1. Chemical Equilibrium
Unit 2

2. One way reactions Reactants react forming products.
The reaction continues until one of the reactants is finished.
The products are not allowed to react to reform reactants.
The reaction stops proceeding any further.

3. Reversible Reactions
Some reactions proceed in both directions -reactants to products and products to reactants.
Example: CuSO4 .5H2O (s) CuSO4(s) H2O(g)
When heated the blue crystals change to white powders which change to blue when water is added.

4. A reaction in equilibrium
The reactants react forming products.
Products react forming reactants.
Finally the rates of the forward and reverse reactions are equal.
The reaction is proceeding in both directions, but at equal rates.
There is no overall change.
The reaction is now in a state of dynamic equilibrium.

5. Dynamic Equilibrium -characteristics
There is no overall change ie the concentrations of the reactants and products overall remain the same.
Rates of the forward and reverse reactions are equal.
The equilibrium can be achieved from either direction.
The equilibrium can be achieved in a closed system

6. How to describe a reaction in equilibrium
Use two concepts: The Equilibrium constant and the Position of the Equilibrium.

7. The Equilibrium Constant
This can be described in terms of the (I) concentrations of each substance in the reaction mixture - Kc, or (ii) The partial pressures of each in the reaction mixture - Kp as the partial pressure of a gas is proportional to its concentration.

8. The position of the equilibrium
This describes the relative proportions of the reactants and products at a given moment in time in the reaction mixture.
When the proportion of products is more than the reactants, the position is said to be to the right.
When the proportion of the reactants is more the position is to the left.

9. Factors affecting Kc or Kp
Only temperature affects Kc or Kp.
Concentration, pressure or the presence of a catalyst does not affect equilibrium constant.
Kc or Kp is constant at a given temperature.

10. Factors affecting the position
Temperature
Concentration
Pressure
Catalyst

11. Le Chatelier’s Principle
If a system in equilibrium is subjected to a change, the position of the equilibrium will shift to minimise the imposed change.

12. Effect of Temperature
Exothermic reaction - An increase in temperature will shift the position of the equilibrium to the left to absorb heat. The concentrations of products will decrease Kc will decrease.
Exothermic reaction - An increase in temperature will shift the position to the right to increase heat. Kc will increase.

13. Effect of concentration
An increase in concentration of one of the reactants will shift the position to the right to use up some of the reactant added. Kc will remain the same.
An increase in concentration of one of the products will shift the position to the left to use up some of product added. Kc will remain the same.

14. Effect of pressure
If there are more moles of reactants than products, an increase in pressure will shift the position to the right to reduce pressure. There will be no change in the value of Kc
If the moles of reactants and products are equal, pressure has no effect on the position of the equilibrium.

15. Effect of a Catalyst
A catalyst has no effect either on the position of the equilibrium or the value of Kc or Kp.
A catalyst will affect the forward and the reverse reaction equally. The equilibrium will be achieved quicker.

16. Writing an expression for Kc
For the reaction : aA + bB cC + dD Kc=[C]c [ D]d/[A]a[B]b
The unit for Kc depends on the expression for Kc for a specific reaction.
Some reactions may not have a unit for Kc.