1. Dynamic Equilibrium

When a chemical reaction takes place in a container which prevents the entry or escape of any of the substances involved in the reaction, the quantities of these components change as some are consumed and others are formed. Eventually this change will come to an end, after which the composition will remain unchanged as long as the system remains undisturbed. The system is then said to be in its equilibrium state, or more simply, "at equilibrium".

There are two forms of equilibrium: static and dynamic equilibrium.

In static equilibrium, the entire system is not moving.

Chemical equilibrium is a state of dynamic balance where the rate of the forward reaction is the same as the rate of the backward reaction. These are reversible reactions. The rates of the forward and the reverse reaction are equal, the conc of each of the species remains constant.

 Examples of reactions at equilibrium

         N2      +    3H2      2NH3 

          2NO2(g)                N2O4(g)

Characteristics of the equilibrium state:

It is dynamic: molecules of reactants are continually being converted to products and molecules of products are continually being converted to reactants.

- At eqm, the rate of the forward reaction equals the rate of the backward reaction.

- The conc of reactants and products at eqm do not change. They are constant. This is because the rates of the forward and backward reactions are the same.

- Eqm only occurs in a closed system. This is one in which none of the reactants or products escapes from the mixture.

The two diagrams below show how the concentrations of the three components of this chemical reaction change with time. Examine the two sets of plots carefully, noting which substances have zero initial concentrations, and are thus "products" of the reaction equations shown. Satisfy yourself that these two sets represent the same chemical reaction system, but with the reactions occurring in opposite directions. Most importantly, note how the final (equilibrium) concentrations of the components are the same in the two cases.

Whether we start with an equimolar mixture of H₂ and I₂ (left) or a pure sample of hydrogen iodide (shown on the right, using twice the initial concentration of HI to keep the number of atoms the same), the composition after equilibrium is attained (shaded regions on the right) will be the same.

2. Le Chatelier's Principle

Le Chatelier’s Principle: when any of the conditions affecting the position of a dynamic equilibrium are changed, then the position of that equilibrium will shift to minimize that change.

If the conc of the products is increased after the disturbance then the position of the eqm will shift to the left to counteract that change and likewise if the conc of reactants increase after the change then the eqm will shift to the right to counteract the change.

In a gaseous system as the pressure increases the equilibrium will shift to the side with fewer molecules.

Reversible reactions​

• A reversible reaction is one which can be made to go in either direction depending on the conditions.​

• If you pass steam over hot iron the steam reacts with the iron to produce a black, magnetic oxide of iron called triiron tetroxide, Fe3O4.​

• ​The hydrogen produced in the reaction is swept away by the stream of steam.​

• Under different conditions, the products of this reaction will also react together. Hydrogen passed over hot triiron tetroxide reduces it to iron. Steam is also produced.​

• ​

• This time the steam produced in the reaction is swept away by the stream of hydrogen.​

• These reactions are reversible, but under the conditions normally used, they become one-way reactions. The products are not left in contact with each other, so the reverse reaction can't happen.​

• A closed system is one in which no substances are either added to the system or lost from it. Energy can, however, be transferred in or out at will.​

• In the example we've been looking at, you would have to imagine iron being heated in steam in a closed container. Heat is being added to the system, but none of the substances in the reaction can escape. The system is closed.​

• ​As the triiron tetroxide and hydrogen start to be formed, they will also react again to give the original iron and steam. So, if you analysed the mixture after a while, what would you find?​

• You would find that you had established what is known as a dynamic equilibrium. To explain what that means, we are going to use a much simpler example​

​Dynamic equilibria​

Getting a visual feel for a dynamic equilibrium​

• Imagine a substance which can exist in two forms - a blue form or an orange form - and that each form can react to give the other one. We are going to let them react in a closed system. Neither form can escape.​

• Assume that the blue form turns into the orange one much faster than the other way round. In any given time, these are the chances of the two changes happening:​

• You can simulate this very easily with some coloured paper cut up into small pieces (a different colour on each side), and a dice.​

• The following are the real results of a "reaction” starting with 16 blue squares. Each was looked at in turn and decided whether it should change colour by throwing a dice.​

• A blue square was turned into an orange square (the bit of paper was turned over!) if I threw a 4, 5 or 6​

• An orange square was turned into a blue square only if I threw a 6 while I was looking at that particular square.​

• Once I had looked at all 16 squares, I started the process all over again - but obviously with a different starting pattern. The diagrams show the results of doing this 11 times (plus the original 16 blue squares).​

• You can see that the "reaction" is continuing all the time. The exact pattern of orange and blue is constantly changing. However, the overall numbers of orange and of blue squares remain remarkably constant - most commonly, 12 orange ones to 4 blue ones.​

• The reaction has reached equilibrium in the sense that there is no further change in the numbers of blue and orange squares. However, the reaction is still continuing. For every orange square that turns blue, somewhere in the mixture it is replaced by a blue square turning orange.​

• This is known as a dynamic equilibrium. The word dynamic shows that the reaction is still continuing.​

• You can show dynamic equilibrium in an equation for a reaction by the use of special arrows. In the present case, you would write it as:​

• ​

• ​

• It is important to realise that this doesn't just mean that the reaction is reversible. It means that you have a reversible reaction in a state of dynamic equilibrium.​