Chap 8C - Chemical equilibria

Describe (prediction using Le Chateller’s Principle) when temperature decrease for N2(g) + 3H2(g) ⇌ 2NH3(g) (Enthalpy < 0) 

• By Le Chatelier’s Principle, the system will counteract the decrease in temperature by favouring the forward exothermic reaction to release heat, so position of equilibrium will shift right

• In the new equilibrium mixture, the [N2] and [H2] would be lower than the previous one while [NH3] would be higher than the previous one

• Kc increase

Describe (Effect on rates of reactions) when temperature decrease for N2(g) + 3H2(g) ⇌ 2NH3(g) (Enthalpy < 0) + draw graph

• Since the rates of both forward and backward reactions decrease, equilibrium is reached after a longer time

Draw conc time graph when temperature decrease for N2(g) + 3H2(g) ⇌ 2NH3(g) (Enthalpy < 0)

Describe (prediction using Le Chateller’s Principle) when temperature increase for N2(g) + 3H2(g) ⇌ 2NH3(g) (Enthalpy < 0) 

• By Le Chatelier’s Principle, the system will counteract the increase in temperature by favouring the backward endothermic reaction to absorb the added heat, so position of equilibrium will shift left

• High [N2] and [H2] and lower [NH3] at new equilibrium 

• Kc decreases

Describe (Effect on rates of reactions) when temperature increase for N2(g) + 3H2(g) ⇌ 2NH3(g) (Enthalpy < 0) 

• Both forward and backward reactions will become faster as KE of reactants and products increase 

• Backward rate increases more than forward rate as backward endothermic reaction is favoured in order to remove heat

• More H2 and N2 formed and so the forward rate will start to increase while the backward rate will decrease until both rates are the equal

• Since the rates of both forward and backward reactions increase, equilibrium is reached faster

Draw conc time and rate time graphs when temperature increase for N2(g) + 3H2(g) ⇌ 2NH3(g) (Enthalpy < 0) 

Describe effect of catalyst on POE

• A catalyst lowers the activation energy of both forward and backward reactions by the same amount

• Hence, both forward and backward reaction rates increase by the same extent.

• The time taken to reach equilibrium is shortened.

• Position of equilibrium and value of K is not affected

Explain under what conditions can inert gases be added under + its effect

1. Constant volume

• The partial pressure (or concentration) of the reactants and products do not change though there is an increase in total pressure

• Reaction is not disturbed, and the equilibrium position is unchanged

2. Constant total pressure 

• The volume of the system must increase to keep total pressure constant when an inert gas is added -> partial pressure of the reactants and products will decrease

• pT = Preactant + Pproduct + Pinert gas

• By Le Chatelier’s Principle, the system will counteract by shifting the position of equilibrium to the side with more gas particles present (left) to raise the partial pressures in order to overcome the disturbance

NOTE: The effect of adding inert gas at constant pressure on the equilibrium position is the

same as increasing the volume of vessel

Describe 450 degree condition for Haber process (effect on ROR at lower temp)

• Since the forward reaction is exothermic, a low temperature would result in a higher yield of NH3

1. Rate of reaction is too slow at low temperature, and it takes a long time to establish equilibrium

• High temperature increases the rate of production but results in lower yield and higher production cost

• Compromise is needed and a moderately high temperature of 450°C is used to ensure a reasonable rate of production and yield

Describe 200atm of Haber process

• The forward reaction takes place with a reduction in the number of gaseous particles

• A high pressure will favour the desired reaction (increase yield)

• Too high a pressure increases the cost of production (more expensive and stronger equipment that could withstand the high pressure needs to be used)

Describe

Catalyst

Continuous removal of NH3

Molar ratio of N2 : H2 = 1 : 3

of haber process

Catalyst	Finely divided Fe catalyst with Al2O3 as promoter NOTE: Catalyst does not affect the percentage of NH3 in the equilibrium mixture
Continuous removal of NH3	Shifts the position of equilibrium right -> increasing the yield of NH3
Molar ratio of N2 : H2 = 1 : 3	The molar ratio used is similar to that of the stoichiometric ratio to minimise excess