3.1.6.1 Chemical equilibria and Le Chatelier’s principle

What reactions can achieve dynamic equilibrium?

All reversible reactions

Example of reversible reaction I must memorise

N2 + 3H2 ⇌ 2NH3

2 Features of dynamic equilibrium

Forward and backwards reactions are occurring at equal rates

The concentrations of reactants and products stay constant.

Le Chatelier’s principle

If an external condition is changed, the equilibrium will shift to oppose the change

Effect of temperature of equilibrium

If the temperature is increased/decreased

Equilibrium will shift to oppose this change

And move in the endothermic/exothermic, backward/forward direction to try and increase/decrease temperature.

The position of equilibrium will shift to the left/right.

Why are compromise temperatures used if a low temperature may increase the yield?

Low temperature gives low rates of reaction

So a compromise temperature is used that gives a reasonable yield and rate.

Effect of pressure on equilibrium

If pressure is increased/decreases

equilibrium will shift to oppose the change and move to the side with fewer moles of gas to try and reduce the pressure.

The position of equilibrium will shift towards left/right as there is () moles on left/right but only () moles on left/right

What happens to the position of equilibrium when pressure is changed and the number of moles of gas is the same of both sides of the equation?

No effect on the position of equilibrium

Effect of concentration on equilibrium

If concentration of a reactant/product is increased/decreased,

the equilibrium will shift to oppose this change

and move in the forward/backward reaction

to increase/decrease the concentration of reactant/product

Position of equilibrium will shift to left/right

Effect of catalysts on equilibrium

No effect on the position of equilibrium but will speed up the rate at which the equilibrium is achieved.

This is because it speeds up the rates of the forward and backward reactions by the same amount