Equilibrium Le Chatelier’s Principle. Equilibrium Brown gasColourless gas

Equilibrium

Le Chatelier’s Principle

Equilibrium

Brown gasColourless gas

Equilibrium

Imagine we begin with a container that has only N2O4 gas. Some of these molecules will successfully collide and produce NO2.

Equilibrium

The colour of the gas mixture will get browner as less N2O4 and more NO2 will be present, until equilibrium is achieved.

Equilibrium

Equilibrium

At equilibrium there will be NO2 and N2O4 present. The colour of the mixture will be determined by the amount of NO2 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.


For this reaction the equilibrium expression is:


The square bracket represents concentration.

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

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


The value for this particular reaction, at 100oC is always 0.21.

Le Chatelier’s PrincipleIf a sy stem at eq u i l ibriu m has a chan g e im posed on i t, then the sy stem wi l l partial ly oppose the chan g e in su ch a way as to re-establ ish the eq u i l ibriu m .


If the equilibrium concentrations were

[NO2] = 0.205, [N2O4] = 0.2


and we added more N2O4 to bring it’s concentration up to 0.3 mol L-1. The system would adjust to get back to where K = 0.21


This would happen by trying to get the [N2O4] partially back to its original value. At the same time the [NO2] would increase.



The concentrations of each at the new equilibrium are:

[NO2] = 0.245 mol L-1

[N2O4] = 0.28 mol L-1


If at these new concentrations the volume of the container was halved:

What would be the new concentrations?


If at these new concentrations the volume of the container was halved:

What would happen to the pressure in the container?


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


This could be achieved by favouring the reaction which produces fewer gas molecules. ie reverse reaction


0oCroom temp


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

Le Chatelier’s PrincipleWhy?

What does Le Chatelier predict will happen when you lower the temperature of a system.

Le Chatelier’s PrincipleThe 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 PrincipleThe 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 PrincipleAt room temperature

At zero degrees

[N2O4] 0.049 0.098

[NO2] 0.102 0.005


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Equilibrium Le Chatelier’s Principle. Equilibrium Brown gasColourless gas