Topic 7 & 17 - Equilibrium

Question 1

Outline the characteristics of a chemical system in equilibrium

Answer 1

If both the forward and reverse reactions reach the same fixed state, the reaction is said to be equilibrium. For example hydrogen-iodide solution first goes purple but then reaches a constant colour

Question 2

Why is the dynamic equilibrium called dynamic

Answer 2

Because the forward and reverse reactions still occurr and theferore are dynamic

Question 3

Outline the characteristics of a physical system in equilibrium

Answer 3

In a closed system the rate of evaporation is equal to the rate of condenstation. There is no net change in the amounts of liquid and gas present.

Question 4

What is the only thing that can change the value of the equilibrium constant, K_c?

Answer 4

Temperature.

Question 5

Deduce the equilibrium constant expression

Answer 5

Question 6

What does the magnitude of the equilibrium constant give information about?

Answer 6

About how far a reaction goes at a particular temperature (not how fast it will achieve equilibrium)

Question 7

Deduce the extent of a reaction from the magnitude of the equilibrium constant

Answer 7

K_c >> 1 → The reaction goes almost completion

K_c << 1 → The reaction hardly proceeds

Question 8

How does a system at equilibrium respond to a change?

Answer 8

In such a way as to minimise the effect of the change

Question 9

Predict the effect of change in temperature on the position of equilibrium and the value of K_c

Answer 9

Decrease in temperature

Exothermic reaction → equilibrium shifts to the right, value of K_c increases (higher yield at lower temperature)

Endothermic reaction → equilibrium shifts to the left, value of K_c decreases

Question 10

How does increasing temperature change the value of K_c?

Answer 10

Endothermic reactions → increase

Exothermic reaction → decrease

Question 11

Predict the effect of change in concentration on the position of equilibrium and the value of K_c

Answer 11

Increasing the concentration of reactants shifts the equilibrium to the right. Decreasing or removing a reactant or product shifts the equilibrium to that side. K_c remains unchanged.

Question 12

Why is the product in industrial processes often removed as it forms?

Answer 12

To ensure that the equilibrium is constantly shifted to the right resulting in a higher yield.

Question 13

Predict the effect of change in pressure on the position of equilibrium and the value of K_c

Answer 13

An increase in pressure causes the system to respond by shifting the position to the side with the smaller number of molecules. A decerase in pressure will shift the position to the side with larger no. of molecules. K_c does not change. If the numbers of gas molecules are equal on both sides, the position does not change

Question 14

What is the condition for pressure having an impact on the position of the equilibrium?

Answer 14

Changes in pressure will affect the position only if it involves a change in the number of gas molecules.

Question 15

What is the effect of a catalyst on an equilibrium reaction?

Answer 15

A catalyst lowers the activation energy and enables equilibrim to be reached more quickly but does not affect the position of the equilibrium or the value of K_c.

Question 16

Write down the Haber process

Answer 16

N₂(g) + 3 H₂(g) ⇔ 2 NH₃(g) DH = -93 kJmol^-1

Question 17

Outline the optimal conditions for the Haber process

Answer 17

Concentration: the product is removed as it forms to make the equilibrium shift to the right and produce more yield

Pressure: forward reaction involves a decrease in molecules → favoured by high pressure, usually 200 atm is used

Temperature: forward reaction is exothermic → favoured by low temperature, a moderate T of 450°C is used (too low a T would cease the reaction)

Catalyst: Fine iron and aluminium and magnesium, increases rate of reaction

Question 18

Write down the Contact process

Answer 18

2 SO₂(g) + O₂(g) ⇔ 2 SO₃(g) DH = -196 kJmol^-1

Question 19

Outline the optimal conditions for the Contact process

Answer 19

Pressure: High pressure will favour product, 2 atm is used

Temperature: Forward reaction is exothermic, a T of 450°C is used

Catalyst: V₂O₅, increases the rate

Question 20

Define equilibrium vapour pressure

Answer 20

The pressure exerted by a vapour on its liquid when the rate of condensation is equal to the rate of evaporation

Question 21

What are the two things vapour pressure is dependent on?

Answer 21

Temperature and intermolecular forces

Question 22

What is vapour pressure not affected by?

Answer 22

1. The surface area of the liquid
2. The volumes of liquid or vapour in the container

Question 23

How is vapour pressure affected by temperature?

Answer 23

Increasing temperature causes an increase in evaporation (liquid ⇔ vapour eqlb shifts to the right) and vapour pressure increases

Question 24

Define escape kinectic energy

Answer 24

The amount of energy particles at the surface of a liquid need to have in order to overcome the forces of attraction with neighbouring particles and form vapour

Question 25

Describe the vapour pressure-temperature graph

Answer 25

It is exponential

Question 26

When does a liquid boil?

Answer 26

When its vapour pressure is equal to the external pressure

Question 27

State the relationship between enthalpy of vaporisation, boiling point, intermolecular forces, and vapour pressure

Answer 27

Stronger intermolecular forces → higher enthalpy of vaporisation → lower vapour pressure → higher boiling point

Weaker intermolecular forces → lower enthalpy of vaporisation → higher vapour pressure → lower boiling point

Question 28

Summarise the ideal conditions for the vapour pressure to be high

Answer 28

The higher the temperature and the weaker the intermolecular forces, the higher the vapour pressure

Question 29

How can we manipulate the value of K_c in order to make different reaction equations?

Answer 29