CH0002 - Lecture 6

Where there is no tendency for a further change to occur.

• A reversible reaction is one which can be made to go in either direction depending on the conditions.
• The happen in Closed Systems!

A dynamic equilibrium is a chemical equilibrium between a forward reaction and the reverse reaction where the rate of the reactions are equal.

• Br_{2 (l)}  <----> Br_{2 (g)} 
• Solubilisation
• PbCl_{2 (s)}  <---->  PbCl_{2 (aq)} 

The Macroscopic properties are constant (for given T, P and initial concentrations)

The Processes on a molecular scale are in balance, therefore no net change occurs

The equilibrium can be obtained from either side of the equation.

The equilibrium only exists in a closed system.

A reaction has a fixed equilibrium constant which describes the extent to which the reaction proceeds.

If you allow the reaction to reach equilibrium and then measure the equilibrium concentrations of everything, you can combine these concentrations into an expression known as an equilibrium constant.

• [A] = concentration of A in mol dm^-3 
• ^a  = Mole numbers in front of each substance in the chemical equation.

If a system at equilibrium is subjected to a change, processes occur which tend to counteract the change.

• Changes in temperature and pressure.
• Changes in concentration

According to Le Chatelier, the position of equilibrium will move in such a way as to counteract the change. That means that the position of equilibrium will move so that the pressure is reduced again.

Pressure is caused by gas molecules hitting the sides of their container. The more molecules you have in the container, the higher the pressure will be. The system can reduce the pressure by reacting in such a way as to produce fewer molecules.

In this case, there are 3 molecules on the left-hand side of the equation, but only 2 on the right. By forming more C and D, the system causes the pressure to reduce.

The equilibrium will move in such a way that the pressure increases again.

In this case, increasing the pressure has no effect whatsoever on the position of the equilibrium.

Because you have the same numbers of molecules on both sides, the equilibrium can't move in any way that will reduce the pressure again.

Increasing the temperature of a system in dynamic equilibrium favours the endothermic reaction. The system counteracts the change you have made by absorbing the extra heat.

Decreasing the temperature of a system in dynamic equilibrium favours the exothermic reaction. The system counteracts the change you have made by producing more heat.

The solubility product is the equilibrium constant for a solubility equilibrium.

It is written as K_sp 

No terms are included for the solid, since molar concentration of a pure solid is constant independent of the amount present.