Equilibrium Le Chatelier’s Principle

Equilibrium Brown gasColourless gas

Equilibrium Imagine we begin with a container that has only N 2 O 4 gas. Some of these molecules will successfully collide and produce NO 2.

Equilibrium The colour of the gas mixture will get browner as less N 2 O 4 and more NO 2 will be present, until equilibrium is achieved.

Equilibrium

At equilibrium there will be NO 2 and N 2 O 4 present. The colour of the mixture will be determined by the amount of NO 2 present.

Equilibrium At equilibrium the macroscopic properties, such as pressure and colour will be constant.

Equilibrium At equilibrium the forward and reverse reactions will both be occurring.

Equilibrium

Equilibrium expression An equilibrium expression gives an indication of the relative amounts of products to reactants at equilibrium.

Equilibrium expression For this reaction the equilibrium expression is:

Equilibrium expression The square bracket represents concentration.

Equilibrium expression Solids and liquids do not have a concentration and therefore would not be included in the expression.

Equilibrium expression Any co-efficient in the balanced equation appears as an indice in the equilibrium expression.

Equilibrium expression The value for this particular reaction, at 100 o C is always 0.21.

Le Chatelier’s Principle

If the equilibrium concentrations were [NO 2 ] = 0.205, [N 2 O 4 ] = 0.2

Le Chatelier’s Principle and we added more N 2 O 4 to bring it’s concentration up to 0.3 mol L -1. The system would adjust to get back to where K = 0.21

Le Chatelier’s Principle This would happen by trying to get the [N 2 O 4 ] partially back to its original value. At the same time the [NO 2 ] would increase.

Le Chatelier’s Principle

The concentrations of each at the new equilibrium are: [NO 2 ] = mol L -1 [N 2 O 4 ] = 0.28 mol L -1

Le Chatelier’s Principle If at these new concentrations the volume of the container was halved: What would be the new concentrations?

Le Chatelier’s Principle If at these new concentrations the volume of the container was halved: What would happen to the pressure in the container?

Le Chatelier’s Principle When pressure increases Le Chatelier’s principle says that the system will try to partially restore the original pressure.

Le Chatelier’s Principle This could be achieved by favouring the reaction which produces fewer gas molecules. ie reverse reaction

Le Chatelier’s Principle

Lowering the temperature favours the reverse reaction, since the mixture gets lighter in colour.

Le Chatelier’s Principle Why? What does Le Chatelier predict will happen when you lower the temperature of a system.

Le Chatelier’s Principle The principle predicts that the reaction will try and increase the temperature. It can do this by producing heat. ie an exothermic reaction would be favoured.

Le Chatelier’s Principle The reaction will attempt to re- establish its original equilibrium but will not be able to. The equilibrium constant for a reaction is only constant at one temperature.

Le Chatelier’s Principle At room temperature At zero degrees [N 2 O 4 ] [NO 2 ]

Le Chatelier’s Principle