C3.5 The production of ammonia

Question 1

Where is nitrogen obtained from for the Haber process?

Answer 1

obtained from the air

Question 2

Where is hydrogen obtained from for the Haber process?

Answer 2

obtained from natural gas or other sources such as crude oil

Question 3

What does the Haber process produce?

Answer 3

Ammonia

Question 4

Describe the Haber process

Answer 4

Purified gases (hydrogen and nitrogen) are passed over a catalyst of iron at a high temperature (about 450 degrees)

and a high pressure (about 200 atmospheres).

Some of the hydrogen and nitrogen react to form ammonia

On cooling, the ammonia liquefies and is removed.

The remaining hydrogen and nitrogen are recycled

Question 5

What is the word equation for the Haber process?

Answer 5

nitrogen + hydrogen ——-reversible——— ammonia

Question 6

What happens to the equilibrium when a reversible reaction occurs in a closed system?

Answer 6

Equilibrium is reached when the reactions occur at exactly the same rate in each direction

Question 7

What is a closed system?

Answer 7

nothing can enter and nothing can leave the reaction

Question 8

What conditions are needed in the Haber process?

Answer 8

pressure of 200 atmospheres

Temperature: 450 degrees

Catalyst: Iron

Question 9

What is a reversible reaction?

Answer 9

A reaction where the products of the reaction can themselves react to produce the original reactants

Question 10

What is Equilibrium?

Answer 10

The amounts of reactants and products will reach a certain balance and stay there

Reactions are still taking place in both directions but there is no overall effect

as the forward and reverse reactions cancel each other out

Question 11

What happens if the temperature is raised in a reversible reaction?

Answer 11

The endothermic reaction will increase to use up the extra heat

the yield of the endothermic reaction increases and the yield from the exothermic reaction decreases

Question 12

What happens if the temperature is lowered in a reversible reaction?

Answer 12

The exothermic reaction will increase to give out more heat

the yield from the endothermic reaction decreases and the yield from the exothermic reaction increases

Question 13

In gaseous reactions, what happens if you raise the pressure?

Answer 13

It will encourage the reaction which produces less volume

Question 14

What factors are important when determining the optimum conditions in industrial processes

Answer 14

reaction rate

temperature

pressure

Question 15

What is the type of reaction between hydrogen and nitrogen to produce ammonia?

Answer 15

reversible

Question 16

In the Haber process, why is the pressure set as high as possible?

Answer 16

As high pressures favour the forward reaction

so pressure is set as high as possible to give the best % yield without making the plant too expensive to build (it’d be too expensive to build a plant that’d stand pressure over 1000 atmospheres for example)

Question 17

In the Haber process, is the forward direction (to make ammonia) exo or endothermic?

Answer 17

Exothermic

Question 18

What effect does increasing the temperature of the forward reaction (exothermic to make ammonia) have?

Answer 18

Increasing the pressure will move the equilibrium the wrong way (away from ammonia and towards hydrogen and ammonia)

Therefore the yield would be less if the temperature increases

Question 19

In the Haber process, why is 450 degrees a ‘compromise’?

Answer 19

because the forward reaction to make ammonia is exothermic, high temperatures move the equilibrium the wrong way - away from ammonia and towards hydrogen and nitrogen

however, low temperature means a slower rate of reaction

so they increase the temperature anyway, to get a much faster reaction

This means they do not get maximum yield

Question 20

What does the iron catalyst do in the Haber process and why is it important

Answer 20

it increases the rate of reaction, which gets it to the equilibrium proportions more quickly

However, it does NOT affect the position of equilibrium (the % yield)

It is important because with out the catalyst, the temperature would have to be raised even further to get a quick enough reaction, and that would reduce the percentage yield even further

Question 21

In gaseous reactions, what happens if you lower the pressure?

Answer 21

It will encourage the reaction which produces more volume

Question 22

Catalysts speed up both the forward and backward reactions by…….

Answer 22

The same amount

Question 23

Adding a catalyst means the reaction reaches equilibrium………., but you end up with ……… as you would without the catalyst

Answer 23

quicker

the same amount of product

Question 24

What are the raw materials for the Haber process?

Answer 24

nitrogen

hydrogen

Question 25

Why is a temperature of 450 degrees an optimum for the Haber process?

Answer 25

because a higher temperature would reduce yield

and a lower temperature would reduce the rate of reaction

Question 26

Explain when a reversible reaction reaches equilibrium

Answer 26

equilibrium is achieved when the rate of the forward reaction is exactly the same as the rate of the backward reaction

Question 27

Explain why the best yield for ammonia is produced at a high pressure

Answer 27

a high pressure will favour the reaction that produces the least number of molecules

(eg the forward reaction to form ammonia)

Question 28

Explain why, when a reversible reaction reaches equilibrium, the reaction appears to have stopped

Answer 28

forward and backward reactions have the same rate

so there is no overall change in quantities of reactants and products

Question 29

Explain how a catalyst speeds up a reaction

Answer 29

provides an alternative reaction pathway

which has a lower activation energy