Chemical Equilibria

Dynamic Equilibrium

Reversible process in a closed system at equilibrium in which the rate of the forward rxn is equal to the rate of the backward rxn, and there is no net change in the concentration of reactants and products

Kc factor

only affected by temperature

Qc is less than Kc

initial [products] to [reactants'] is too small, more products must be formed. forward rxn is favoured and equilibrium position shifts to the right

Qc is more than Kc

initial [products] to [reactants] is too big, more reactants must be formed. backward rxn is favoured and POE shifts left

Le Chatelier’s Principle

if a closed system at dynamic equilibrium is subjected to a change that disturbs the equilibrium, the system responds in such a way to counteract some effects of the change

Effect of change of concentration

conc increases/decreases, the position of equilibrium shifts in the direction that consumes/replenishes some of it

Effect of change in pressure(only for gases)

when the volume decreases, POE will shift in the direction that reduces the pressure by decreasing the total number of gaseous particles in the system

Effect of temperature

When the temperature increases, POE will shift in the direction that favours the endothermic reaction to absorb some of the added heat

Less products formed

Kc/Kp decreases

More products formed

Kc/Kp increases

Effect of catalysts

increase the rates of both the forward and backward reactions by the same extent, thus does not affect the POE and Kc, but only increases the rate

Standard Gibbs free energy and equilibrium position

Gstandard=-RTlnK

Haber process eqn

N2(g)+3H2(g)←→2NH3 H=-92.4kJmol-1

Haber process (temperature)

foward rxn is exo, so decreasing temp will shift POE to the right and increase yield. but too low and the rxn would be too slow, thus optimum temp is 450

Haber process (pressure)

increasing pressure will shift POE to the right and yield increases, but cost is high so 250 atm

Haber process(catalyst)

To compensate for the slower reaction at lower temp, a finely divided iron catalyst is used