2.4 Equilibrium

Question 1

What are irreversible reactions?

Answer 1

Irreversible reactions are reactions where the reactants convert to products and the products cannot convert back to the reactants. These reactions continue until one of the reactants runs out.

Question 2

What is a reversible reaction shown by?

Answer 2

A reversible reaction is shown by a half arrow to the right (forward reaction) and a half arrow to the left (backward reaction).

Question 3

What is the definition of a reversible reaction?

Answer 3

A reversible reaction is a reaction where reactants can change into products and the products can change back into the reactants.

Question 4

What is a closed system?

Answer 4

A closed system is one in which no substances can get in or out.

Question 5

What is an open system?

Answer 5

An open system allows entry and exit of substances.

Question 6

What is the definition of an equilibrium reaction?

Answer 6

An equilibrium reaction is considered to be a closed system where only the reactants and products are present.

Question 7

What are the conditions for dynamic equilibrium to occur?

Answer 7

-In a closed system
-When the rates of the forward and backward reactions are equal.
-When the amounts of reactants and products are equal.

Question 8

What is the definition of position of equliibrium?

Answer 8

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 9

What does Le Chatelier’s Principle state

Answer 9

Le Chatelier’s Principle states that if 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 10

What is the summary for the effect of changing concentration?

Answer 10

-An increase in reactant concentration shifts equilibrium to the right to reduce the concentration of reactant.
-An increase in product concentration shifts equilibrium to the left to reduce the concentration of product.

-A decrease in reactant concentration or removing a reactant shifts equilibrium to the left to increase concentration of reactant.
-A decrease in product concentration or removing product shifts equilibrium to right to increase the concentration of product.

Question 11

What happens to the equilibrium position as the temperature is increased and decreased?

Answer 11

Increased- in order to increase the temperature, the equilibrium position moves in the exothermic direction

Decreased- in order to decrease the temperature, the equilibrium position moves in the endothermic direction.

Question 12

What happens to the equilibrium, products and reactants when the temperature is increased or decreased (if the forward reaction is exothermic)?

Answer 12

Increase in temp:

Equilibrium- moves to the left
Products- decrease in products
Reactants- increase in reactants.

Decrease in temp:
Equilibrium- moves to the right
Products- increase in products
Reactants- decrease in reactants.

Question 13

What happens to the equilibrium, products and reactants when the temperature is increased or decreased (if the forward reaction is endothermic)?

Answer 13

Increase in temp:

Equilibrium- moves to the right
Products- increase in products
Reactants- decrease in reactants.

Decrease in temp:

-Equilibrium- moves to the left
Products- decrease in products
Reactants- increase in reactants.

Question 14

What is the summary for the effect of changing pressure?

Answer 14

-Increasing pressure shifts equilibrium to the side of the equation which has fewer gas moles.
-Decreasing pressure shifts equilibrium to the side of the equation which has more gas moles.

Question 15

What is the effect of a catalyst on equilibrium?

Answer 15

The addition of a catalyst does not cause a shift in equilibrium position. A catalyst is a substance that affects the rate of reaction, but is not consumed in the reaction. It does this by providing an alternative mechanism for the reaction with lower activation energy. Therefore, a catalyst has no effect on equilibrium position but allows dynamic equilibrium to be attained more quickly.

Question 16

What are the three conditions used to ensure a good yield of ammonia in the Haber process?

Answer 16

-Temperature (450 degrees celcius)
-Pressure (200 atm)
-Iron catalyst

The reaction is reversible in the Haber process.

N2 (g)+3H2 (g) –> 2NH3 (g)
<–

Question 17

Describe why the temperature used in the Haber process is 450 degrees celcius

Answer 17

TEMPERATURE (450 °C):

-The forward reaction (the production of ammonia) is exothermic.
-According to Le Chatelier’s Principle, this will be favoured if you lower the temperature. The yield of ammonia will increase as temperature is decreased. Therefore, in order to get as much ammonia as possible in the equilibrium mixture, you need as low a temperature as possible. However, carrying out the reaction at low temperatures would slow down the rate meaning that the reaction would happen very slowly. This is no good when you are trying to make a profit!
-Therefore, a compromise between yield and rate must be reached. A temperature of 450 °C represents a compromise giving a good rate of reaction and keeping temperature low enough to obtain a reasonable yield of ammonia.

Question 18

Describe why the pressure used in the Haber process is 200 atm

Answer 18

PRESSURE (200 atm):

N2 (g)+ 3H2 (g) <=> 2NH3 (g)

-Notice that there are 4 molecules on the left-hand side of the equation, but only 2 on the right. According to Le Chatelier’s Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules. That will cause the pressure to fall again. In order to get as much ammonia as possible in the equilibrium mixture, you need as high a pressure as possible.
-Rate considerations: Increasing the pressure brings the molecules closer together. In this particular instance, it will increase their chances of hitting and sticking to the surface of the catalyst where they can react. The higher the pressure the better in terms of the rate of a gas reaction.
-Economic considerations: Very high pressures are extremely expensive to produce for two reasons.
* You have to build extremely strong pipes to withstand the high pressure. That increases capital costs when the plant is built.
* A lot of energy is needed to compress the gas. High pressures cost a lot to produce and maintain.
That means that the running costs of the plant are very high.
-The compromise: 200 atmospheres is a compromise pressure chosen on economic grounds.

Question 19

Describe why an iron catalyst is used in the Haber process

Answer 19

Because it has no effect on the position of equilibrium, it simply speeds up the rate of reaction.

Question 20

What is the definition of ‘dynamic equilibrium’?

Answer 20

A dynamic equilibrium occurs when the rate of the forward reaction is equal to the rate of the reverse reaction resulting in the amount of reactants and products remaining constant.

Question 21

What is the defintion of a ‘homogeneous reaction’?

Answer 21

A homogeneous reaction (or system) is one in which all the reactants and products are in the same state.