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A second set of questions on equilibrium: predicting shifts and the Haber compromise. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
H₂ + I₂ ⇌ 2HI has the same number of gas molecules on each side. What is the effect of increasing the pressure on the position of equilibrium?
Answer: no change
N₂O₄ ⇌ 2NO₂ is endothermic in the forward direction. What happens to the amount of NO₂ when the mixture is heated?
Answer: it increases
A concentration-time graph for a reversible reaction in a closed container has both lines flat after 8 minutes. What does this show?
Answer: equilibrium has been reached
State two raw materials for the Haber process and where each comes from.
Answer: nitrogen from the air and hydrogen from natural gas
Explain why the Haber process uses a temperature of about 450 °C and not a much lower temperature.
Answer: a lower temperature gives a higher yield but the rate would be too slow
Explain why a catalyst is used in the Haber process even though it does not increase the yield.
Answer: it speeds up the reaction so equilibrium is reached faster
After 30 minutes, the concentrations of reactants and products in a closed flask are constant, but they are not equal. A student says the reaction has stopped. Explain why this is wrong.
Answer: both reactions are still happening at the same rate
Explain why unreacted nitrogen and hydrogen are recycled in the Haber process.
Answer: only some of the gases react each time, so recycling saves raw materials
Short questions with options. Choose one before you open the answer.
What does the symbol ⇌ mean in an equation?
The reaction is reversible and can go in both directions.
What is dynamic equilibrium?
The forward and backward reactions happen at the same rate, so the amounts stay constant.
What is a closed system?
One in which nothing can enter or leave.
If the forward reaction is exothermic, what is the backward reaction?
Endothermic, with the same size of energy change.
What does it mean if the position of equilibrium is to the right?
There is more product than reactant at equilibrium.
What does Le Chatelier's principle say?
If a change is made to a system at equilibrium, the system shifts to oppose the change.
What happens to the position if more reactant is added?
It shifts right, to use up the extra reactant.
What happens to the position if product is removed?
It shifts right, to make more product.
Which side does higher pressure favour?
The side with fewer gas molecules.
When does pressure not change the position of equilibrium?
When both sides have the same number of gas molecules, or no gases are involved.
Which direction does higher temperature favour?
The endothermic direction.
Which direction does lower temperature favour?
The exothermic direction.
What does a catalyst do to an equilibrium?
It speeds up both directions equally, so equilibrium is reached faster but the position is the same.
What do flat lines on a concentration-time graph show?
Equilibrium has been reached, so the concentrations are no longer changing.
What is the equation for the Haber process?
N₂ + 3H₂ ⇌ 2NH₃, which is exothermic in the forward direction.
Where do the raw materials for the Haber process come from?
Nitrogen from the air and hydrogen from natural gas.
What conditions does the Haber process use?
About 450 °C, 200 atmospheres and an iron catalyst.
Why is a temperature of 450 °C used in the Haber process?
It is a compromise: lower temperatures give more ammonia but are too slow.
Why is 200 atmospheres used and not a much higher pressure?
A higher pressure raises the yield, but the equipment costs more and is less safe.
What happens to the unreacted nitrogen and hydrogen in the Haber process?
They are recycled back into the reactor.
How is ammonia separated from the gases in the Haber process?
It is cooled until it liquefies and is run off.
What happens to the colour when N₂O₄ ⇌ 2NO₂ is heated?
It becomes browner, because the endothermic forward reaction makes more NO₂.
How many gas molecules are on each side of N₂ + 3H₂ ⇌ 2NH₃?
4 on the left and 2 on the right.
Why does a higher temperature reduce the yield in an exothermic equilibrium?
It favours the endothermic direction, which is the reverse reaction.
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