Chemical Equilibria

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Static equilibrium

is a state in which there is no change to both macroscopic and microscopic properties in a system or when a force acting on an object at rest are balanced

Dynamic equilibrium

refers to the state of a reversible rxn when both forward and backward rxns are taking place simultaneously at the same rate in a closed system at a constant temperature and there is no change in the conc of reactants and products

when a reversible rxn has reached dynamic equilibrium

ratef = rateb

conc of rxts and pdts do not change

Reaction quotient, Q

Q = [C]^c[D]^d/[A]^a[B]^b

C,D - products

A,B - reactants

a,b,c,d - stoichiometric coefficients

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at dynamic equil, Q is a constant = Kc

Kc = [C]eqm^c[D]eqm^d / [A]eqm^a[B]eqm^b

Kc = Kf/Kb

Qc < Kc (Kc is large) => forward rxn favoured

Qc = Kc => system is at equilibrium

Qc > Kc (Kc is small) => backward rxn favoured

Equilibrium constant for gases => Kp

Kp = (PC)^c(PD)^d / (PA)^a(PB)^b

P = partial pressrre

• PC = nc/nt x PT

• PT = PA + PB + PC + PD

• Homogeneous equilibrium - all substances involved are in the same phase(state)

• Heterogeneous equilibrium - substances involved are not in the same phase(state)

=> conc. of pure solid & pure liquid are constant -> omitted from Kc and Kp expression

To calculate Kc

write equation

ICE table

calculate amt of rxts and pdts

calculate concentration

write Kc expression and substitute

ICE table

• Initial amt

• Change in amt

• Equilibrium amt

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Degree of dissociation, a

is the fraction of reactants that has undergone dissociation

a = amount dissociated / total initial amt

• fraction or percentage

• increases with temperature

Kc/Kp is affected by changes in temperature

Kc/Kp not affected by:

• changes in conc of rxt/pdt or addition/removal of rxt/pdt

• changes in pressure (partial or total)

• presence of absence of catalyst

=> Kc/Kp is constant

Le Chatelier's Principle

states that if a system at dynamic equilibrium is subjected to a change which disturbs equilibrium, the system will respond in such way so as to counteract the change and re-establish the equilibrium

To explain equilibrium observation

• Write equilibrium eqn

• Write "According to Le Chatelier's Principle,"

• State

-the change

-how the position of equilibrium shifts left/right to counteract the effect of the change

-whether foward or backward rxn is favoured

• Write down the new observation

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Changing concentrations of species

N2 + 3H2 <> 2NH3

According to Le Chatelier's Principle, when more H2 is added to the system, the position of equilibrium shifts right to reduce the increased concentration of H2. Forward rxn is favoured. At the same time, N2 is consumed and more NH3 is formed.

Hence at the re-established eqm:

• [H2]new > [H2] old

• [N2]new < [N2]old

• [NH3]new > [NH3]old

Effect of pressure changes:

(a)Changes in partial pressure

• adding/removing gas

• similar to that of changing conc

PV=nRT

P = (n/V)RT

= [A]RT

(b)Changes in total pressure/volume of system

• change by compression (increase in total pressure by decreasing volume) or expansion (decrease in total pressure by increasing volume)

• Increase in pressure => favours rxn which produces fewer no. of moles of gas

• Decrease in pressure => favours rxn which produces more no. of moles of gas

2SO2 + O2 <> 3SO3

{When total pressure is increase (volume decrease)}

According to Le Chatelier's Principle, the equilibrium position shifts to the right to reduce the total pressure by reducing the total number of gaseous molecules present in the system.

•When there is no change in total no. of moles of gas in a rxt, then changes in total pressure has no effect on the position of equilibrium

• When an inert gas is added to eqm mixture at constant volume, the total pressure of the system is increased. The partial pressures of each reacting species and the total pressure of the reacting species remains the same => no shift in eqm

Effect of temperature changes on equilibrium position:

• Increase in temperature favours endothermic rxn

• Decrease in temperature favours exothermic rxn

Co(H2O)6^2+(aq) + 4Cl-(aq)

<> CoCl4^2-(aq) + 6H2O(l) ◇H>0

pale pink -> deep blue => violet solution

{When temperature is increased}

According to Le Chatelier's Principle, when the temperature is increased, the eqm position shifts right to absorb the additional heat energy, favouring the forward endothermic reaction. Hence, the resulting soln is deep blue.

The increase in temp causes the rates of both the forward and backward rxn to increase. Since the forward rxn is favoured, the ratef increases more than rateb. Hence, Kc increases.

Effect of Catalyst on Equilibrium position:

• Catalyst increases the rate of reaction by providing an alternative pathway with lower activation energy, Ea.

• lowers both forward Ea (Ef) and backward Ea (Eb) by the same extent

• As the rates of both forward & backward rxn increase, eqm is attained faster

• A catalyst has no effect on the composition of an eqm mixture. Hence, eqm position is not affected and Kc remains unchanged

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Haber Process: Manufacture of Ammonia

N2(g) + 3H2(g) <> 2NH3(g) ◇H=-92 kJmol-1

Temp: 450°C

Pressure: 200 atm

Catalyst: Finely divided iron

Temp: 450°C

By Le Chatelier's Principle, a low temp would favour the forward rxn, which is exothermic, to release more energy. Hence, obtain a higher yield of NH3. However, a low temp would slow down the rate of prod and cause equilibrium to be achieved slowly. Hence, a low temp is not economically effective. A high temp increases the rate of prod, but results in lower yield and high prod cost. Hence, a moderate temp of about 450 is used to speed up the rxn, although the % of NH3 produced at eqm is reduced.

Pressure: 200 atm

By Le Chatelier's Principle, a high pressure shift the equilibrium position to the right, where there is less gaseous species, to reduce the pressure. Hence, obtain a higher yield of NH3. However, high pressure would increase the cost and maintenance of equipments. Thus, an optimum pressure of 200 atm is used.

Catalyst: finely divided Fe with Al2O3 as promoter

Catalyst is added to increase the rate of rxn so that eqm is reached faster. Furthermore, a catalyst permits lower operating temperatures which is more economical. Finely divided iron(heterogeneous catalyst) is mixed with small amounts of promoters (such as K2O and Al2O3) to enhance the effectiveness of the Fe catalyst

Continual removal of NH3

The removal of ammonia shifts the position of eqm to the right thereby increasing the yield of ammonia. This is achieved by cooling the rxn mixture to -50°C to liquefy the ammonia formed (b.p. of NH3(l) is -33°C)

Molar ratio of N2: H2 of 1:3

The molar ratio used is similar to that of the stoichiometric ratio to minimise excess

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