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 and forward rxns occur simultaneously
• rate of both reactions are equal

Question 3

physical equilibrium

Answer 3

equilibrium set up in physical processes

e.g. vaporization ↔ condensation

Question 4

chemical equilibrium

Answer 4

equilibrium set up in chemical processes

e.g. decomposition of CaCO3

Question 5

closed system

Answer 5

• system in which neither matter nor energy is gained or lost from the system
• this allows an equilibrium to be reached

Question 6

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

Answer 6

• most of its particles will have enough energy to evaporate
• but this causes conc of bromine vapour to increase in the closed system
• vapour can’t escape so many of its particles will condense back
• this is possible because bromine is a volatile liquid with a b.pt close to room temp

Question 7

equilibrium constant

Answer 7

symbol: Kc
- tells us the equilibrium position
- i.e. the proportion of reactants and products in the equilibrium mixture
- at a given temp, ratio of conc of products (raised to the power of molar coefficients) : conc of reactants (raised to the power of molar coefficients) is constant

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

Question 8

homogeneous reaction

Answer 8

reaction in which all constituents (reactants and products) are of same state

Question 9

Meaning of Kc’s value

Answer 9

• if Kc > 0, products are favoured over reactions
• vice versa
• if Kc > 1, the reaction almost goes to completion
• if Kc < 1, the reaction is barely proceeding
  e. g. if Kc = 4, conc. of products is 2x that of reactants
• DOES NOT tell us anything about how quickly equilibrium will be reached

Question 10

situation where Kc won’t apply

Answer 10

non-reversible rxns

Question 11

Reaction quotient

Answer 11

symbol: Q
- measures relative amount of reactants and products during a rxn at a particular point in time
- helps figure out which direction a rxn is likely to proceed based on the pressure and conc of reactants

Question 12

difference between Kc and Q

Answer 12

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

Question 13

Meaning of Q’s value

Answer 13

• if Kc > Q: forward rxn favored

- vice versa

Question 14

effect of inverting the rxn on Kc

Answer 14

inverts Kc value

1/Kc

Question 15

effect of doubling rxn coefficients on Kc

Answer 15

square the expression Kc

Kc^2

Question 16

effect of halving reaction coefficients

Answer 16

square root of Kc

Kc^(1/2)

Question 17

effect of adding together 2 rxns on Kc

Answer 17

multiply the 2 Kc values

Kc1 x Kc2

Question 18

Le Chatelier’s principle

Answer 18

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

Question 19

Factors affecting equilibrium

Answer 19

• concentration
• pressure
• temp
• catalyst

Question 20

Factors affecting equilibrium: concentration

Answer 20

• if reactant conc. > product conc., forward rxn favored

- vice versa

Question 21

Factors affecting equilibrium: pressure

Answer 21

• ↑ pressure = rxn resulting in ↓ no of moles favored

- vice versa

Question 22

factors affecting equilibrium: temp

Answer 22

• ↑ temp = endothermic side favored
• vice versa
• change in temp = change in Kc

Question 23

why does only temp change Kc?

Answer 23

• pressure and conc affect Q (NOT Kc)
• the equilibrium shifts to oppose the change
• this results in equal rates on both sides again, leaving Kc unchanged
• however, activation energies differ between forward and backward rxn
• and the energy change is not the same on both sides
• altering temp allows one rxn to proceed faster than the other
• this alters Kc

Question 24

factors affecting equilibrium: catalyst

Answer 24

• catalysts have no overall effect on equilibrium position

- it only speeds up the attainment of equilibrium

Question 25

Haber’s Process

Answer 25

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

Question 26

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

Answer 26

Favourable:

• low temp
• high pressure

Actual:
450°C at 200 atm

Question 27

Why are moderate conditions taken instead of favourable conditions?

Answer 27

• low temp = slow rate of rxn = more time taken to produce yield = inefficient
• high pressure = risk of explosion = costly to rebuild infrastructure

Question 28

Contact process

Answer 28

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 29

Production of Methanol

Answer 29

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

- catalyst: Al2O3 OR CuO3 OR ZnO3

Question 30

conditions required to achieve equilibrium

Answer 30

closed system

Question 31

concentrations of reactants and products at equilibrium

Answer 31

• constant but not necessarily equal

- depends on the reaction equation and the conditions

Question 32

why do different reactions have different equilibrium positions?

Answer 32

• depends on the amount of gibbs free energy (∆G*) – i.e. work – available in the system
• the farther the reaction is from equilibrium, the more negative (i.e. viable) the forward reaction is
• ∆G* = 0 at equilibrium

*generally comes with a theta to denote standard conditions