Chemistry unit 2 - Energy

A chemical system is in dynamic equilibrium when:

• The concentrations of the reactants and the products remain the same.
• The rate of the forward reaction is the same as the rate of the reverse reaction.

An equilibrium only applies as long as the system is isolated.

Factors affecting the position of equilibrium:

• Concentrations of the reactants or products
• Pressure in reactions involving gases
• Temperature

Increasing the concentration of a reactant

• The system opposes the change by decreasing the concentration of the reactant by removing it.
• The position of equilibrium moves to the right-hand side, forming more products.
• Minimising change.

 Increasing the concentration of a product.

• The system opposes the change by decreasing the concentration of products by removing it.
• The position of the equilibrium will move to the left-hand side forming more reactants.

Changing the total pressure of a system will only change the position of equilibrium if there are gases present.

N2 + 3H2          ><        2NH3

4 mol                         2mol 

Higher pressure        Lower pressure

The side with the greater amount of moles has the highest pressure.

Increasing the pressure  

Position of equilibrium moves to the side with the fewer gas molecule, as will decrease pressure.

Decreasing the pressure 

Position of equilibrium moves to the side with the greater gas molecules, as will increase the pressure.

[Cu(H2O6)]2+ + 4Cl >< [CuCl4]2+ + 6H2O

Pale blue colour            Yellow colour  

Increasing the concentration of chloride ions causes the position of equilibrium to move in the direction that decrease concentration. The system opposes the change by decreasing the concentration of chloride ions, by removing them. The position of equilibrium moves to the right-hand side forming more products. You would observe the pale blue colour replaced by yellow colour.