Chemical equilibria

Question 1

Defn. Reversible reaction

Answer 1

a reaction that can proceed in both the forward and reverse directions. The reaction mixture contains quantities of all reaction species in the system

Question 2

Defn. Dynamic equilibrium

Answer 2

occurs in a reversible system, where the rates of the forward and reverse reactions are the same. There is no net change in the concentration of the reactants and products

Question 3

Defn. Le Chatelier’s Principle

Answer 3

states that when a stress is applied to a reversible system at equilibrium, the position of equilibrium will shift so as to minimise the stress

Question 4

Factors affecting equilibrium

Answer 4

• Change in concentration (addition/ removal of reactants or products)
• Change in pressure (partial/ total pressure)
• Change in temperature
• Addition of a catalyst

Question 5

Factors: Change in conc. / Change in partial pressure of a gaseous component

Answer 5

• When reactant W is added,
• by Le Chatelier’s Principle, the forward reaction is favoured
• to reduce the added W.
• Hence, position of equilibrium shifts to the right. The equilibrium mixture will contain more W, Y and Z but less X.
• When reactant W is removed,
• by Le Chatelier’s Principle, the reverse reaction is favoured
• to compensate partially for the W removed.
• Hence, position of equilibrium shifts to the left. The equilibrium mixture will contain less W, Y and Z but more X.

Question 6

Factors: Change in total pressure of a system (amount of gaseous reactants and products are different)

Answer 6

• Total pressure of the system is increased by decreasing volume of the container.
• By Le Chatelier’s Principle, the reverse reaction is favoured
• to reduce the total pressure by producing less amount of gas particles.
• Hence, the position of equilibrium shifts to the left.

Question 7

Factors: Change in total pressure of a system (amount of gaseous reactants and products are the same)

Answer 7

• There is no change in the number of moles of gaseous particles before and after the reaction.
• A change in the total pressure of the above system will not have any effect on the position of equilibrium
• Partial pressure of all gases increase
• The number of gaseous particles per unit volume increases, hence increasing the frequency of effective collisions. According to the collision theory, the rate of reaction increases
• The rate of forward and reverse reactions increase. Dynamic equilibrium is established in a shorter time

Question 8

Factors: Change in temp

Answer 8

• When temperature increases,
• By LCP, the forward endothermic reaction is favoured
• to reduce the added heat.
• Hence the position of equilibrium will shift to the right. New equilibrium mixture contains more Y and Z, less W and X.

Question 9

Factors: Addition of catalyst

Answer 9

• A catalyst is a substance that alters the rate of a chemical reaction while remaining chemically unchanged at the end of the reaction
• Addition of a catalyst has no effect on the position of equilibrium, composition of equilibrium mixture and Kc value.
• However, a catalyst lowers the activation energy of both the forward and reverse reactions to the same extent, therefore, increasing the rate of the forward and reverse reactions to the same extent.
• equilibrium is reached in shorter time

Question 10

Factors affecting Kc

Answer 10

Temperature only

Question 11

Kc, Kp formula

Answer 11

pg 5 notes

Question 12

Gibbs free energy formula

Answer 12

(NOT IN SYLLABUS)_
^G = ^G0 +RT lnQ

At equilibrium, ^G=0, so
^G0 = -RT lnK

Question 13

Haber process conditions

Answer 13

500 degree celsius
250 atm
finely divided iron catalyst

By LCP, the conditions of high pressure and low temperature would maximise the yield of ammonia at equilibrium

Question 14

Haber process: temp

Answer 14

Yield:
- At a low temp, by LCP, the forward exothermic reaction is favoured
- to produce more heat.
- Hence, POE shifts to the right to produce more ammonia

Rate:
- however, at low temp, rate of rxn is slow. System will take a long time to reach equilibrium which makes the process uneconomical
- Hence, an optimal temperature of 500 degree celsius and a catalyst is used to ensure that both the yield and rate are high

Question 15

Haber process: pressure

Answer 15

Yield:
- At a high pressure, by LCP, the forward reaction is favoured
- to reduce the pressure by producing less amounts of gas particles.
- Hence, POE shifts to the right to produce more ammonia

Economic cost:
- however, extremely high pressures will involve a higher cost of production and equipment maintenance. Stronger and more expensive equipment have to be used to withstand the high pressure
- Hence, an optimal pressure of 250 atm is used

Question 16

Haber process: use of catalyst

Answer 16

Rate:
- Finely divided iron mixed with aluminium oxide as promoter is used to increase the rate of rxn
- Hence, equilibrium yield can be achieved within a shorter time