Chemical Equilibria

Question 1

Reversible reactions

Answer 1

Reactions that proceed in both forward and backward directions

Not complete, attain a state of dynamic equilibrium, obtain mixture of both reactants anDyd products

Question 2

Dynamic equilibrium

Answer 2

State in reversible system where forward and backward reactions continue at the same rate, resulting in non net change in macroscopic properties

Question 3

Conditions for equilibrium

Answer 3

Achieved in closed system
Can be attained from either direction
Temperature remains constant
System remains in this state until system is disturbed

Question 4

Reaction quotient

Answer 4

Ratio of concentrations of reactants and products raised to stoichiometric ratios. changes as concentrations of reactants and products change continually until equilibrium is reached
IF aA + bB -><- cC + dD
Q(C)= ([C]^c [D]^d)/ ([A]^a [B]^b)
(product/ reactant)

Question 5

Equilibrium constant K(c)

Answer 5

When dynamic equilibrium is attained at a given temperature Q becomes a constant and is the equilibrium constant

Question 6

Position of equilibrium

Answer 6

Relative composition of products and reactants present in reaction mixture at equilibrium

Question 7

Equilibrium constant K(p)

Answer 7

Same as K(c) other than changing concentration for partial pressures

Question 8

Rate constant vs equilibrium constant

Answer 8

At dynamic equilibrium, forward rate = backward rate
k(f)/ k(b)= K(c)
Equilibrium constant = rate of forward reaction/ rate of backward reaction

Question 9

Variations in forms of K
- forward and backward reaction
- overall reaction
- coefficients multiplied by common factor

Answer 9

Equilibrium constant of backward reaction is reciprocal of that of forward reaction (vice versa)

Overall equilibrium constant is product of equilibrium constants for the steps

K is raised to the power of the factor of multiplication

Question 10

Factor affecting equilibrium constant

Answer 10

Equilibrium constant is constant at specific temperature and varies ONLY with temperature, not affected by anything else

Question 11

Significance of equilibrium constant

Answer 11

Small -> Yield little products when equilibrium is reached, reaction favours reactants over products at equilibrium

Very small -> Position of equilibrium lies very much to left, almost “no reaction”

Large-> Very little reactants remain when equilibrium is reached. reaction favours products over reactants at equilibrium

Very large-> Position of equilibrium lies very much to right, reaction almost complete

Question 12

What does K(c)/ K(p) expressions exclude

Answer 12

Concentration (or partial pressure) of pure solids and pure liquids because they have fixed density and fixed concentration (same number of particles per unit volume)

Concentration of water when it is present in large amount (as a solvent) in aqueous solution as it would remain approximately constant

Question 13

Relation between equilibrium position and standard Gibbs free energy change of reaction

Answer 13

Reaction mixture tends to adjust composition until Gibbs free energy is at a minimum

Gradient of change would also be 0

If ΔG <0, forward reaction tending towards equilibrium is spontaneous, rate of forward reaction is faster than backward reaction

If ΔG > 0, backward reaction tending toward equilibrium is spontaneous, rate of backward reaction is faster than forward reaction

If ΔG=0, system is at dynamic equilibrium where forward rate equals to backward rate

When ΔG<0, K>1, when ΔG>0, K<1

At equilibrium, if there are more products than reactants, position of equilibrium lies to right, implying that K>1, meaning that ΔG<0 VICE VERSA

Question 14

Answer questions regarding changes in concentration by Le Chateliers Principle

Answer 14

By Le Chatelier’s Principle, equilibrium position will shift left/ right to partially offset the decrease in [Z] by producing more [Z] until a new equilibrium is reached

Question 15

Answer questions regarding changes in pressure

Answer 15

For system with different number of gaseous reactants and products:
When total pressure of system is increased/ decrease, system will try to counteract change in pressure by favouring the forward/ backward reaction that decreases/ increases total pressure, as it would produce fewer/ more gas particles, equilibrium position will shift left/ right in direction that leads to decrease/ increase in number of gas particles

For system with equal number of gaseous reactants and products,
change in total pressure of system does not affect equilibrium position

For changes in partial pressure:
Effect of changing partial pressure of any gaseous components is similar to that of changing concentration

Question 16

Answer questions regarding changes in volume

Answer 16

When volume of reaction system is increased/ decreased under constant T
1. Total pressure decreases/ increases since gas particles are further/ closer
2. According to Le Chatelier’s Principle, system counteracts by favouring backward/ forward reaction which produces more/ less gas particles
3. Position of equilibrium shifts to left/ right
4. Partial pressures of all gases decrease/ increase since p is directly related to concentration, concentration decreases/ increases, rate of both forward and backward reactions decreases/ increases
5. Reaction takes a longer/ shorter time to reach equilibrium

Question 17

Answer questions regarding changes in temperature

Answer 17

When temperature is increased/ decreased
1. According to Le Chatelier’s Principle, system will try to counteract the change in temperature by favouring the (forward/ backward) endothermic/ exothermic reaction that absorbs/ releases heat
2. Equilibrium position shifts right/ left favouring formation of more products/ reactants
3. Rate constants of both forward and backward increases/ decreases since number of particles with energy greater than/ equal to activation energy increases/ decreases
4. Reaction takes a shorter/ longer time to reach equilibrium

Question 18

Answer questions regarding catalyst

Answer 18

1. Catalyst lowers activation energy of both forward and backward reactions to same extent, rates of both reactions increased to same extent
2. No effect on equilibrium constant and composition of equilibrium mixture
3. Equilibrium reached faster but equilibrium position remains the same

Question 19

Conditions of Haber Process

Answer 19

Moderate temperature:
- Forward reaction is exothermic, lower temperature results in higher yield of ammonia, but rate of production is slow at low temperature
- To ensure reasonable rate of production and yield, compromise is needed and moderately high temperature is used

Moderate pressure:
- High pressure favours desired reaction
- Too high pressure increases cost of production and increase safety concerns

Catalyst:
- Increases production rate

Continual removal of ammonia:
- Removal of ammonia shifts position of equilibrium to right, increasing yield of ammonia