. The system opposes the increase by moving to the side with fewer molecules.
Slip: The yield decreases because the reaction is exothermic. Exothermic matters for temperature changes. For pressure only the number of gas molecules counts.
Example 3
PCl₅ ⇌ PCl₃ + Cl₂ is endothermic in the forward direction. All three are gases. Predict the effect of increasing the temperature and of increasing the pressure on the yield of PCl₃.
Temperature: the forward direction is endothermic, so a higher temperature shifts the position right and the yield of PCl₃ increases.
Pressure: there is 1 gas molecule on the left and 2 on the right.
A higher pressure shifts the position left, towards fewer molecules, so the yield of PCl₃ decreases.
Answer: Higher temperature increases the yield of PCl₃. Higher pressure decreases the yield of PCl₃.
Common slips
Slip: Temperature: the forward direction is endothermic, so heating shifts left. Heating favours the endothermic direction, which is the forward reaction here, so it shifts right.
Slip: Pressure: 2 molecules on the left and 1 on the right. PCl₅ is one molecule on the left. PCl₃ and Cl₂ are two on the right.
Slip: Higher pressure increases the yield of every product. It favours the side with fewer molecules. Here that reduces the products.
Questions and answers
Equilibrium: predicting shifts and the Haber compromise practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
What name is given to the symbol ⇌ in an equation?
Show answer
Answer: reversible arrow
It shows the reaction goes both ways.
It is the reversible arrow.
Question 2 Easy
A reaction is endothermic in the forward direction. Is the backward reaction exothermic or endothermic?
Show answer
Answer: exothermic
The energy changes are opposite.
So the backward reaction is exothermic.
Question 3 Easy
At dynamic equilibrium, what is the same for the forward and backward reactions?
Show answer
Answer: the rate
Both are still happening.
Their rates are the same.
Question 4 Easy
In N₂ + 3H₂ ⇌ 2NH₃, how many gas molecules are on the left-hand side?
Show answer
Answer: 4
1 N₂ and 3 H₂.
1 + 3 = 4.
Question 5 Easy
What is the effect of a catalyst on the position of equilibrium?
Show answer
Answer: none
A catalyst speeds up both directions equally.
So the position does not change.
Question 6 Medium
The forward reaction is exothermic. What happens to the yield of products when the temperature is increased?
Show answer
Answer: it decreases
A higher temperature favours the endothermic direction.
That is the backward reaction, so the yield decreases.
Question 7 Medium
In 2SO₂ + O₂ ⇌ 2SO₃, what happens to the yield of SO₃ when the pressure is increased?
Show answer
Answer: it increases
3 gas molecules on the left, 2 on the right.
Higher pressure favours fewer molecules, so the yield increases.
Question 8 Medium
For CO + 2H₂ ⇌ CH₃OH, more hydrogen is added. Which way does the position of equilibrium shift?
Show answer
Answer: to the right
The system uses up the extra hydrogen.
So the position shifts to the right.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
2SO₂ + O₂ ⇌ 2SO₃. The forward reaction is exothermic.
(a) Explain the effect of increasing the pressure on the yield of SO₃. [2 marks]
(b) Explain the effect of increasing the temperature on the yield of SO₃. [2 marks]
Show answer and marking points
Answer: (a) The yield increases because there are fewer gas molecules on the right, so the position shifts right. (b) The yield decreases because a higher temperature favours the endothermic direction, so the position shifts left.
(a) yield increases (1)
(a) fewer gas molecules on the right so the position shifts right (1)
(b) yield decreases (1)
(b) higher temperature favours the endothermic direction, which is the reverse (1)
Exam-style question 26 marks
The Haber process makes ammonia: N₂ + 3H₂ ⇌ 2NH₃. The forward reaction is exothermic. Explain why the process uses about 450 °C, 200 atmospheres and an iron catalyst. [6 marks]
Show answer and marking points
Answer: A lower temperature would give a higher yield, because the forward reaction is exothermic, but the rate would be too slow. So about 450 °C is a compromise between yield and rate. A high pressure gives a higher yield, because there are fewer gas molecules on the right, and it increases the rate. A much higher pressure would cost more and be less safe, so 200 atmospheres is used. The iron catalyst speeds up the reaction so equilibrium is reached faster, but it does not change the yield.
lower temperature gives higher yield as the forward reaction is exothermic (1)
but the rate would be too slow, so 450 °C is a compromise (1)
high pressure gives a higher yield because there are fewer gas molecules on the right (1)
a very high pressure costs more or is less safe, so 200 atmospheres is a compromise (1)
the iron catalyst speeds up the reaction (1)
the catalyst does not change the yield (1)
Common mistakes
Saying the reaction stops at equilibrium
Both reactions carry on at the same rate. The concentrations stay constant, which is why it is called dynamic.
Saying a catalyst increases the yield at equilibrium
A catalyst speeds up both directions equally. Equilibrium is reached sooner but the yield is unchanged.
Counting only one reactant when working out the gas molecules for pressure
Add all the gas molecules on each side using the balancing numbers: N₂ + 3H₂ is 4.
Using the exothermic or endothermic label to answer a pressure question
Pressure depends only on the number of gas molecules. The energy change matters for temperature.
Mixing up rate and yield when temperature is raised
A higher temperature increases the rate, but it lowers the yield of an exothermic reaction. Say both when asked.
Questions students ask
Is Equilibrium: predicting shifts and the Haber compromise on the Foundation or Higher tier?
Equilibrium: predicting shifts and the Haber compromise is Higher tier content for GCSE Combined Science Chemistry, aimed at around grade 7.
What is a common mistake on Equilibrium: predicting shifts and the Haber compromise?
Saying the reaction stops at equilibrium. Both reactions carry on at the same rate. The concentrations stay constant, which is why it is called dynamic.
Where are the answers to the Equilibrium: predicting shifts and the Haber compromise questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.