What does the contact process make?
sulphuric acid (H₂SO₄)
What are the raw materials used in the contact process?
Where is suplhur available?
in fossil fuels and suphide ores
What is a three step brief summary of the contact process?
make sulphur dioxide
convert the sulphur dioxide into sulphur trioxide (reversible reaction)
convert the sulphur trioxide into concentrated sulphuric acid
What is stage 1?
Making the sulphur dioxide
What are the two ways in which you can make sulphur dioxide? (Stage 1)
This can either be made by:
burning sulphur in an excess of air
by heating sulphide ores, like pyrite, in an excess of air
When making the sulphur dioxide, why is an excess of air is used? (stage 1)
so that the sulphur dioxide produced is already mixed with oxygen for the next stage
What is the equation for burning sulphur in an excess of air? (Stage 1)
S(s) + O2(g) → SO2(g)
What is the equation for heating sulphide ores in an excess of air? (Stage 1)
4FeS2(s) + 11O2(g) → 2Fe2O3(s) + 8SO2(g)
What is stage 2?
Converting the sulphur dioxide into sulphur trioxide:
sulfur dioxide reacts with more oxygen to make sulfur trioxide
What is the equation for making sulphur trioxide? What type of reaction is this? (Stage 2)
2SO2(g) + O2(g) ⇌ 2SO3(g)
This is a reversible reaction
In stage 2, which direction is exothermic?
2SO2(g) + O2(g) ⇌ 2SO3(g)
the formation of sulphur trioxide is exothermic (to the right)
What are the conditions of stage 2?
pressure: 1 atmospheres - 2 atmospheres
catalyst: vanadium (V) oxide, V2O5
Explain the conditions of stage 2: temperature
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)
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)
Explain the conditions of stage 2: pressure
There are 3 molecules on the left-hand side of the equation, but only 2 on the right:
2SO2(g) + O2(g) ⇌ 2SO3(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
Explain the conditions of stage 2: catalyst
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
What is stage 3?
Converting the sulphur trioxide into sulphuric acid
What happens during stage 3?
The sulphur trioxide is first dissolved in concentrated sulphuric acid:
H2SO4(l) + SO3(g) → H2S2O7(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) → 2H2SO4(l)
What is the equation for fuming sulphuric acid or oleum?
In stage 3, why can't converting the sulphur trioxide into sulphuric acid be done by simply adding water to the sulphur trioxide?
The reaction is so uncontrollable that it creates a fog of sulphuric acid
What are four uses of sulphuric acid?
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
Explain what fertilisers are used for and the problems that they cause when they are washed into rivers
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
Write the balanced equation for the combustion of hydrogen sulphide (H2S) to form sulphur and water
2H2S + O2 → 2H2O + 2S
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)
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
State one advantage of using a pressure higher than two atmospheres for the oxidation of sulphur dioxide
Explain you answer
the same amount of gas particles into a smaller volume
there are more frequent, successful collisions in the same amount of time
Write an equation for the reaction of sulphur trioxide with water
SO3 + H2O → H2SO4