F322 - Equilibrium

Le Chatelier’s Principle

When a system in equilibrium is subject to a change, the equilibrium will shift so as to counteract the change.

What is dynamic equilibrium?

When a reversible reaction has appeared to have stopped, it has reached an equilibrium. At equilibrium the rate of the forward and reverse reaction occurs at the same rate. This means that the concentrations of the reactants and products are the same. There is no observable change. The extent of how far the reaction has gone towards the products is called 'the position of the equilibrium'. Equilibrium is stable under fixed conditions and in a closed system.

The effect of temperature on the position of equilibrium

Increasing the temperature cause the position of equilibrium to move in the endothermic (ΔH = +ve) direction because this takes in the extra heat.

Decreasing the temperature causes the position of equilibrium to move in the exothermic (ΔH = -ve) direction because this release heat.

Pressure

• Increasing the total pressure of the system causes the position of equilibrium to move to the side with fewer gas molecules as this will decrease the pressure.

• Decreasing the total pressure of the system causes the position of the equilibrium to move to the side with more gas molecules as this will increase the pressure.

Concentration

• Increasing the concentration of a reactant cause the position of the equilibrium to move in the direction which decreases this change, so moves to the side so to produce more products.

• Increasing the concentration of a product cause the position of the equilibrium to move in the direction which decreases this change, so moves to the side so to produce more reactants.

Catalysts

A catalyst has no effect on the position of the equilibrium.

Catalysts speeds up the rate of a reaction so it will only increase the rate at which equilibrium is achieved. This is true for the forward and reverse reaction.

Concentration

• If the concentration of the reactants is increased then the rate of reaction also increases. This gives more molecules in the same volume. The molecules will be closer together and therefore there’s a greater chance of successful collisions. The collisions will be more frequent.

Pressure

• If the pressure of a gas is increased, the molecules will be closer together. The same number of molecules will occupy the same volume. More collisions will occur and there will be more collisions with energy above the activation energy. So there will be more frequent collisions and the rate of reaction increases.

Effect of Temperature

As the temperature of the reaction is increased, the rate of reaction increases. This is because the molecules have more kinetic energy, and a higher proportion of the molecules have energy greater than the activation energy (E_a ). Therefore there will be more successful collisions.

Effect of Catalyst

A catalyst lowers the activation energy required for a reaction to take place. More molecules will have the activation energy and will be able to react.

So there will be more frequent successful collisions and the rate of reaction will increase.

Compromise with Conditions

• The synthesis of ammonia is an exothermic reaction which needs to be performed at an industrial scale.

N_2(g)+ 3H_2(g) 2NH_3(g)        ∆H= -92kJmol^-1

Because it’s an exothermic reaction, lower temperatures favour the forward reaction, so more products are produced. But with lower temperatures this is a slower rate of reaction. Higher pressures also favour the forward reactions, however high pressures are expensive as they require special equipment to contain the reaction.

With temperature change

With a higher temperature

If you increase the temperature, then the molecules have more kinetic energy so there are more frequent collisions. This means that more molecules have the activation energy, so there are more successful collisions.