Topic 7: Equilibrium

Question 1

reversible rxn

Answer 1

rxn in which products can react with one another under suitable conditions to produce rxns

Question 2

dynamic equilibrium

Answer 2

• conc. of reactants and products don’t change over time
• backward & forward rxns are occurring simultaneously
• with the rate of both reactions equal

Question 3

physical equilibrium

Answer 3

equilibrium set up in physical processes

e.g. melting of solids, evaporation of bromine

Question 4

chemical equilibrium

Answer 4

equilibrium set up in chemical processes

e.g. decomposition of CaCO3

Question 5

what happens when bromine is placed in a sealed container at room temp?

Answer 5

• bromine is a volatile liquid with a boiling point close to room temp
• significant number of particles will have enough energy to evaporate
• concentration of bromine vapour increases in the closed system
• as the vapour can’t escape, many of its particles will condense back

Question 6

equilibrium law

Answer 6

at a given temp, the ratio of conc of products (raised to the power of molar coefficients) to conc of reactants (raised to the power of molar coefficients) is a constant

this constant is called Kc

NOTE: in aqueous rxns, the conc of the solvent won’t appear in the equilibrium constant expression as its conc won’t change

Question 7

Meaning of Kc’s value

Answer 7

• if Kc > 0 at a given temp, products are favoured over reactions
• if Kc

Question 8

situation where Kc won’t apply

Answer 8

non-reversible rxns

Question 9

Reaction quotient

Answer 9

• denoted by Q
• measures relative amount of reactants & products during a rxn at a particular point in time
• helps in figuring out which direction a rxn is likely to proceed using pressure/conc of reactants

Question 10

difference between Kc and Q

Answer 10

Kc: describes rxn at equilibrium
Q: describes rxn not at equilibrium

Question 11

Meaning of Q’s value

Answer 11

• if Kc > Q: rxn is in favour of products (forward)
• if Kc = Q: equilibrium
• if Kc

Question 12

effect of inverting the rxn on Kc

Answer 12

inverts the value of Kc

1/Kc

Question 13

effect of doubling rxn coefficients on Kc

Answer 13

square the expression Kc

Kc^2

Question 14

effect of halving reaction coefficients

Answer 14

square root of Kc

Kc^(1/2)

Question 15

effect of adding together 2 rxns on Kc

Answer 15

multiply the 2 Kc values

Kc1 x Kc2

Question 16

Le Chatelier’s principle

Answer 16

a system of equilibrium, when subjected to change, will respond to minimise the effect of the change

Question 17

Factors affecting equilibrium

Answer 17

• concentration
• pressure
• change in temp
• catalyst

Question 18

Factors affecting equilibrium: concentration

Answer 18

• increase in product conc: backwards reaction favoured
• decrease in product conc: forwards reaction favoured
• increase in reactant conc: forwards reaction favoured
• decrease in reactant conc: backwards reaction favoured

Question 19

factors affecting equilibrium: pressure

Answer 19

• increase in pressure = shift towards side with less no of moles
• decrease in pressure = shift towards side with higher no of moles

Question 20

factors affecting equilibrium: change in temp

Answer 20

• increase in temp = shift to endothermic side (ΔH = +tive)
• decrease in temp = shift to exothermic side (ΔH = -tive)
• due to temp having diff effects on forward/backward rxns due to diff activation energies

Question 21

factors affecting equilibrium: catalyst

Answer 21

• catalysts have no overall effect on equilibrium position

- it only speeds up the attainment of equilibrium

Question 22

Haber’s Process

Answer 22

N2 (g) + 3H2 (g) 2NH3 (g) [ΔH = -93 kJ/mol]

Question 23

conditions for Haber’s process, Contact process, methanol production

Answer 23

Favourable:

• low temp
• high pressure

Actual:
- moderate conditions
450 degrees and 200 atm

Question 24

Why are moderate conditions taken instead of favourable conditions?

Answer 24

low temp = slow rate of rxn = more time taken to produce yield = inefficient

high pressure = risk of explosion = costly to rebuild infrastructure

Question 25

Contact process

Answer 25

1. Combustion of sulphur-containing compounds
   S(s) + O2 (g) -> SO2 (g)
2. Oxidation of SO2 to SO3
   2SO2 (g) + O2 2SO3 (g) [ΔH = -196 kJ/mol]
   catalyst: vanadium (V) oxide
3. SO3 + H2SO3 (concentrated) -> H2S2O7 (oleum)
   H2S2O7 + H2O -> H2SO4

Alternatively,
SO3 + H2O -> H2SO4
But it’s highly exothermic and produces corrosive mist so it’s generally not used.

Question 26

Production of Methanol

Answer 26

CO (g) + 2H2 (g) CH3OH (g) [ΔH = -90 kJ/mol]

- catalyst: Al2O3 OR CuO3 OR ZnO3