07 Chemical Equilibria

Question 1

Reversible Reactions

Definition

Answer 1

Reactions that proceed in both the forward and backward directions.

Question 2

Dynamic Equilibrium, DE

Definition

Answer 2

A state in a reversible reaction within a closed system in which the forward and backward reactions are continuing at the same rate, resulting in no net change in the macroscopic properties of the reactants and products.

Question 3

Reaction Quotient, Q_c

Definition

Answer 3

The ratio of the concentrations of reactants and products raised to their stoichiometric ratios.

Question 4

Reaction Quotient, Q_c

Formula

Given aA + bB –> cC + dD

Answer 4

Q_c = [C]^c [D]^d/ [A]^a [B]^b = concentration of products/ concentration of reactants

[A/ B/ C/ D] = concentration in mol dm^-3 at any given time

Question 5

Equilibrium Constant, K_c

Given aA + bB –> cC + dD aka must be accompanied by an equation as stoichiometric ratios are needed in K_c.
K_c = Q_c when DE is attained at a given temperature.
K_c is only affected by temperature based on Arrhenius equation.

Answer 5

K_c = [C]^c_eqm [D]^d_eqm/ [A]^a_eqm [B]^b_eqm

For K_p, use partial pressure instead of concentration.
K_c & K_p acceptable for gaseous systems.

K_c measures the extent of a reaction.

Question 6

Position of Equilibrium, POE

Definition

Answer 6

The relative composition of the products and reactants present in a reaction mixture at equilibrium.

Question 7

Equilibrium Constant, K_c

Significance

Answer 7

1. Small K_c
   a. Little pdts yielded aka rxts favoured over pdts at equilibrium
   b. POE lies on the left
   c. Forward rxn does not proceed to any appreciate extent
2. Big K_c
   a. Little rxts remain aka pdts favoured over rxts at equilibrium
   b. POE lies on the right
   c. Forward rxn goes into completion ≠ completed (irreversible rxn)
3. Intermediate K_c
   Significant amounts of both pdts and rxts present at equilibrium

Magnitude of K_c does not give any information on the rates of forward and backward rxn.

Question 8

Homogenous Equilibrium

Definition

Answer 8

An equilibrium system in which all the substance involved are in the same phase.

Question 9

Heterogenous Equilibrium

Definition

Answer 9

An equilibrium system in which all the substance involved are not in the same phase.

Question 10

Expression of K_c/ K_p in Heterogenous Equilibrium

Answer 10

Exclude:
1. Concentration of pure solids & liquids as it remains constant at the given temperature
2. Concentration of water when present in large amounts such as as a solvent –> [H₂O] is approximately constant

Rewrite K_c/ K_p as K’_c/ K’_p

Question 11

Calculation of K_c/ K_p

Answer 11

1. Given concentrations,
   a. Construct I.C.E table where I = Initial Concentration, C = Change in Concentration, E = Equilibrium Concentration
2. Given K_c,
   a. Let volume of the mixture be V
   b. Construct I.C.E table
3. Given partial pressures,
   a. Construct I.C.E table
4. Given K_p,
   a. Let partial pressure of unknown variable be x
   b. Solve for x

When equilibrium quantities all species is given in the question, construction of I.C.E table not needed.

Question 12

Degree of Dissociation, α

Definition

Answer 12

The fraction of a reactant that has dissociated at a particular temperature

can be expressed as a fraction or percentage.

Question 13

Degree of Dissociation, α

Formula

Answer 13

α = amount dissociated/ total initial amount

Question 14

Relationship between Gibbs Free Energy and Position of Equilibrium

Formula: ΔG = -RT ln K not within syllabus

Answer 14

1. ΔG = 0
   a. System is at equilibrium
2. ΔG < 0
   a. K > 1
   b. POE lies on the right
   c. Pdts favoured over rxts aka forward reaction is favoured over backward reaction
   d. Reaction goes to completion
3. ΔG > 0
   a. K < 1
   b. POE lies on the left
   c. Rxts favoured over pdts aka backward reaction is favoured over forward reaction
   d. No reaction seen.

Question 15

Le Chaterlier’s Principle, LCP

Definition

Answer 15

When a system at equilibrium is disturbed by subjecting it to a change, the system will react to counteract the change imposed so as to re-establish equilibrium aka in a manner to re-establish the equilibrium.

Changes include:
1. Concentration
2. Pressure/ Volume
3. Temperature
4. Addition of catalysts

Question 16

Determining the reaction of the system upon disturbance

Answer 16

1. LCP (Preferred)
   a. State change
   b. Favoured reaction
   c. Reason
   d. Shift of POE
2. Analysis of forward and backward rxn rates using CT
3. Comparing Q_c to K_c
   a. Q_c = K_c
   System is at equilibrium
   b. Q_c > K_c
   Backward reaction favoured
   c. Q_c < K_c
   Forward reaction favoured

Question 17

Changes in concentration

Answer 17

1. Addition of rxt
   a. POE shifts right
   b. Forward rxn favoured
   c. Other rxt is depleted and pdts increase
   d. New equilibrium [X] is higher than before
2. Removal of rxt
   a. POE shifts left
   b. Backward rxn favoured
   c. Other rxt increases and pdts deplete
   d. New equilibrium [X] is lower than before
3. Addition of pdt
   a. POE shifts left
   b. Backward rxn favoured
4. Removal of pdt
   a. POE shifts right
   b. Forward rxn favoured

Question 18

Changes in pressure

Partial Pressure, P_X

Only affects systems with gases involved

Answer 18

1. Increased P_{rxt X}
   POE shifts right
2. Decreased P_{rxt X}
   POE shifts left
3. Increased P_{pdt X}
   POE shifts left
4. Decreased P_{pdt X}
   POE shifts right

Increased/ Decreased P_X –> adding/ removing X

P_X ∝ [X]

Question 19

Changes in pressure

Total Pressure of a System, P_T

Only affects systems with gases involved

Answer 19

1. Different number of gaseous rxts and pdts
   a. Increased P_T
   i. Reaction that decreases pressure is favoured i.e decreases number of gaseous particles
   b. Decreased P_T
   i. Reaction that increases pressure is favoured i.e increases numbers of gaseous particles
2. Equal number of gaseous rxt and pdts
   POE not affected

Increased/ Decreased –> compression/ expansion

Question 20

Changes in pressure

Total Pressure of a System, P_T

Only affects systems with gases involved

Answer 20

Addition of inert gases
1. At constant volume
a. P_T increased
b. P_X remains constant
c. Q_c = K_c
d. POE not affected
2. At constant pressure
a. Total volume of system increased
b. P_X decreased
c. POE shifts to the side that increases the number of gaseous particles

For systems with equal number of gaseous particles, addition of inert gas has no effect on POE or equilibrium composition.

Question 21

Changes in volume

Answer 21

1. Increase in volume
   a. P_T decreases i.e concentration decreases
   b. Rxn that increases number of gaseous particles is favoured
   i. Rate of both forward and backward rxn decreases
   ii. Rxn takes a longer time to reach equilibrium
2. Decrease in volume
   a. P_T increases i.e. concentration increases
   b. Rxn that decreases number of gaseous particles is favoured
   i. Rate of both forward and backward rxn increases
   ii. Rxn takes a shorter time to reach equilibrium

Question 22

Changes in temperature

Answer 22

1. Endothermic Forward Rxn
   a. Temperature increased
   i. Forward endothermic rxn favoured
   ii. POE shifts right
   iii. K_c increases
   b. Temperature decreased
   i. Backward exothermic rxn favoured
   ii. POE shifts left
   iii. K_c decreases
2. Exothermic Forward Rxn
   a. Temperature increased
   i. Backward endothermic rxn favoured
   ii. POE shifts left
   iii. K_c decreases
   b. Temperature decreased
   i. Forward exothermic rxn favoured
   ii. POE shifts right
   iii. K_c increases

For increase in temperature, rate of both forward and backward rxn increases, time taken to reach equilibrium is shorter and vice versa.

Question 23

Addition of catalyst

Answer 23

1. E_A lowered
2. Rates of both forward and backward rxn increase by the same amount
3. No change in K_c and composition of equilibrium mixture
4. Time taken to reach equilibrium is shorter

Question 24

Haber Process

Conditions

Answer 24

1. Compromise temperature of 450°C
   a. Lower T, increased yield of NH₃ but low rate of production
2. Moderate pressure of around 200 atm
   a. Higher pressure, higher yield of NH₃ but increased cost of production, COP & safety concerns
3. Finely divided Fe catalyst with Al₂O₃ as promoter
   a. Increased rate of production
4. Continuous removal of NH₃ through cooling of the mixture
   a. Increased yield of NH₃
5. Molar ratio of N₂: H₂ = 3:1
   a. Minimise excess