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Student Activity: Rates of Reaction - Catalysts and Enzymes (LA0039)

Author: . Cook
Published: 12th September 2002
Lesson plan:  LP0039
Introduction
The rate of a reaction is a measure of how fast it happens. Many factors can affect the rate of a reaction, the addition of a catalyst or enzyme is one of them. In the following experiments we will investigate how the addition of a catalyst or enzyme affects the rate of a reaction.
The decomposition of hydrogen peroxide (LF0039a)
Open the Crocodile Chemistry simulation file LF0039a.

In this simulation we are going to look at the decomposition of hydrogen peroxide. The word equation for this reaction is:

hydrogen peroxide water + oxygen

  1. Look at test tube A. Can you see anything happening?
  2. By itself hydrogen peroxide decomposes very slowly. Now add the catalyst (manganese(IV) oxide) to test tube B. What do you see happening in this test tube? What gas is being produced?
  3. What do these observations tell us is happening in test tube B? What does the addition of the catalyst do to the rate of the reaction?
  4. At the end of the reaction the manganese(IV) oxide remains chemically unchanged.
A catalyst is a substance that increases the rate of a reaction but remains chemically unchanged at the end of the reaction.
Fermentation (LF0039b)

Open the Crocodile Chemistry simulation file LF0039b.

Enzymes are catalysts in biological systems (inside living things). For example there are lots of reactions inside our body that depend on enzymes to make them happen.

In this simulation we are going to look at the fermentation of sugar solution. The word equation for this reaction is:

sugar ethanol + carbon dioxide
  1. Look at test tube containing the sugar solution. Can you see anything happening? The test for carbon dioxide gas is that it turns limewater milky. Look at test tube containing the limewater. Is the limewater milky?
  2. Yeast is a living thing (it is a fungus) and so it contains enzymes. Now add the yeast to test tube containing the sugar solution. What do you see happening in the test tube? Look at test tube containing the limewater. Is the limewater milky?
  3. What do these observations tell us is happening? What does the addition of the yeast (containing enzymes) do to the rate of the reaction?
  4. There are lots of enzymes and catalysts used in everyday life and in industry. For example a catalyst is used in the manufacture of ammonia and enzymes are used in baking. Can you think of some more examples of where catalysts and enzymes are used?
Enzymes are catalysts in biological systems.
Extension activity
  1. In the activity 'Fermentation' above, sugar solution is fermented to produce ethanol. However the amount of ethanol in the mixture cannot go above roughly 15%, as at this level the yeast is poisoned and the reaction stops. Can you think of a way to get pure ethanol from this mixture? Try your ideas out in the simulator.
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