Equilibria

Question 1

What does ⇌ mean in chemistry?

Answer 1

Reaction is reversible forward and reverse occur

Question 2

How can direction of a reversible reaction be changed?

Answer 2

By changing reaction conditions

Question 3

What happens when NH₄Cl is heated?

Answer 3

Decomposes to NH₃ and HCl gases

Question 4

What happens when NH₃ and HCl cool?

Answer 4

Recombine to form NH₄Cl solid

Question 5

Example of reversible reaction with copper sulfate

Answer 5

CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O

Question 6

Colour of hydrated copper(II) sulfate

Answer 6

Blue crystals

Question 7

Colour of anhydrous copper(II) sulfate

Answer 7

White powder

Question 8

Is dehydration of hydrated copper(II) sulfate endothermic or exothermic?

Answer 8

Endothermic energy taken in

Question 9

Is rehydration of anhydrous copper(II) sulfate endothermic or exothermic?

Answer 9

Highly exothermic energy released

Question 10

What is water of crystallisation?

Answer 10

Water included in salt structure during crystallisation

Question 11

What is dynamic equilibrium?

Answer 11

When forward and reverse reactions occur at same rate

Question 12

Two features of a system at equilibrium

Answer 12

Rates of forward and reverse reactions equal concentrations stay constant

Question 13

Why is equilibrium dynamic?

Answer 13

Reactants and products keep interchanging at equal rates

Question 14

Does concentration change at equilibrium?

Answer 14

No remains constant if conditions unchanged

Question 15

What type of system is needed for equilibrium?

Answer 15

Closed system

Question 16

Why must equilibrium be in a closed system?

Answer 16

No substances can enter or leave

Question 17

Example of reaction reaching equilibrium

Answer 17

Haber process N₂ + 3H₂ ⇌ 2NH₃

Question 18

What happens to forward reaction rate as Haber process proceeds?

Answer 18

Decreases as N₂ and H₂ concentrations fall

Question 19

What happens to reverse reaction rate as Haber process proceeds?

Answer 19

Increases as NH₃ concentration rises

Question 20

When is equilibrium reached in Haber process?

Answer 20

When forward and reverse rates become equal

Question 21

What is Le Chatelier’s Principle?

Answer 21

If conditions change equilibrium shifts to oppose the change

Question 22

What does position of equilibrium mean?

Answer 22

Relative amounts of reactants and products at equilibrium

Question 23

Effect of increasing temperature on equilibrium

Answer 23

Favours endothermic direction

Question 24

Effect of decreasing temperature on equilibrium

Answer 24

Favours exothermic direction

Question 25

Effect of increasing pressure on equilibrium

Answer 25

Favours side with fewer gas molecules

Question 26

Effect of decreasing pressure on equilibrium

Answer 26

Favours side with more gas molecules

Question 27

If gas molecules equal on both sides what is pressure effect?

Answer 27

No change in position of equilibrium

Question 28

What does shift to the right mean?

Answer 28

Forward reaction favoured more products formed

Question 29

What does shift to the left mean?

Answer 29

Reverse reaction favoured more reactants formed

Question 30

Example of exothermic forward reaction

Answer 30

ICl (l) + Cl₂ (g) ⇌ ICl₃ (s)

Question 31

Colour change when heating ICl + Cl₂ mixture

Answer 31

Becomes more brown equilibrium shifts left

Question 32

Example of pressure effect

Answer 32

2NO₂ (g) ⇌ N₂O₄ (g)

Question 33

Colour change when pressure increases in 2NO₂ ⇌ N₂O₄

Answer 33

Becomes colourless equilibrium shifts right

Question 34

Does a catalyst affect position of equilibrium?

Answer 34

No only speeds up reaching equilibrium

Question 35

Why does catalyst not change equilibrium position?

Answer 35

Increases rate of forward and reverse equally

Question 36

What is a closed system?

Answer 36

A system where no reactants or products can enter or leave but energy can be exchanged with surroundings

Question 37

Explain the Haber Process

Answer 37

N from air + H from natural gas 200 atm, 450°C Iron catalyst NH3 gas forms is condensed (has highest BP) N and H gas recycled back

Question 38

How is the Haber Process optimised to maximise yield

Answer 38

Compromise conditions. High pressure around 200 atm favours forward reaction but is expensive. Moderate temperature around 450°C balances rate and yield because lower temperature gives higher yield but slower rate. Iron catalyst speeds up reaction without changing equilibrium position. Continuous removal of ammonia shifts equilibrium to the right

Question 39

Explain the Contact Process

Answer 39

Used to make sulfuric acid. Burn sulfur in air to make SO₂. Oxidise SO₂ to SO₃ using V₂O₅ catalyst at 450°C and 2 atm. Reaction is exothermic and reversible 2SO₂ + O₂ ⇌ 2SO₃ Absorb SO₃ in concentrated H₂SO₄ to form oleum then add water to make H₂SO₄ Conditions chosen for good yield and reasonable rate