Chapter 53
Catalysis and industrial processes
53.1 Characteristics of catalysts
53.2 How do catalysts work?
53.3 Effect of catalyst on reversible reactions
53.4 Industrial applications of catalysts
Key terms
Progress check
Summary
Concept map
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53.1 Characteristics of catalysts
A catalyst changes (usually increases) the rate
of a reaction without itself being chemically
changed at the end of the reaction.
Learning tip
Catalysts that speed up reactions are called
positive catalysts. Some catalysts can
slow down fast reactions. They are known
as negative catalysts or inhibitors.
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General characteristics of catalysts
1. Chemically unchanged at the end of reaction
A + catalyst → A−catalyst (unstable intermediate)
+)
A−catalyst + B → A−B + catalyst (regenerated)
____________________________________________________________________
Overall
reaction:
A + B → A−B
The catalyst remains chemically unchanged at
the end of the reaction.
53.1 Characteristics of catalysts
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2. Needed in very small amounts
Example
MnO2(s)
2H2O2(aq) → 2H2O(l) + O2(g)
spatula
a small amount
of MnO2(s)
H2O2(aq)
bubbles of oxygen gas
Figure 53.1 Addition of a small
amount of MnO2(s) catalyses
the decomposition of H2O2(aq).
Learning tip
Increasing the amount of catalyst would increase the
reaction rate. However, there is a limit to this, beyond
which larger amounts of catalyst have no further effect.
53.1 Characteristics of catalysts
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3. Very specific in action
Example
Manganese(IV) oxide catalyses the decomposition
of hydrogen peroxide much better than zinc oxide
and copper(II) oxide do.
4. Improved catalytic effect as the surface area
increases
Example
finely divided Fe(s)
N2(g) + 3H2(g)
2NH3(g)
Think about
53.1 Characteristics of catalysts
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5. Poisoned by small amounts of impurities
Example
Platinum catalyst used in the catalytic converters in
motor cars is easily poisoned by lead compounds.
Catalytic converters can only be used in motor cars
which run on unleaded petrol.
53.1 Characteristics of catalysts
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Key point
General characteristics of catalysts:
• They remain chemically unchanged at the end
of the reaction.
• They are needed in very small amounts.
• They are very specific in action.
• Their catalytic effect can be improved by an
increase in the surface area.
• They may be poisoned by small amounts of
impurities.
Experiment 53.1
Experiment 53.2
Experiment 53.1
Class practice 53.1
53.1 Characteristics of catalysts
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Potential energy
53.2 How do catalysts work?
Providing an alternative pathway with lower
activation energy
Ea’ of
catalysed
reaction
Ea of uncatalysed reaction
intermediates
catalysed reaction
uncatalysed
reaction
reactants
products
Reaction coordinate
Figure 53.2 Energy profiles of a catalysed reaction (two-step) and
an uncatalysed reaction (single-step).
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The activation energy, Ea’, of the catalysed reaction
is lower than that of the uncatalysed reaction (Ea).
Learning tip
The Ea’ of the first step of the catalysed reaction
is the highest energy barrier for the reactants to
overcome before the reaction can occur.
Therefore, it is taken for comparison with the Ea
of the uncatalysed reaction.
53.2 How do catalysts work?
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A catalyst
does not lower the activation energy of
the original reaction pathway.
allows the reaction to proceed in a different
way (an alternative pathway) with lower
activation energy.
Think about
53.2 How do catalysts work?
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Number of particles
fraction of particles having energy
equal to or greater than Ea’
fraction of particles having energy
equal to or greater than Ea
Ea’
Ea
Kinetic energy
Figure 53.4 More particles have energy equal to or greater than the
activation energy of a catalysed reaction.
In the presence of a catalyst, there is an increased
number of effective collisions per unit time.
Reaction rate ↑
53.2 How do catalysts work?
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Key point
A catalyst increases the rate of reaction by
providing an alternative pathway with lower
activation energy for the reaction to proceed.
Example 53.1
Class practice 53.2
53.2 How do catalysts work?
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53.3 Effect of catalyst on reversible reactions
2SO2(g) + O2(g)
V2O5(s)
2SO3(g)
450–600°C
1 atm
Vanadium(V) oxide, the catalyst
increases the rates of both forward and
backward reactions to the same extent.
only shortens the time for the reaction to
reach equilibrium.
The concentrations of the reacting substances
do not change when equilibrium is reached.
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Key point
Catalysts increase the rates of both forward and
backward reactions in an equilibrium to the same
extent. They have no effect on the equilibrium
position.
Class practice 53.3
53.3 Effect of catalyst on reversible reactions
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53.4 Industrial applications of catalysts
Iron as catalyst in Haber process – the manufacture
of ammonia
Haber process
finely divided Fe(s)
N2(g) + 3H2(g)
2NH3(g)
400–450°C, 200 atm
This reaction has very high activation energy
because the reactant molecules have to break the
strong H–H bond and the very strong N≡N bond.
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The reaction proceeds extremely slowly in the
absence of a catalyst even at a very high
temperature.
Adsorption of N2 and H2 molecules onto the
surface of the iron catalyst helps weaken the
strong covalent bonds.
This provides a reaction pathway with lower
activation energy.
53.4 Industrial applications of catalysts
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N2
(a)
H2
surface of iron
(catalyst)
(b)
NH3
(d)
Iron atom
(c)
nitrogen atom
hydrogen atom
Figure 53.5 The catalytic action in the Haber process. (a) N2 and H2
molecules diffuse to the iron surface. (b) N2 and H2 molecules are adsorbed
on the iron surface. (c) N2 and H2 molecules dissociate into atoms as
covalent bonds break. Then the N and H atoms combine to form NH3
molecules. (d) NH3 molecules diffuse away from the iron surface.
53.4 Industrial applications of catalysts
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Ammonia is a raw material for the production of
nitrogenous fertilizers (e.g. ammonium nitrate
and ammonium sulphate)
NH3(aq) + HNO3(aq) → NH4NO3(aq)
ammonium nitrate
2NH3(aq) + H2SO4(aq) → (NH4)2SO4(aq)
ammonium sulphate
Figure 53.6 A bag of ammonium
sulphate fertilizer.
53.4 Industrial applications of catalysts
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Enzymes as catalyst in the production of alcoholic
drinks
In the brewing industry, various enzymes
(biological catalysts) in yeast are used to
produce alcoholic drinks.
Learning tip
Enzymes are proteins which catalyse biochemical
reactions in living systems. They are biological
catalysts.
53.4 Industrial applications of catalysts
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Fermentation
is the biological process by which starch or
sugar is changed to ethanol.
requires the action of enzymes on starch
or sugar in the absence of oxygen.
a
e
b
c
f
d
g
Figure 53.7 Some raw
materials for fermentation.
(a) Grape (b) Wheat (c)
Potato (d) Apple (e) Barley
(f) Rice (g) Maize.
53.4 Industrial applications of catalysts
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diastase (an enzyme from malt)
2 ( C6H10O5 )n(s) + nH2O(l)
nC12H22O11(aq)
starch
maltose
maltase (an enzyme from yeast)
C12H22O11(aq) + H2O(l)
maltose
2C6H12O6(aq)
water
glucose
zymase (an enzyme from yeast)
C6H12O6(aq)
glucose
2C2H5OH(aq) + 2CO2(g)
ethanol
carbon dioxide
53.4 Industrial applications of catalysts
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(a)
(b)
Figure 53.8
(a) Fermentation during the brewing of beer. The foam is produced by the
escaping of carbon dioxide. (b) In modern brewing, the fermentation is carried
out in enclosed containers.
53.4 Industrial applications of catalysts
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The fermentation produces a dilute solution
(roughly 10%) of ethanol in water.
The concentration cannot be higher than 15%
because the yeast dies at higher concentrations.
By distillation, the concentration of ethanol in the
product can be increased.
Figure 53.9 Beer (alcohol content: 3–8%)
and table wine (alcohol content: ~11%)
are produced by fermentation alone.
Brandy (a spirit, alcohol content: 30–60%)
has to be obtained by distillation of the
fermented liquid.
53.4 Industrial applications of catalysts
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Cider
Apple
3–6
Beer
Barley
3–8
Table wine
Grape
∼11
Whisky
Brandy
Vodka
Rum
Barley
Grape
Potatoes
Sugar cane
30–60
Table 53.1 Percentage volume of ethanol in different alcoholic drinks.
53.4 Industrial applications of catalysts
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short rubber tubing
Experiment 53.3
delivery tube
Experiment 53.3
Class practice 53.4
conical flask
solution of
glucose with yeast
Figure 53.10 Fermentation of
limewater glucose in the laboratory.
This reaction is carried out between 25°C and 45°C.
The enzymes in yeast are denatured and stop
working at higher temperatures.
The end of delivery tube is put under limewater.
This provides an air-lock, thus preventing
oxidation of ethanol formed to ethanoic acid.
53.4 Industrial applications of catalysts
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Key terms
1.
2.
3.
4.
catalyst 催化劑
enzyme 酶
fermentation 發酵作用
Haber process 哈柏法
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Progress check
1. What is a catalyst?
2. What are the general characteristics of catalysts?
3. How do catalysts work to increase the rate of a
chemical reaction?
4. What is the difference in the energy profile of a
catalysed reaction and an uncatalysed reaction?
5. How does the Maxwell-Boltzmann distribution
curve help explain the increase in the rate of
chemical reaction as a result of using a catalyst?
6. What is the effect of catalysts on reversible
reactions?
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7. Can you suggest two industrial applications of
catalysts?
8. What is produced in the Haber process? What is
the catalyst used in the process?
9. What is fermentation? What is the catalyst used in
fermentation?
Progress check
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Summary
53.1 Characteristics of catalysts
1.
A catalyst is a substance that changes (usually
increases) the rate of a chemical reaction and
remains chemically unchanged at the end of
the reaction.
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2.
General characteristics of catalysts:
• Chemically unchanged at the end of the
reaction
• Needed in very small amounts
• Very specific in action
• Increased catalytic effect as the surface area
increases
• Poisoned by small amounts of impurities
Summary
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53.2 How do catalysts work?
3.
4.
A catalyst increases the rate of reaction by
providing an alternative pathway with lower
activation energy for the reaction to proceed.
For a catalysed reaction with lower activation
energy, more particles will have energy equal to
or greater than the activation energy of the
catalysed reaction. Therefore, there is an
increased number of effective collisions per unit
time. The reaction rate is thus increased.
Summary
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53.3 Effect of catalyst on reversible reactions
5.
Catalysts have no effect on the equilibrium
position of an equilibrium system because the
rates of both the forward and backward
reactions are increased to the same extent.
53.4 Industrial applications of catalysts
6.
7.
Haber process is the industrial preparation of
ammonia using finely divided iron as the
catalyst.
Enzymes are biological catalysts widely used in
the production of alcoholic drinks by
fermentation.
Summary
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Concept map
CATALYSTS
Increase the rates
of both forward and
backward reactions
same extent
to the ______
No effect on
equilibrium position
General characteristics:
unchanged at
• chemically __________
the end of the reaction
small
• needed in very _______
amounts
• specific in action
• improved catalytic effect as
surface area increases
the ____________
impurities
• poisoned by ___________
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CATALYSTS
working principle
Industrial applications
Provide alternative
reaction pathway
lower Ea
with ______
More particles have
energy equal to or
greater than Ea
Faster reaction
______
Haber
process
Fermentation
produces
produces
Alcoholic
drinks
NH3
catalyst used
catalyst used
Finely divided
iron
__________
Enzymes
__________
in yeast
Concept map
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