6. Chemical Equilibria

Question 1

What is a reversible reaction?

Answer 1

A reaction that can take place in both directions and is therefore incomplete

Question 2

Why is the reaction rate fast at the beginning of a reversible reaction?

Answer 2

There are only reactants

Question 3

How is a dynamic equilibrium reached in a reversible reaction?

Answer 3

• beginning - only reactants so fast rate
• reactants produce products so forward reaction slows
• products then react to form reactants
• continues until equilibrium reached

Question 4

What happens at dynamic equilibrium?

Answer 4

• rates of forward and reverse reactions are the same

* net concentrations of the components of the reaction mixture remain constant

Question 5

What condition must be true for dynamic equilibrium?

Answer 5

Reaction takes place in a closed system

Question 6

If the concentration of products at equilibrium is high compared to that of the reactants, where does equilibrium lie?

Answer 6

To the right

Question 7

If the concentration of reactants at equilibrium is high compared to that of the products, where does equilibrium lie?

Answer 7

To the left

Question 8

What can change the position of equilibrium?

Answer 8

• temperature
• concentration
• pressure

Question 9

What principle can be used to explain which way equilibrium will shift?

Answer 9

Le Chatelier’s

Question 10

What does Le Chatelier’s principle state?

Answer 10

That if a factor affecting the position of equilibrium is altered, the position of equilibrium shifts to oppose the effect of the change

Question 11

Which way does equilibrium shift when the temperature of a reaction is increased?

Answer 11

In the endothermic direction (in order to ↓ the temperature)

Question 12

Which way does equilibrium shift when the temperature of a reaction is decreased?

Answer 12

In the exothermic direction (in order to ↑ the temperature)

Question 13

The reaction N₂ + 3H₂ ⇌ 2NH₃ has a -ve enthalpy change. What happens when the temperature is increased?

Answer 13

• equilibrium shifts left
• in endothermic direction
• to take in heat energy to oppose the change and ↓ temperature

Question 14

Would a high or low temperature increase the yield of ammonia? [N₂ + 3H₂ ⇌ 2NH₃]

Answer 14

Lower - will shift equilibrium to produce more products - but rate will be slower

Question 15

When will changing pressure have an effect on equilibrium?

Answer 15

When reactions involve gases

Question 16

Which way does equilibrium shift when the pressure of a reaction is increased?

Answer 16

In the direction with fewer moles of gas to oppose the change (i.e. ↓ pressure)

Question 17

Which way does equilibrium shift when the pressure of a reaction is decreased?

Answer 17

In the direction with the most moles of gas to oppose the change (i.e. ↑ pressure)

Question 18

What happens if pressure is increased but the number of moles is the same on both sides of the reaction?

Answer 18

The position of equilibrium remains unaffected, but equilibrium will be reached faster

Question 19

What is the position of equilibrium for reactions in the liquid/aqueous phase affected by?

Answer 19

Concentration, not pressure

Question 20

What happens to equilibrium when the concentration of a reactant is increased?

Answer 20

Shift position to the right

Question 21

What happens to equilibrium when the concentration of a product is increased?

Answer 21

Shift position to the left

Question 22

In the equation BiOCl + 2HCl ⇌ BiCl₃ + H₂O, what happens when more acid is added?

Answer 22

Equilibrium will shift right to counteract the increase in HCl

Question 23

In the equation BiOCl + 2HCl ⇌ BiCl₃ + H₂O, what happens when more water is added?

Answer 23

Equilibrium will shift left to counteract the increase in H₂O

Question 24

How does a catalyst increase the rate of reaction?

Answer 24

• lowering activation energy

* provide alternate route for the reaction

Question 25

What happens to equilibrium when a catalyst is added?

Answer 25

Position not affected, but rate increases so time to reach equilibrium decreases

Question 26

What happens if pressure is changed in a reaction with gases and solids as reactants, and only gases as products?

Answer 26

Only count the moles of gas when deciding which way equilibrium shifts

Question 27

How can ethanol be produced industrially?

Answer 27

Ethene (produced from cracking of crude oil) can be hydrated

Question 28

In the equation to produce ethanol using hydration of ethene [C₂H₄ + H₂O ⇌ C₂H₅OH], what is the theoretical yield?

Answer 28

100%

Question 29

Will the reaction C₂H₄ + H₂O ⇌ C₂H₅OH have an 100% yield in reality?

Answer 29

No

Question 30

Why may some reversible reactions with a theoretical yield of 100% not actually have 100% yield?

Answer 30

May produce some side reactions

Question 31

What is the initial yield of ethanol?

Answer 31

5%

Question 32

What could the yield of the production of ethanol by hydration of ethene be increased by?

Answer 32

* ↑ pressure * ↑ concentration of steam as its cheaper than ethene * ↓ temperature

Question 33

What temperatures are used in the hydration of ethene to ethanol? Why?

Answer 33

* 573K and 6500kPa | * compromised between best yield and fastest rate

Question 34

What is the equilibrium constant?

Answer 34

The ratio of concentrations of the reactants and products in a reaction mixture

Question 35

What can the equilibrium constant be denoted by?

Answer 35

Kc

Question 36

What does the c in Kc represent?

Answer 36

Molar concentration

Question 37

When does the value of Kc remain constant?

Answer 37

Only if the temperature remains constant

Question 38

For any reaction that reaches equilibria, how can the equation be written (with regard to Kc)?

Answer 38

ᵃA + Bᵇ ⇌ ᶜC + ᵈD

Question 39

How can the equilibrium constant be written using the equation ᵃA + Bᵇ ⇌ ᶜC + ᵈD (provided temperature is constant)?

Answer 39

Kc = [C]ᶜ [D]ᵈ / [B]ᵇ [A]ᵃ

Question 40

How can Kc be shown in an expression?

Answer 40

Kc = products/ reactants

Question 41

What is the units for Kc?

Answer 41

Vary but worked out by cancelling units for each term in the expression

Question 42

What does homogenous mean?

Answer 42

All reactants and products are in the same phase

Question 43

What happens to the equilibrium position if Kc is relatively large?

Answer 43

It lies well over to the product (right) side

Question 44

What happens to the equilibrium position if Kc is relatively small?

Answer 44

It lies well over to the reactant (left) side

Question 45

How will change in temperature affect Kc?

Answer 45

Will ↑ or ↓ depending on whether forward reaction is exothermic or endothermic

Question 46

Where will equilibrium lie if Kc > 1?

Answer 46

Concentration of products > reactants so equilibrium lies to the right

Question 47

Where will equilibrium lie if Kc < 1?

Answer 47

Concentration of reactants > products so equilibrium lies to left

Question 48

If temperature increases for an exothermic reaction, what happens to Kc?

Answer 48

Decreases

Question 49

If temperature increases for an endothermic reaction, what happens to Kc?

Answer 49

Increases

Question 50

If temperature decreases for an exothermic reaction, what happens to Kc?

Answer 50

Increases

Question 51

If temperature decreases for an endothermic reaction, what happens to Kc?

Answer 51

Decreases

Question 52

Will increasing or decreasing concentration have an effect on the value of Kc?

Answer 52

No

Question 53

Why will a change in concentration not affect Kc?

Answer 53

Kc is dependent on concentrations of reactants and products changing e.g. if concentration of reactants is increased, then equilibrium will shift until equilibrium is restored - so no overall change in concentration of reactants or products

Question 54

Will increasing or decreasing pressure have an effect on the value of Kc?

Answer 54

No

Question 55

Will adding a catalyst have an effect on the value of Kc?

Answer 55

No - increases rate of forward and reverse reactions by the same amount

Question 56

Which reactions are regarded as going to completion?

Answer 56

Those with Kc having a value greater than 10¹⁰

Question 57

What reactions are regarded as not taking place at all?

Answer 57

Those with Kc having a value less than 10⁻¹⁰