Chapter 7: Equilibrium

Question 1

Define reversible reactions

Answer 1

REVERSIBLE REACTIONS: reactions that can proceed both forwards and backwards (forward and backward reaction) (⇌)

Question 2

Define equilibrium and dynamic equilibrium, and explain why a reversible reaction can only come to a state of equilibrium in a closed system

Answer 2

OPEN SYSTEM:
- Gaseous products are able to escape the system, and will not be available to reform the reactant through the backwards reaction

CLOSED SYSTEM:
- Gaseous products are unable to escape into the atmosphere and are able to reform the reactant such that there are constant amounts of reactants and products
- Equilibrium: concentrations of reactants and products are constant (no further changes appear to occur)
- Dynamic equilibrium: concentrations of reactants and products are constant (no further changes appear to occur as the rate of the forward reaction is equal to the rate of the reverse reaction)

Question 3

Graph a reaction that has reached state of equilibrium

Answer 3

1) Graph concentration of reactants and products against time
- Dotted line: state of equilibrium –> concentration of both reactants and products are constant

2) Graph rate of forward and backward reactions against time
- Dotted line: state of equilibrium –> rate of both reactions are constant and equal to each other

Question 4

List the 5 characteristics of the equilibrium state

Answer 4

1) Concentration of reactants and products are constant
2) Rate of forward and backward reactions are equal (and do not equal to 0)
3) Attained in a closed system
4) All species in the chemical equation are present in the reactant mixture (none have a concentration of 0)
5) Can be attained in either direction

Question 5

Describe how state of equilibrium can be attained through physical equilibria

Answer 5

PHYSICAL EQUILIBRIA: attained through change of state

Eg. When water evaporates and to form vapour molecules which condense to reform water molecules, the rate of condensation will equal the rate of evaporation, reaching dynamic equilibrium

Question 6

Define position of equilibrium

Answer 6

POSITION OF EQUILIBRIUM: relative amounts of reactants and products at state of equilibrium

(At equilibrium, the concentrations of reactants and products may be constant, but are not necessarily equal to each other)

Question 7

Define and apply Le Chatelier’s Principle to predict the position of equilibrium

Answer 7

LE CHATELIER’S PRINCIPLE: if a system at equilibrium is subjected to a change, the position of equilibrium will shift in order to minimise the effect of that change

1) TEMPERATURE
- If forward reaction is endothermic (ΔH>0), backward reaction will be exothermic (ΔH<0)
- Temperature increase will favour endothermic reactions (which would decrease the temperature), whereas temperature decrease will favour exothermic reactions (which would increase the temperature)
- Position of equilibrium would shift to the side of reaction that is favoured

Eg. According to LCP, an increase/decrease in temperature will cause the system to favour the endothermic/exothermic reaction, as this would decrease/increase the temperature. Hence, the forward/backward reaction would be favoured, causing the position of equilibrium to move to the right/left.

2) PRESSURE (only for reactions involving gaseous reactants/products)
- No. of mols ∝ Pressure of gas
- Pressure increase will cause system to favour reaction producing fewer number of moles of gas (as this would lower the pressure)

Eg. According to LCP, an increase/decrease in pressure would cause the system to favour the reaction that produces the fewer/greater number of moles of gas, as this would decrease/increase the pressure. Hence, since the amount of gaseous reactants (_mol) is greater/lesser than the amount of gaseous products (_mol), the forward/backward reaction is favoured, and the position of equilibrium will shift to the right/left.

3) CONCENTRATION
- Increase in concentration would cause position of equilibrium to move to the opposite side to reduce the concentration of that reactant

Eg. According to LCP, if ___ was added to/removed from the mixture and the concentration of __ increased/decreased, the system would favour the reaction that uses up/produces ___, as this would decrease/increase the concentration of ___. Hence, the forward/backward reaction is favoured and the position of equilibrium will shift to the right/left.

Question 8

State the expression for the equilibrium constant (K꜀)

Answer 8

For reaction
aA+bB –> cC+ dD

K꜀ = [C]ᶜ[D]ᵈ/[A]ᵃ[B]ᵇ

• If K꜀>1, position of equilibrium is to the right (reaction proceeds towards products)
  If K꜀<1, position of equilibrium is to the left (reaction proceeds towards reactants)

Question 9

State the expression for the reaction quotient (Q)

Answer 9

Q = [C]ᶜ[D]ᵈ/[A]ᵃ[B]ᵇ
(concentrations are not equilibrium concentrations)

• If Q<K꜀ : reaction must proceed towards products to reach equilibrium
• If Q>K꜀ : reaction must proceed towards reactants to reach equilibrium
• If Q=K꜀ : reaction is at equilibrium

Question 10

Explain the effect of changing factors on K꜀

Answer 10

K꜀ will only be affected by change in temperature
- Change in temperature –> position of equilibrium will shift –> concentration of reactants/products will change (numerator/denominator of K꜀ will increase/decrease) –> K꜀ will increase/decrease

Catalysts have no effect on K꜀:
- Catalysts increase reaction rate for both forward and backward reactions –> no effect on position of equilibrium –> no effect on K꜀
- Only reduces time to reach equilibrium