Equilibrium

Question 1

Irreversible reaction definition

Answer 1

A reaction where reactants can change into products but the products can change back into the reactants

Question 2

Position of equilibrium definition

Answer 2

The position of equilibrium in a reversible reaction is a measure of how far the reaction has proceeded to the right (towards the products) or has it remained to the left (towards the reactants)

Question 3

What happens as concentration increases ?

Answer 3

As concentration increases a shift to the right will occur in order to counteract the increasing concentration of the reactants by producing more product

Question 4

What happens as concentration decreases?

Answer 4

The concentration of reactant decreases and thus to re-establish equilibrium a shift to the left occurs by decomposing product to increase the concentration of reactant

Question 5

What reactions does change in pressure not affect?

Answer 5

• Reactions where the equilibrium mixture is made up of only liquids and/or solids will not be affected by changes in pressure

Question 6

What is the ratio of moles in this reaction?

2SO2 + O2 —>< 2SO3

Answer 6

2 molecules of SO2 and 1 molecule of O2, giving 2 molecules of SO3

Question 7

What happens as pressure is increased ?

Answer 7

• An increase in pressure will move the position of equilibrium in the direction of the smaller number of gas molecules, to minimise the effect of the increase in pressure

Question 8

What happens as pressure is reduced?

Answer 8

• A decrease in pressure will move the position of equilibrium in the direction of the larger number of gas molecules

Question 9

What happens when pressure is added to a reaction where the number of moles of gases on both sides of the equation are equal?

Answer 9

Changes in pressure will not affect the position of equilibrium

Question 10

What happens when the forward reaction is:
- exothermic
- temperature increases/ decreases ?
(4)

Answer 10

• If the forward reaction is exothermic and temperature is increased, the position of equilibrium moves to the reverse reaction (endothermic) in order to absorb the additional heat,
• The yield of products is decreased
• If the temperature is decreased the position of equilibrium moves towards the exothermic reaction in order to release more heat
• the yield of products is increased

Question 11

What happens when the forward reaction is:

• endothermic
• temperature increases / decreases ?

Answer 11

• the yield of products is increased

- If the temperature decreases the yield of products is decreased

Question 12

What is the Haber process ?

Answer 12

The industrial manufacture of ammonia

Question 13

Why is a higher temperature chosen for the Haber process if the yield of ammonia is better at lower temperatures ?

Answer 13

The reaction runs much faster at high temperatures so a compromise temperature of 450 degrees is chosen.
High enough to get a reasonable rate of reaction but not so high that the yield of ammonia is low

Question 14

Why is there a limit to the pressure that can be used industrially ?

Answer 14

Very high pressures require very strong and expensive equipment
This means that a compromise pressure is chosen - high enough to get a good yield of ammonia but not so high that it would add too much to the costs of the process

Question 15

How does a catalyst affect equilibrium ?

Answer 15

• It reduces the time taken to reach equilibrium
• It doesn’t change the position of the equilibrium
• Catalysts increase the rates of the forward and reverse reactions by the same amount

Question 16

What catalyst is used in the Haber process?

Answer 16

Iron

Question 17

Conditions used in the manufacture of ammonia? (3)

Answer 17

• temperature 450 degrees (compromise temperature )
• pressure 200atm (high pressure to favour NH3 production )
• Iron catalyst (equilibrium attained faster )

Question 18

What is an open system ?

Answer 18

When a chemical reaction happens in a container where one or more of the reactants or products can escape

Question 19

Closed system

Answer 19

A chemical reaction happening in a container where none of the reactants or products can escape. Only the reactants and products are present

Question 20

Reversible reactions that happen in a closed system eventually reach …

Answer 20

Equilibrium

Question 21

Dynamic equilibrium definition

Answer 21

Occurs in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the amounts of reactants and products remain constant

Question 22

Name features of reversible reactions (4)

Answer 22

• Has a double two-way arrow in equation
• Direction of a reversible reaction can be changed by altering reaction conditions e.g temperature
• Has a position of equilibrium showing how far the reaction has proceeded to the right or left
• Most studied reactions are homogeneous (those where all the reactants and products are gases) or aqueous (homogenous aqueous)

Question 23

Explain Le Châtelier’s principle

Answer 23

When a change is made to the conditions of a system at equilibrium, then the position of the equilibrium moves to oppose that change in conditions

Question 24

Explain why this reaction is a homogenous equilibrium

N2 (g) + 3H2 (g) —>< 2NH3 (g)

Answer 24

Reactants and products are all in the same physical state, gases

Question 25

What happens if there are the same number of molecules of gas on each side of the reaction during pressure change ?

Answer 25

A change in pressure will have no effect on the position of equilibrium

Question 26

How is equilibrium achieved ?

Answer 26

When the amount of reactants and products remains constant

Question 27

Two points for dynamic equilibrium :

Answer 27

• Amount of reactants and products remains constant

- Rate of forward reaction is equal to reverse reaction

Question 28

How does temperature affect equilibrium ?

Answer 28

• An increase in temperature will move the position of equilibrium in the direction of the endothermic reaction, to absorb the heat
• a decrease in temperature will move the position of equilibrium in the direction of the exothermic reaction, to release more heat

Question 29

Describe the Haber process and ideal conditions for it

Answer 29

• Ammonia is produced industrially in the Haber process from nitrogen and hydrogen
• The reaction is a homogenous, gaseous equilibrium

N2 (g) + 3H2 (g) —<> 2NH3 (g)

Energy change is -92kJ so the forward reaction is exothermic

• A decrease in temperature will move the position of equilibrium to the right towards the exothermic reaction so more ammonia is produced
• An increase in pressure will moves the position of equilibrium to the right so there is more ammonia produced
• The presence of iron catalyst means that equilibrium is attained more quickly