The Contact Process

Question 1

What does the contact process make?

Answer 1

sulphuric acid (H₂SO₄)

Question 2

What are the raw materials used in the contact process?

Answer 2

suplhur

air

water

Question 3

Where is suplhur available?

Answer 3

in fossil fuels and suphide ores

Question 4

What is a three step brief summary of the contact process?

Answer 4

make sulphur dioxide

convert the sulphur dioxide into sulphur trioxide (reversible reaction)

convert the sulphur trioxide into concentrated sulphuric acid

Question 5

What is stage 1?

Answer 5

Making the sulphur dioxide

Question 6

What are the two ways in which you can make sulphur dioxide? (Stage 1)

Answer 6

This can either be made by:

burning sulphur in an excess of air

by heating sulphide ores, like pyrite, in an excess of air

Question 7

When making the sulphur dioxide, why is an excess of air is used? (stage 1)

Answer 7

so that the sulphur dioxide produced is already mixed with oxygen for the next stage

Question 8

What is the equation for burning sulphur in an excess of air? (Stage 1)

Answer 8

S_(s) + O_2(g) → SO_2(g)

Question 9

What is the equation for heating sulphide ores in an excess of air? (Stage 1)

Answer 9

4FeS_2(s) + 11O_2(g) → 2Fe₂O_3(s) + 8SO_2(g)

Question 10

What is stage 2?

Answer 10

Converting the sulphur dioxide into sulphur trioxide:

sulfur dioxide reacts with more oxygen to make sulfur trioxide

Question 11

What is the equation for making sulphur trioxide? What type of reaction is this? (Stage 2)

Answer 11

2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

This is a reversible reaction

Question 12

In stage 2, which direction is exothermic?

Answer 12

2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

the formation of sulphur trioxide is exothermic (to the right)

Question 13

What are the conditions of stage 2?

Answer 13

temperature: 450^oC
pressure: 1 atmospheres - 2 atmospheres
catalyst: vanadium (V) oxide, V₂O₅

Question 14

Explain the conditions of stage 2: temperature

Answer 14

The position of equilibrium needs to shift to the right in order to produce more sulphur trioxide

The forward reaction (the production of sulphur trioxide) is exothermic

If you lower the temperature, the p.o.e. will move to counteract this (move to the right) to produce more heat (Le Chatelier’s Principle)

However:

If the temperature is too low, the rate of reaction becomes too slow; it is not economical

A compromise is chosen:

450°C is a compromise temperature producing a fairly high proportion of sulphur trioxide (percentage yield), but in a very short time (rate of reaction)

Question 15

Explain the conditions of stage 2: pressure

Answer 15

There are 3 molecules on the left-hand side of the equation, but only 2 on the right:

2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

If you increase the pressure, the p.o.e. will move to counteract this (move to the right) to lower the pressure (Le Chatelier’s Principle)

High pressure also speeds up the reactions as a higher pressure pushes the same amount of gas particles into a smaller volume ensuring that there are more frequent, successful collisions in the same amount of time

However: Economic Considerations

Even at these relatively low pressures, there is a 99.5% conversion of sulphur dioxide into sulphur trioxide. The very small improvement that you could achieve by increasing the pressure isn’t worth the expense of producing those high pressures

Question 16

Explain the conditions of stage 2: catalyst

Answer 16

The catalyst does not effect the p.o.e.

Its only function is to speed up the reaction by lowering the activation energy

Without the catalyst, the reaction is too slow

Question 17

What is stage 3?

Answer 17

Converting the sulphur trioxide into sulphuric acid

Question 18

What happens during stage 3?

Answer 18

The sulphur trioxide is first dissolved in concentrated sulphuric acid:

H₂SO_4(l) + SO_3(g) → H₂S₂O_7(l)

The product is known as fuming sulphuric acid or oleum

This can then be reacted safely with water to produce concentrated sulphuric acid:

H₂S₂O₇(l) + H₂O_(l) → 2H₂SO_4(l)

Question 19

What is the equation for fuming sulphuric acid or oleum?

Answer 19

H₂S₂O_{7 (l)}

Question 20

In stage 3, why can’t converting the sulphur trioxide into sulphuric acid be done by simply adding water to the sulphur trioxide?

Answer 20

The reaction is so uncontrollable that it creates a fog of sulphuric acid

Question 21

What are four uses of sulphuric acid?

Answer 21

to make fertilisers for farmers to improve the amount of nutrients in the soil - this increases plant growth

to manufacture detergents

paints - sulphuric acid is used to make titanium dioxide, which is a white pigment that’s used in paints

dyes

Question 22

Explain what fertilisers are used for and the problems that they cause when they are washed into rivers

Answer 22

fertilisers are used to increase crop yield by providing nutrients

when washed into rivers, this causes eutrophication: plants in the rivers grown and use up oxygen so the fish die

unsuitable drinking water

Question 23

Write the balanced equation for the combustion of hydrogen sulphide (H₂S) to form sulphur and water

Answer 23

2H₂S + O₂ → 2H₂O + 2S

Question 24

Calculate the volume of sulphur dioxide and the volume of oxygen needed to produce 100 dm³ of sulphur trioxide

(You should assume 100% conversion and that all volumes are measured at the same temperature and pressure)

Answer 24

volume of sulphur dioxide:

SO₂ : SO₃

2 : 2

1 : 1

100 : 100

= 100 dm³

volume of oxygen:

O₂ : SO₃

1 : 2

50 : 100

= 50 dm³

Question 25

State one advantage of using a pressure higher than two atmospheres for the oxidation of sulphur dioxide

Explain you answer

Answer 25

faster reaction

the same amount of gas particles into a smaller volume

there are more frequent, successful collisions in the same amount of time

Question 26

Write an equation for the reaction of sulphur trioxide with water

Answer 26

SO₃ + H₂O → H₂SO₄