Chap 8B - Chemical equilibria

Describe relationship of K and gibbs free

• A negative value for ∆G represents a driving force in the forward direction, and POE lies to the right

• A positive value represents a driving force in the reverse direction, and POE lies to the left

Define Le Chatelier’s Principle 

• (Def.): Le Chatelier’s Principle states that when a system at equilibrium is disturbed, the system will counteract by shifting its position of equilibrium in a direction that will reduce that disturbance in order to re-establish the equilibrium

Describe effect of conc on reaction mechanisms

• If a substance is added to a system at equilibrium, the system will adjust itself to remove the substance added

• If a substance is removed from a system at equilibrium, the system will adjust itself to replace the substance removed

Describe effect of conc (Prediction using Le Chateller’s Principle)

• The system will counteract the increase in the amount of H2 by favouring the forward reaction -> POE shift right.

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

Describe effect of conc (Effect on rates of reactions)

• When more H2 is added, system will adjust to reduce [H2]

• The rate of forward reaction will increase, leading to formation of more NH3

• The forward rate will then start to decrease while the backward rate starts to increase with

• time until both are equal again

• At this stage, the system re-established its equilibrium at t2

Describe conc time and rate time graph for N2(g) + 3H2(g) ⇌ 2NH3(g)

Explain why there is no change to the value of Kc when [H2] increase in N2(g) + 3H2(g) ⇌ 2NH3(g)

• When [H2] increase, Qc will drop causing Qc < Kc (system no longer at equilibrium) -> POE shifts right to lower [H2] 

• [N2] will start to decrease while [NH3] will start to increase -> Qc will start to increase until it is equal to Kc 

• After spiking, the forward rate will then drop as [H2] and [N2] start to fall and the backward rate will increase as [NH3] starts to increase, at new equilibrium, Qc = Kc

Explain effect on POE when more CaCO3 or CaO added in CaCO3(s) ⇌ CaO(s) + CO2(g)

• Adding more CaCO3 or CaO will not affect the position of equilibrium because the position of equilibrium is unaffected by pure solids or liquids since their concentrations are considered constant

Describe effect of pressure

• Only affect reactions with gases involved 

• Pressure of gas directly proportional to concentration 

• The effect of changing partial pressure of A on position of equilibrium is the same as changing [A] 

Explain how partial pressure can be changed

1. Adding or removing any of the gaseous components at constant temperature

• Removal of some NH3 will lower the partial pressure of NH3.

• By Le Chatelier’s Principle, the system will counteract the decrease in partial pressure of NH3 by favouring the forward reaction in order to increase the partial pressure of NH3.

• POE will shift right

2. Changing the volume of the vessel at constant temperature

• Compression increases partial pressure of all gasses 

• Effect on POE depends on stoichiometry of reaction 

Describe effect when volume of vessel is increased for PCl3(g) + Cl2(g) ⇌ PCl5(g) 

• increase and so both forward and backward rates will spike

• The system will counteract by favouring forward reaction to reduce the total amount of gas particles in the system -> POE shifts right 

• As more PCl5 forms, the backward rate will rise while the forward rate will fall until they are equal again

• At this stage (t2), equilibrium is re-established

Draw graph for effect when volume of vessel is decreased for PCl3(g) + Cl2(g) ⇌ PCl5(g) 

Draw graph for effect when volume of vessel is increased for PCl3(g) + Cl2(g) ⇌ PCl5(g) 

Describe effect on POE if pressure of gases change H2(g) + I2(g)  ⇌ 2HI(g)

• Since the number of gaseous particles on both sides of the system is equal, changes in partial pressure of gases favour neither the forward nor the backward reaction

• Position of equilibrium is unaffected, and the system remains at equilibrium

• No change in the composition of the equilibrium mixture