Equilibrium and reversible reactions

In the last unit we looked at the energy changes involved in chemical reactions and talked about the energy barrier which particles must overcome before they can react

In principle there is no reason why a reaction cannot overcome the energy barrier in the reverse direction and indeed this happens in many cases. For most reactions, however, the forward reaction happens nearly completely and so we do not consider the reverse process. But in some reactions the reverse reaction occurs at a reasonable rate and must be taken into consideration.

Consider the reactions:

A		B
B		A

At the beginning of the reaction there is only A present and therefore the rate of the reaction is at its highest level. There is no B present and so the reverse reaction cannot occur.

· During the course of the reaction A gets slowly used up and B is formed. The rate of the forward reaction (A to B) decreases as the concentration of A gets smaller, and the rate of the reverse reaction (B to A) increases as the concentration of B increases.

· A situation will be reached when the rate of the forward reaction equals the rate of the reverse reaction.

· When this point is attained the concentration of the reactants and the products (A and B) cannot change any more (they are being used up at the same rate as they are being formed). We call this point the equilibrium situation. Remember the reaction has not stopped it's just that its going both forwards and backwards at the same rate. For that reason we say that it is dynamic.

To give an analogy to the situation, its like a house party.

Initial situation:
At the beginning of the party the house is empty and the guests are all in the garden with the barbecue. As they satisfy their hunger they start to drift inside and the house starts to fill up.

Some of the guests inside dance and get hungry and decide to go into the garden to eat. People are still going into the house from the garden but as there are now fewer people in the garden this happens at a slower rate.

As more people fill up the house, so the number getting hungry inside the house increases until eventually the number going into the garden equals the number going into the house.

At this point the total number of people in the house doesn't change but guests are still entering and leaving so the actual people in the house and garden at any one time are different.

Situation at equilibrium
Its a dynamic situation (constantly changing) and yet the population of the house and the garden remains at the same number. This is dynamic equilibrium.

Changing the conditions:

It suddenly gets very cold in the garden and so the number entering the house increases and the amount of people in the garden goes down. A new point will be reached at which only very hungry people will dare to venture outside and so the people ratio house:garden will settle down so as to reflect the new conditions.

Summarising:

The fact that the numbers of people inside and out does not change at equilibrium does not mean that they are the same people - different ones are entering and leaving continuously (dynamically).

In a chemical equilibrium the amounts of reactants and products remain the same although there is constant movement in both directions. Change the conditions and you change the relative amounts of reactants and products at equilibrium.

It may take time for the system to respond but it will eventually arrive at the new equilibrium situation.

Addition of more reactants or products to the equilibrium

If we artificially add products to the equilibrium mixture then this will be seen by the equilibrium as an unbalanced situation and reaction will take place in the reverse direction so as to re-establish the original equilibrium proportions.
If we artificially add more reactants (left hand side) to the equilibrium mixture then this will be seen by the equilibrium as an unbalanced situation and reaction will take place in the forward direction so as to re-establish the original equilibrium proportions.

This also changes the proportions of reactants and products at equilibrium.

Heat up the equilibrium (this makes more heat energy available)

The proportions of the reactants and products will change in an attempt to absorb the extra heat energy available. In other words the reaction will move towards the direction of endothermic change (heat energy is hence turned into chemical energy and the extra heat is absorbed by the system).

Reducing the temperature will have the opposite effect - the equilibrium moves towards the direction of exothermic change and releases heat energy to restore the original temperature.