4.13Reversible reactions
In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions, and are represented with a double arrow rather than a single one:
A + B ⇌ C + D
The direction of reversible reactions can be changed by changing the conditions.
The standard example is hydrated copper sulfate. Heating the blue crystals drives off the water and leaves a white powder; adding water to the white powder turns it blue again and releases heat:
hydrated copper sulfate ⇌ anhydrous copper sulfate + water (blue) ⇌ (white)
Common mistake
A reversible reaction is not the same as a reaction that can be undone by a separate process. Both directions here are the same reaction run each way, which is why one equation with a double arrow describes them both.
4.13Energy changes and reversible reactions
If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The same amount of energy is transferred in each case.
For hydrated copper sulfate, the forward reaction — driving the water off by heating — is endothermic, and the reverse reaction — adding water to the anhydrous powder — is exothermic by exactly the same amount. Nothing else is possible if energy is conserved: whatever energy had to be put in to break the substance apart comes back out when it re-forms.
Worked example
The forward reaction of a reversible reaction takes in 92 kJ/mol. State the energy change for the reverse reaction, with reasoning.
- Taking energy in means the forward reaction is endothermic.
- If a reversible reaction is exothermic in one direction, it is endothermic in the other, so the reverse reaction is exothermic.
- The same amount of energy is transferred in each case.
- Exothermic changes are written with a negative sign.
Answer: The reverse reaction is exothermic and releases 92 kJ/mol, written as −92 kJ/mol.
4.14Equilibrium
When a reversible reaction occurs in apparatus which prevents the escape of reactants and products, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.
Two parts of that definition are easy to lose. The system must be closed — nothing can escape — or the reverse reaction has nothing to work with. And equilibrium is about equal rates, not equal amounts: at equilibrium the concentrations stop changing, but there is no reason for there to be as much product as reactant.
This is why equilibrium is described as dynamic. Both reactions are still happening, at the same rate in each direction, so nothing appears to change even though the reactions have not stopped.
- Closed system
- Apparatus which prevents the escape of reactants and products.
- Equilibrium
- The state in which the forward and reverse reactions occur at exactly the same rate.
- Position of equilibrium
- How far the reaction lies towards the products or the reactants when equilibrium is reached.
Common mistake
At equilibrium the reactions have not stopped, and the amounts of reactant and product are not necessarily equal. Say that the rates are equal and the concentrations no longer change.
4.15–4.16The Haber process
The Haber process makes ammonia, which is the starting point for fertilisers, and it is the reversible reaction you are most likely to be asked about.
nitrogen + hydrogen ⇌ ammonia
N₂ + 3H₂ ⇌ 2NH₃
The nitrogen is extracted from the air, which is about 78% nitrogen. The hydrogen comes from natural gas, or from other sources such as steam reforming. Both are purified before they enter the reactor, because impurities poison the catalyst.
Three conditions have to be recalled exactly:
• a temperature of about 450 °C • a pressure of about 200 atmospheres • an iron catalyst.
The reaction never goes to completion, because it is reversible and reaches an equilibrium. The mixture leaving the reactor is cooled so that the ammonia condenses to a liquid and can be run off, while the unreacted nitrogen and hydrogen are gases and are recycled back into the reactor. Nothing is wasted, which is a large part of why the process is economic.
Common mistake
The catalyst does not increase the yield of ammonia. It gets the mixture to equilibrium faster, and that is all — the position of the equilibrium is unchanged.
4.17The effect of changing conditions on equilibriumHigher tier
The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.
If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change. The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier's Principle.
You should be able to make qualitative predictions about the effect of changes on systems at equilibrium when given appropriate information. "Counteract the change" is the phrase to reason from every time: whatever you do to the system, it shifts in the direction that partly undoes what you did.
4.17The effect of changing concentrationHigher tier
If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium, and the concentrations of all the substances will change until equilibrium is reached again.
• If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again. • If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.
You should be able to interpret appropriate given data to predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium.
Worked example
For A + B ⇌ C + D at equilibrium, predict what happens if more B is added, and if D is removed as it forms.
- Adding more B increases the concentration of a reactant.
- The system counteracts the increase by using B up, so the equilibrium shifts to the right and more C and D form.
- Removing D decreases the concentration of a product.
- The system counteracts the decrease by making more D, so the equilibrium again shifts to the right.
Answer: Both changes shift the equilibrium to the right, giving more product — which is why industrial processes both feed in extra reactant and remove the product as it forms.
4.17The effect of temperature changes on equilibriumHigher tier
If the temperature of a system at equilibrium is increased:
• the relative amount of products at equilibrium increases for an endothermic reaction • the relative amount of products at equilibrium decreases for an exothermic reaction.
If the temperature of a system at equilibrium is decreased:
• the relative amount of products at equilibrium decreases for an endothermic reaction • the relative amount of products at equilibrium increases for an exothermic reaction.
All four follow from counteracting the change. Raising the temperature is countered by the direction that takes energy in — the endothermic direction. Lowering it is countered by the direction that gives energy out.
You should be able to interpret appropriate given data to predict the effect of a change in temperature on given reactions at equilibrium.
Worked example
The forward reaction of a reversible reaction is exothermic. The temperature is increased. Predict the effect on the yield of product, and explain.
- Identify which direction is endothermic: the forward reaction is exothermic, so the reverse reaction is endothermic.
- Increasing the temperature is counteracted by the endothermic direction, which takes energy in.
- So the equilibrium shifts backwards, towards the reactants.
- That means less product at equilibrium.
Answer: The yield of product decreases, because the equilibrium shifts in the endothermic direction — here the reverse reaction — to counteract the rise in temperature.
Common mistake
Raising the temperature always makes both reactions faster, but that is about rate, not yield. The question of which way the equilibrium shifts is decided only by which direction is endothermic.
4.17The effect of pressure changes on equilibriumHigher tier
For gaseous reactions at equilibrium:
• an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules, as shown by the symbol equation for that reaction • a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules, as shown by the symbol equation for that reaction.
Again this is counteracting the change: fewer gas molecules exert less pressure, so shifting that way partly relieves an increase.
You should be able to interpret appropriate given data to predict the effect of pressure changes on given reactions at equilibrium.
Worked example
N₂(g) + 3H₂(g) ⇌ 2NH₃(g). Predict the effect of increasing the pressure on the yield of ammonia.
- Count the gas molecules on each side using the balanced equation: 1 + 3 = 4 on the left, 2 on the right.
- The right-hand side has the smaller number of molecules.
- An increase in pressure shifts the equilibrium towards the side with fewer molecules.
- That side is the products.
Answer: The equilibrium shifts to the right, so the yield of ammonia increases.
Common mistake
Count molecules of gas only, and take the numbers from the balanced equation, not from the formulae. If both sides have the same number of gas molecules, changing the pressure has no effect on the position of equilibrium at all.
Before you practise
- A reversible reaction is shown by ⇌ and can go both ways depending on the conditions.
- If the forward reaction is exothermic, the reverse is endothermic by exactly the same amount.
- At dynamic equilibrium both reactions are still happening, at the same rate, so the amounts stop changing — it needs a closed system.
- The Haber process: N₂ + 3H₂ ⇌ 2NH₃, at about 450 °C, 200 atmospheres, over an iron catalyst.
- Raise the temperature and the endothermic direction is favoured; lower it and the exothermic direction is.
- Raise the pressure and the side with fewer gas molecules is favoured.
- Increase the concentration of a reactant and the equilibrium shifts to use it up.
- A catalyst changes nothing about the position of an equilibrium — it only gets there faster.