Equilibium

CHEMICAL EQUILIBRIUM

Many reactions are not reversible and just occur in one direction. For example it would be rather difficult to get the combustion of petrol to go in the opposite direction! However, some reactions and physical processes are reversible i.e. can occur in the forward or reverse direction. When the products are never fully formed and the reactants never used up a state of equilibrium is reached. At this point the rate of the forward and back reactions is the same. For example if bromine is put in a gas jar, initially the liquid evaporates at a faster rate than the gas condenses and the gas gets observably thicker. However after a while the gas density and volume of liquid remain constant. This does not mean that nothing is happening but there is no observable change as the system has reached equilibrium. This means that the forward and back directions are occurring at the same rate. The system is said to be in a state of dynamic equilibrium and for this to occur there must be:

1. A closed system

2 .No observable change

3. Both directions occurring at an equal rate

4. The equilibrium can be achieved starting from either direction

This is rather like walking up the escalators at the same rate as they are travelling down, you are both moving but you remain in the same position. Of course this can occur at different points on the escalators, the same is true in chemistry i.e. you can have different positions of equilibrium. For example if some bromine is removed from a gas jar by connecting another vessel,  then the bromine will evaporate at a faster rate and a new position of equilibrium will be established. Every equilibrium system has an equilibrium constant Kc which is simply the concentration of the products divided by the concentration of the reactants raised to the power of their stoichiometric coefficients. Kc has the advantage over the position of equilibrium as it is only affected by temperature i.e. nothing else will affect the value of Kc.  If more of one of the reactants is added then the position of equilibrium will move towards the right i.e. the products and equilibrium will again be established. When the new concentrations of reactants and products are fed into the  equilibrium constant expression the value of Kc will be the same. If more product is added then the position of equilibrium will move to the left and equilibrium will again be established with the Kc value remaining constant. Only temperature affects Kc values and so they are useful to compare equilibrium systems. A large Kc value means that that the equilibrium is towards the products and a small one indicates that the equilibrium is towards the reactants.