Explain Le Chatelier's Principle and the factors that affect the position of equilibrium

Le Chatelier's principle states that is there is a change in concentration, pressure or temperature, the equilibrium will move in order to counteract the change. 

Firstly temerature - in an reversible reaction, one reaction will be exothermic (i.e. gives out heat, to indicate bond making) and the other reaction will be endothermic (requires heat to be taken in, and is bond breaking). If there is for example, a rise in temperature, the equilibrium will shift in the endothermic direction in order to oppose the change in temperature. This is because if there is an increase in temperature, the system will be disturbed and so will produce a change in order for the equilbirum to return back to normal. So if there is an increase in temperature, the eqilibrium will shift in the endothermic direction so that more heat is taken in (and removed). Similarly, if there is a decrease in temperature, the equilibrium will shift in the exothermive direction to generate more heat. You can always tell which direction is exothermic as when a reversible reaction is written, the enthalpy change will be quoted by the side. If the enthalpy change is negative it means the forward reaction is exothermic, and so the backward reaction will be endothermic. If the enthalpy chnage is positive, the forward reaction will be endothermic, and backward reaction exothermic. 

If there is an increase in pressure the equilibrium will shift to the side with fewer moles. These can be indicated by the numbers in front of the chemical compounds. For example, in the reaction 2SO₂ + O₂ <--> 2SO₃ the left hand side has a total of 3 moles, and the right hand side a total of 2 moles. Therefore, in this reaction an increase in pressure will cause the equilibrium to shift to the right, as it is the side with fewer moles in order to oppose the change in concentration. This is because if you increase the pressure, it means you have more molecules in a smaller amount of space. By shifting the equilibrium to the side with fewer moles, it will result there being fewer molecules in the smaller space generated by the high pressure, thus acting to decrease the pressure and return the system back to normal. Similarly, a decrease in pressure will cause the equilibrium to shift to the side with more moles. 

Finally, a change in concentration of any of the reactants or products in an equilibrium mixture will cause the equilibrium to shift in the opposite direction in order to use up more of the extra component. For example, in the above example, if more oxygen is added to the mixture the equilibrium will shift to the right, in order to use up more of the oxygen, and oppose the change in concentration, and more of the SO₃ will be produced. Therefore, if there has been a decrease in a component, the equilibrium will also shift to oppose the change, and to produce more of that component.