Chemistry Reference
In-Depth Information
of molecules of gas, it will be favoured by a
high pressure. In reality, the process is run at
atmospheric pressure. Under these conditions,
about 96% of the sulfur dioxide and oxygen are
converted into sulfur trioxide. The heat produced
by this reaction is used to heat the incoming gases,
thereby saving money.
•
If this sulfur trioxide is added directly to water,
sulfuric acid is produced. This reaction, however,
is very violent and a thick mist is produced.
sulfur trioxide
SO
3
(
g
)
+
+
water
H
2
O(
l
)
sulfuric acid
H
2
SO
4
(
l
)
→
→
This acid mist is very diffi cult to deal with and
so a different route to sulfuric acid is employed.
Instead, the sulfur trioxide is dissolved in
concentrated sulfuric acid (98%) to give a
substance called
oleum
.
sulfuric acid
H
2
SO
4
(
aq
)
Figure 12.7
A Contact process plant used for making sulfuric acid.
The process has the following stages.
•
Sulfur dioxide is fi rst produced, primarily by the
reaction of sulfur with air.
+
+
sulfur trioxide
SO
3
(
g
)
oleum
H
2
S
2
O
7
(
l
)
→
→
The oleum formed is then added to the correct
amount of water to produce sulfuric acid of the
required concentration.
oleum
H
2
S
2
O
7
(
l
)
sulfur
S(
s
)
+
+
oxygen
O
2
(
g
)
sulfur dioxide
SO
2
(
g
)
→
→
+
+
water
H
2
O(
l
)
sulfuric acid
2H
2
SO
4
(
l
)
→
→
•
Any dust and impurities are removed from the
sulfur dioxide produced, as well as any unreacted
oxygen. These 'clean' gases are heated to a
temperature of approximately 450 °C and fed
into a reaction vessel, where they are passed over
a catalyst of vanadium(
v
) oxide (V
2
O
5
). This
catalyses the reaction between sulfur dioxide
and oxygen to produce sulfur trioxide (sulfur(
vi
)
oxide, SO
3
).
sulfur
dioxide
2SO
2
(
g
)
Questions
1
Produce a fl ow diagram to show the different processes
which occur during the production of sulfuric acid by
the Contact process. Write balanced chemical equations
showing the processes which occur at the different
stages, giving the essential raw materials and conditions
used.
2
Both the following reactions are reversible:
(ii).
X
2
(
g
)
+
O
2
(
g
)
→
2XO(
g
)
(ii)
2XO(
g
)
+
O
2
(
g
)
→
2XO
2
(
g
)
+
oxygen
sulfur
trioxide
2SO
3
(
g
) Δ
H
= −197 kJ mol
−1
+
O
2
(
g
)
Suggest a reason why an increase in pressure:
a
does not favour reaction (i)
b
increases the amount of XO
2
produced in reaction (ii).
This reaction is reversible and so the ideas of
Le Chatelier (Chapter 11, p. 178) can be used
to increase the proportion of sulfur trioxide in
the equilibrium mixture. The forward reaction
is exothermic and so would be favoured by low
temperatures. The temperature of 450 °C used
is an optimum temperature which produces
suffi cient sulfur trioxide at an economical rate.
Since the reaction from left to right is also
accompanied by a decrease in the number
Uses of sulfuric acid
Sulfuric acid has many uses in industry. It is such an
important
bulk chemical
that the amount of sulfuric
acid which a country uses in one year can be seen as
a measure of that country's economic development,
that is, how modern or wealthy it is.
