3.1.6 Chemical equilibria, Le Chatelier’s principle and Kc

What do we need to reach dynamic equilibrium?

• reversible reaction

• closed system (nothing can enter or leave the reaction)

What does dynamic equilibrium mean?

Dynamic- forward and backward reactions proceed at equal rates

Equilibrium- concentration of reactants and products remains constant

What is Le Chatelier’s principle?

A system at equilibrium will respond to oppose any change imposed upon it

What factors affect equilibrium?

Temperature, pressure and concentration

Why does a catalyst not affect the position of equilibrium?

It increases the rate of forward and backward reactions equally

How does a catalyst affect the rate of a reaction?

It increases the rate of attainment of equilibrium

What are the compromises in industrial processes?

• if a forward reaction is exothermic, a low temperature would mean a greater yield of product. However a higher temperature is actually used to increase the rate of reaction

• if there are more moles on the left, a high pressure would give a higher yield. It also increases the rate of reaction. However a lower pressure is actually used as it is cheaper and safer

How does concentration affect equilibrium?

• if you increase the concentration of a substance, the equilibrium will shift to the opposite side to use it up and decrease the concentration

• if you decrease the concentration of a substance, the equilibrium will shift to the same side to make more of it and increase the concentration

How does pressure affect equilibrium?

• if pressure is increased, equilibrium will shift to the side with fewer moles to oppose the change and decrease the pressure

• if pressure is decreased, equilibrium will shift to the side with more moles to oppose the change and increase the pressure

How does temperature affect equilibrium?

If the forward reaction is exothermic (-ΔH)

• increase in temperature will shift equilibrium left to oppose the change and decrease the temperature

• decrease in temperature will shift equilibrium right to oppose the change and increase the temperature

And vice versa for reactions where the forward reaction is endothermic (+ΔH)

What is the expression for Kc?

Kc= [products]/ [reactants]

Power after brackets if there is more than one mole of the substance

What do the square brackets in Kc represent?

Concentration of a species in mol dm^-3

What is the value of the equilibrium constant affected and not affected by?

It is affected by temperature.

It is not affected by changes in concentration or addition of a catalyst