Chemical Equilibria

Question 1

What is dynamic equilibrium

Answer 1

in which the rate of the forward reaction equals that of the reverse reaction, and there is no net change in composition

Question 2

What is the link between Thermodynamics/Kinetics and Chemical Equilibrium

Answer 2

Thermodynamics: can be used to find the equilibrium composition
Kinetics: can be used to assess how quickly equilibrium is achieved

Question 3

Why does Gibbs free energy of a reaction mixture change with composition

Answer 3

Because the Gibbs free energy of each reactant or product changes with concentration

Question 4

What is effective concentration

Answer 4

Effective concentration of a species refers to the concentration of that species, considering any interactions or derivations from ideal behaviour that may occur in a solution or gas

Question 5

What is the equation for Gibbs Free energy of a reaction when the composition is changing

Answer 5

Where a = effective concentration
G(A) = Gibbs Free energy of pure A

Question 6

For an ideal gas, what is the equation for activity

Answer 6

Where p∘ = 1bar

Question 7

What is an ideal solution

Answer 7

No forces between the molecules in a solution

Question 8

For an ideal solution, what is the equation for activity

Answer 8

Where c∘ = 1mol dm⁻³

Question 9

What is concentration and activity like in a pure solid or liquid

Answer 9

For pure solids or pure liquids concentration is fixed and the activity is 1

Question 9

For a spontaneous reaction to occur

Answer 9

ΔG products < ΔG reactants
ΔG < 0

Question 10

How does Gibbs free energy relate to equilibrium

Answer 10

• For a reaction that reaches equilibrium, the formward reaction is spontaneous until equilibrium is reached
• At equilibrium the Gibbs free energy of the reaction mixture is a minimum
• Beyond equilibrium the production of more products would result in an increase in Gibbs free energy
• At equilibrium the Gibbs free energy of the products equals that of the reactants, so ΔrG = 0

Question 11

Describe the values of ΔrG for the forward and backwards reactions and equilibrium

Answer 11

Forwards = ΔrG < 0
Backwards = ΔrG > 0
Equilibrium = ΔrG = 0

Question 12

The composition of the equilibrium mixture is given by

Answer 12

the equilibrium constant K
Defined by the activities of the reactants and products at equilibrium

Question 13

Equilibrium constant is

Answer 13

A dimensionless concept

Question 14

What is the expression for equilibrium constant for the following reaction in terms of partial pressure

Answer 14

Question 15

What is equilibrium constant for the following reaction, in terms of partial pressure

Answer 15

Question 16

What is the equilibrium constant for the following reaction in terms of concentration

Answer 16

Question 17

What is reaction quotient

Answer 17

The favoured direction for reaction at any stage can be assessed from the reaction quotient which is calculated using the same expressuin for equilibrium constant but substituting non-equilibrium values

Question 18

The difference in Gibbs free energy between the products and reactants at any stage is measured through which equation

Answer 18

If Q<K then ΔrG<0 = forward reaction proceeds
If Q>K then ΔrG>0 = backwards reaction proceeds

Question 19

What is the link between Gibbs Free energy and equilibrium constant

Answer 19

Question 20

When

Answer 20

the products are favoured in the equilibrium mixture

Question 21

When

Answer 21

the reactants are favoured in the equilibrium mixture

Question 22

What is Le Chatelier’s Principle

Answer 22

When a change is made to a sysmtem in dynamic equilibrium, the equilibrium responds so as to minimise the effect of the change

Question 23

Explain in terms of equilibrium constant, why removing products causes more product to form

Answer 23

• This is a way of increasing yield from a reaction which has only a small amount of products in th equilibrium mixture
• For example in the Haber process (N₂(g) + 3H₂(g) → 2NH₃(g)) if ammonia is removed from the reaction mixture p(NH₃) decreases so Q<K and the forwards reaction is favoured

Question 24

In terms of the equilibrium constant, why does increasing the pressure, shift equilibrium in the direction of the fewest moles of gas

Answer 24

• Composition of equilibrium does depend on overall pressure
• If K is to remain constant when the total pressure is increase, then NH₃ must increase relative to N₂ and H₂

Question 25

For an endothermic reaction, what change favours the products

Answer 25

Increasing the temperature as heat is absorbed
K becomes bigger

Question 26

For an exothermic reaction, what change favours the products

Answer 26

decreasing temperature, as heat is released
K becomes bigger

Question 27

What is the Van Hoff Equation and how does it relate to exothermic and endothermic reactions

Answer 27

The Van Hoff equation takes the form of a straight line, so that a graph of Ln(k) over 1/T will have the gradient -ΔrH/ R
For an exothermic reaction the slope is +ve
For an endothermic reaction the slope is -ve

Question 28

How do catalyst affect reactions

Answer 28

A catalyst increases the rate at which a reaction mixture reaches equilibrium, but does not alter the composition of equilibrium mixture
A catalyst can be used to lower the temperature required for a significant rate of reaction

Question 29

For a reaction with a negative ΔrH value, what can be said

Answer 29

a reaction releases more energy than it absorbs
the reaction is exothermic

Question 30

A reaction with a positive ΔrH value, what can be said

Answer 30

a reaction absorbs or uses more energy than it releases
the reaction is endothermic

Question 31

The decomposition reaction has a ΔrH [700K] = +161 kJ mol⁻¹
Would K increase or decrease if the temperature was increease

Answer 31

This is an endothermic reaction, so the slop from the Van Hoff equation is negative
So if Ln(K) increases as 1/T decreases, therefore K increases as T increases
Therefore the products are favoured