5B - Equilibria

Question 1

Define:

Homogeneous equilibrium

Heterogeneous equilibrium

Answer 1

Homogeneous equilibrium = An equilibrium in which all th species making up the reactants and products are in the same physical state

Heterogeneous equilibrium = An equilibrium in which all th species making up the reactants and products are in different physical states

Question 2

Define Le Chatelier’s Principle

Answer 2

When a system in dynamic equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of the change and to restore equilibrium

Question 3

Define the equilibrium law and it’s relationship with Kc = ?

Units of Kc = ?

Answer 3

Units = Change but note that each concentration is moldm^-3

Question 4

What does each include:

1. Kc
2. Kp

Answer 4

Kc = gases, liquids and aqueous compounds

Kp = gases only

Question 5

Equation:

Ideal gas law = ?

Units and common conversions

Answer 5

PV = nRT

P = Pressure / Pa

V = Volume / m³ (1m³ = 1000dm³ = 1 x 10⁶cm³)

n = Moles

R = Gas constant (on data sheet)

T = Temperature / K (+273 to convert from °C → K)

Question 6

Partial pressure of gas A = ?

Answer 6

Partial pressure of gas A = Mole fraction x Total pressure

Mole fraction of gas A = Moles of gas A / Total no’ of moles in mixture

Question 7

Total pressure = ?

Answer 7

P_Total = n_Total x (RT / V)

(rearrangement of PV = nRT)

You could also add up all the partial pressures:

P_Total = n_A + n_B + … x (RT / V)

Question 8

What is the relationship betwwen pressure and the no’ of moles?

Answer 8

Question 9

Define the equilibrium law and it’s relationship with Kp = ?

Units of Kp = ?

Answer 9

Units = Vary but must be in KPa

Question 10

You can make these assumptions only when?

Answer 10

The temperature and volume are constant

Question 11

Define what a dynamic equilibrium means (4)

Answer 11

Dynamic equilibrium = The equilibrium that exists in:

• A closed system
• When the rate of the forward reaction = rate of backward reaction
• Concentration (or anything macrophysical - can be measured) don’t change
• The equilibrium can be approached from both sides

Question 12

Describe an experiment to determine Kc from experimental results - esterification reaction: (5)

Use this as the example:

CH₃CH₂OH + CH₃COOH ⇌ CH₃COOCH₂CH₃ + H₂O

Why does the analysis need to be carried out quickly?

Answer 12

1. In conical flask:
   
   • Mix known moles of ethanol, ethanoic acid, ethyl ethanoate and water with a small known mass of conc H₂SO₄ catalyst
     
     • ethyl ethanoate = 0 at the start
2. Flask is maintained at constant temperature until equilibrium has been reached
3. Further flasks containing different initial amounts are prepared and allowed to equilibrate - put stopper on all flasks.
4. Flasks are titrated against a standard solution of NaOH_(aq)
5. Create a control by adding an equal amount of the conc H₂SO₄ catalyst into a second flask, dilute it with water and titrate against standard solution of NaOH_(aq)​
   
   • Need this because acid catalyst also reacts with the alkali

Analysis must be carried out as fast as you can because as soon as you pipette the solution out, it’s no longer in a closed system ∴ no longer the same equilibrium

Question 13

The value of Kc and how this affects the position of equilibrium

Answer 13

Kc > 1

• Equil favours products - [large] / [small] ∴ Kc increases

Kc < 1

• Equil favours reactants - [small] / [large] ∴ Kc decreases

Kc = 1

• Dynamic equilibrium

Question 14

The value of Kp and how this affects the position of equilibrium

Answer 14

Kp > 1

Equil favours products - [large] / [small] ∴ Kp increases

Kp < 1

Equil favours reactants - [small] / [large] ∴ Kp decreases

Kp = 1

Dynamic equilibrium

Question 15

Factors that affect Kc

Answer 15

Temperature only!

Le Chatelier’s Principle takes care of changes in pressure and concentrations

Exothermic reactions:

• Increase temp
• Equil favours backward reaction as it’s endothermic
• More reactants produced
• Kc decreases - [small] / [large]

Endothermic reactions:

• Increase temp
• Equil favours forward reaction as it’s endothermic
• More prducts produced
• Kc increases - [large] / [small]

Question 16

Factors that affect Kp

Answer 16

Temperature only!

Le Chatelier’s Principle takes care of changes in pressure and concentrations

Exothermic reactions:

• Increase temp
• Equil favours backward reaction as it’s endothermic
• Partial pressures of reactacts increase and partial pressures of products decrease
• Kp decreases - [small] / [large]

Endothermic reactions:

• Increase temp
• Equil favours forward reaction as it’s endothermic
• Partial pressures of reactacts decrease and partial pressures of products increase
• Kp increases - [large] / [small]

Question 17

Affect of catalysts on equilibrium constants? (3)

Answer 17

No affect

• Catalysts affect the rate of a chemical reaction but not the position of equilibrium
• Catalysts speed up both the rate of the forward and backward reactions in the equilibrium by the same factor
• Equilibrium is reached quicker but the equilibrium position in not changed

Question 18

Illustrate through a graph the affect of a change in pressure on Kp

Answer 18

Question 19

Illustrate through a graph the affect of a change in concentration on Kp

Answer 19