Equilibrium

Equilibrium

1. In a closed system the concentrations of all the reactants and products will eventually become constant

   • Such a system is said to be in a state of dynamic equilibrium

2. The forward and reverse reactions continue to occur, but at equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction.

3. Dynamic equilibrium also occurs when physical changes take place

   • In a closed flask, containing some water, equilibrium will be reached between the liquid water and the water vapour. The faster moving molecules in the liquid will escape from the surface to become vapour and the slower moving molecules in the vapour will condense back into liquid
   • Equilibrium will be established when the rate of vaporization equals the rate of condensation

4. A closed system is one in which neither matter nor energy can be lost or gained from the system, that is, the macroscopic properties remain constant.

5. Le Chatelier's principle

   • Provided the temperature remains constant the value for ic must remain constant.
   • If a system at equilibrium is subjected to a small change the equilibrium tends to shift so as to minimize the effect of the change.

6. Change in concentration

   • eg : C2H5OH(I) + CH3COOH(I) <----> CH3COOC2H5(I) + H20(l) If more ethanoic acid is added the concentration of ethanoic acid increases so that at the point of addition:
   • To restore the system so that the equilibrium law is obeyed the equilibrium will move to the right, so that the concentration of ester and water increases and the concentration of the acid and alcohol decreases.

7. Change In pressure

   • If there is an overall volume change in a gaseous reaction then increasing the pressure will move the equilibrium towards the side with less volume
   • This shift reduces the total number of molecules in the equilibrium system and so tends to minimize the pressure.

8. Change in temperature

   • In exothermic reactions heat is also a product. Taking the heat away will move the equilibrium towards the right, so more products are formed.
   • The forward reaction in exothermic reactions is therefore increased by lowering the temperature
   • Unlike changing the concentration or pressure, a change in temperature will also change the value of Kc.
   • . For an exothermic reaction the concentration of the products in the equilibrium mixture decreases as the temperature increases, so the value of Kc will decrease.

9. Adding a catalyst

   • A catalyst will increase the rate at which equilibrium is reached, as it will speed up both the forward and reverse reactions equally,

10. Units of the equlibrium constant

    • The units of K depend on the powers of the concentrations in the equilibrium expression.