7.1.1

Outline the characteristics of chemical and physical systems in a state of equilibrium.

Definition of Equilibrium:
At equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction.

Physical Systems:
An example of this is bromine is placed in a sealed container at room temperature. A layer of bromine gas is formed above the liquid bromine. It occurs in both forms because, due to its boiling point being close to room temperature. Hence some particles have more kinetic energy in order to be in a gas form (evaporation), however when these gas particles collide with the surface of the liquid they lose energy and turn back into liquid (condensation).

Equilibrium is only reached when rate of evaporation is equal to the rate of condensation.

Chemical Systems:

In the reaction of dissociation between hydrogen iodide with H2 and I2, a purple iodine gas is released and the colour increases. However, after a short amount of time colour stops’ increasing indicating equilibrium is reached.

Heating CuSO4 Crystals:
When heating blue copper sulphate (CuSO4) crystals we are converting hydrated CuSO4 crystals to anhydrous CuSO4 crystals. The colour of the crystals changes from blue to white. They have dynamic equilibrium when hydrated crystals are left unconcealed as they lose water due to evaporation however the water vapour condenses back into the crystals; therefore it is a reversible reaction.

Hydrated Copper Sulphate Crystal (blue)



Heating Copper Sulphate Crystal



Anhydrous Copper Sulphate (white)