Reversible reactions and dynamic equilibrium

Chemistry · The rate and extent of chemical change. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

In a reversible reaction the products can react to give back the reactants. It is shown with the sign ⇌. If the reaction is exothermic in one direction, it is endothermic in the other direction, and the same amount of energy is transferred each way.

In a closed system nothing can get in or out. The forward and reverse reactions then reach the same rate. This is dynamic equilibrium: both reactions are still going on, but the amounts of each substance stay the same.

Le Chatelier's principle says that if you change the conditions, the equilibrium moves in the direction that reduces the change.

Rules to remember

At equilibrium: forward rate = reverse rate, and the amounts stay constant (they are not necessarily equal)
Add more of a reactant: the position moves towards the products
Remove a product: the position moves towards the products
Higher temperature favours the endothermic direction; lower temperature favours the exothermic direction
Higher pressure favours the side with fewer gas molecules; lower pressure favours the side with more
Equal numbers of gas molecules on both sides: pressure has no effect on the position
Hydrated copper(II) sulfate (blue) ⇌ anhydrous copper(II) sulfate (white) + water

Common mistake

Students say the concentrations are equal at equilibrium, or that the reactions have stopped. The rates are equal, the amounts are constant, and both reactions are still happening.

Worked example

CO(g) + 2H₂(g) ⇌ CH₃OH(g). The forward reaction is exothermic. Which conditions of temperature and pressure give the highest equilibrium yield of methanol?

The forward reaction is exothermic, so a low temperature favours it.
Count gas molecules: 1 + 2 = 3 on the left and 1 on the right.
A high pressure favours the side with fewer gas molecules, which is the right-hand side.
So both a low temperature and a high pressure move the position to the right.

Answer: A low temperature and a high pressure.

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