10.4 Dynamic Equilibrium And Le Chatliers Principle

Take place in forward and reverse directions

Rate of forward reactions is equal to the rate of backward reactions Concentrations of reactants and products do not change

Isolated from surroundings, temperature, pressure, concentration are unaffected by outside influences

When a system in equilibrium is subjected to an external change, system readjusts to minimise the effect of that change

System must be closed

More products are formed

More reactants are formed

Add solution of yellow potassium dichromate to a beaker Add dilute sulfuric acid dropwise until there is no further change - changes to orange colour Add aqueous sodium hydroxide until no further change, colour changes back to yellow

When adding sulfuric acid, concentration of H+ ions is increased, which increases rate of forward reactions Equilibrium shifts towards the right, so more products are formed and solution turns orange

As concentration of substance is increased, position of equilibrium moves away from that substance

Increase in temperature - equilibrium position shifts in endothermic direction Decrease in temperature - equilibrium position shifts in exothermic direction

Chromate - yellow Dichromate - orange

Dissolve cobalt chloride in water in a boiling tube - add a small quantity of HCl and place in iced water (pink solution) Transfer boiling tube into boiling water in water bath - solution turns blue colour When back in ice water, turns back into pink

When temperature is decreased, there are more backward reactions (exothermic), so the pink solution is formed When temperature is increased, there are more forward reactions (endothermic), so blue solution is formed

Increasing pressure means equilibrium position shifts towards the side with fewer mols

NO2 is a brown gas, N2O4 is colourless 2NO2 >< N2O4 Increase pressure, moves to side with fewer mols, so more N2O4 is formed, change from brown to colourless

Catalyst doesn’t change position of equilibrium Speeds up rate of both forward and backward reactions

N2 + 3H2 >< 2NH3 ΔH = -92KJ/mol Low temperature and high pressure will increase yield Low temperature, low reaction rate - so compromise temperature used High pressure - not safe, risk of explosion, requires strong container and a lot of energy, so compromise pressure is used

350-500oC temperature 100-200 atm pressure Iron catalyst