Catalysts
Introduction

Have you ever heard anyone say that an event was 'a catalyst for change'? A teacher might say it of a test, if a student suddenly improved their performance after receiving poor marks. In a sense, the test result was the catalyst that sparked off a reaction in the student, stirring them into action!

In this unit we will be looking at chemical catalysts that affect the rates of chemical reactions.

Catalysis
We can speed up some reactions by adding a substance known as a
catalyst
A catalyst is a substance that alters (usually speeds up) the rate of a chemical reaction, but remains chemically unchanged itself at the end of the reaction.
catalyst
. Look at the reaction in Fig.1 below. It shows the breakdown of hydrogen peroxide solution:

hydrogen peroxide     water   +  oxygen
2 H2O2(aq)    2 H2O(l)  +  O2(g)

Figure 1.   An example of catalysis
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The reaction in Fig.1 takes place rapidly when manganese(IV) oxide is present in the mixture. However, the manganese(IV) oxide is unchanged when the reaction has finished: it acts as a catalyst.

How would you perform a test to identify the gas given off in the experiment in Fig.1?
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At the end of this reaction, when all the hydrogen peroxide has broken down, the catalyst remains chemically unchanged. We could filter off the manganese(IV) oxide and use it again.

So we can define a catalyst in the following way:

A catalyst is a substance that speeds up a chemical reaction but remains chemically unchanged at the end of the reaction
.

A catalyst does not appear in the chemical equation for the reaction that it catalyses. It is the same substance before and after the reaction. We can show its presence by including it above the arrow in the equation:


Manganese(IV) oxide is also a catalyst for the breakdown of potassium chlorate(V). The potassium chlorate(V) forms potassium chloride and oxygen when heated. Which is the correct
word equation
A word equation is a way of describing what we start with and what is formed in a chemical equation, e.g. magnesium  +  oxygen    magnesium oxide.
word equation
for the catalysed reaction?
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There are also some substances that act as 'negative catalysts', slowing down chemical reactions. We call these inhibitors.

Explaining how catalysts work
In order for colliding particles to react together, they must have enough energy to exceed the activation energy needed for the reaction.

We can show this on an energy level (or energy profile) diagram, as in Fig.2:

Figure 2.   Energy level diagram (for an exothermic reaction).
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But look what happens when we add a catalyst:

Figure 3.   The effect of a catalyst on activation energy.
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The catalyst provides a different route for the reaction – a route with a lower
activation energy
The activation energy is the minimum amount of energy required for reactants to form products in a chemical reaction.
activation energy
. This means that more of the particles in the reaction mixture have sufficient energy to react. It's a bit like lowering the bar in a high-jump competition: a lot more people can get over the bar at the lower height.

Which statement about catalysts is correct?
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Examples of catalysis in industry
Catalysts are very important in many chemical processes in industry. They speed up reactions, making processes more efficient. So lots of research goes into finding the best catalyst for particular reactions. Look at the table below which shows the catalysts used in some industrial processes:


Substance manufactured Catalyst used
ammonia iron
sulfuric acid vanadium(V) oxide
nitric acid platinum/rhodium
margarine nickel
alkenes (to make plastics) silica/aluminium oxide
high-density poly(ethene) Ziegler catalyst (titanium(IV) chloride and
triethyl aluminium)

Which of the catalysts in the table above are transition metals or compounds of a transition metal?
  • Iron
    Vanadium(V) oxide
    Platinum
    Rhodium
    Nickel
    Silica
    Aluminium oxide
    Titanium(IV) chloride
    Triethyl aluminium
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Summary


Catalysts speed up chemical reactions but remain chemically unchanged themselves at the end of the reaction.

Catalysts work by lowering the activation energy of a reaction so that a higher proportion of particles have sufficient energy to react.

Figure 4.   Catalysts lower the activation energy of a reaction.
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Exercises
1. Decide whether each of the following statements is true or false.
  • A catalyst is changed chemically at the end of a reaction.
    A catalyst can increase the rate of a reaction.
    Manganese(IV) oxide increases the rate of all chemical reactions.
    A catalyst remains chemically unchanged itself at the end of a reaction.
    A catalyst raises the activation energy of a reaction.
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2. Insert the correct number into the blanks to balance the following equation.
  • H2O2(aq) H2O(l) + O2(g)
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3. Name a catalyst for the reaction above.
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Figure 5.  
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4. A reaction is done with and without a catalyst. Exactly the same quantities of reactants and conditions are used each time. Which of the curves labelled A–C in the graph shown in Fig.5 could represent the line for the catalysed reaction?
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5. Explain how a catalyst works.
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Well done!
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