Catalysts and Reaction Rates
Catalyst
Adding a catalyst increases the reaction rate by lowering the energy required for a successful reaction to take place. A catalyst speeds up a reaction and is released at the end of the reaction, completely unchanged.
In the first catalyst and reaction rate experiment (with manganese dioxide and hydrogen peroxide) it is important to note that hydrogen peroxide can cause burns. The learners should wear safety equipment, as always when handling chemicals. If the concentration of hydrogen peroxide is too high the liquid hydrogen peroxide may splash out of the container along with the oxygen gas. As a result the learners should be particularly careful around the mouth of the containers.
In the second experiment the learners are again working with a strong acid and should follow all the usual safety procedures.
Optional Experiment: Catalysts and reaction rate
Aim
Hydrogen peroxide decomposes slowly over time into water and oxygen. The aim of this experiment is to determine the effect a catalyst has on the reaction rate.
Apparatus
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3% hydrogen peroxide (\(\text{H}_{2}\text{O}_{2}\)), manganese dioxide (\(\text{MnO}_{2}\)) powder, yeast powder
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two beakers or large measuring cylinders
Method
Warning:
Be careful when handling \(\text{H}_{2}\text{O}_{2}\) as it can burn you. We recommend wearing gloves and safety glasses.
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Pour \(\text{30}\) \(\text{cm$^{3}$}\) \(\text{H}_{2}\text{O}_{2}\) into two seperate containers.
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Add a spatula tip of yeast to one container.
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Time how long it takes for the bubbles to stop.
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Repeat with \(\text{MnO}_{2}\) in the second container.
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Compare the effect of the two catalysts.
The balanced equation for this reaction is:
\(2\text{H}_{2}\text{O}_{2}(\text{l})\) \(\to\) \(2\text{H}_{2}\text{O}(\text{l}) + \text{O}_{2}(\text{g})\)
This can also be written:
\(2\text{H}_{2}\text{O}_{2}(\text{l}) + \text{catalyst}\) \(\to\) \(2\text{H}_{2}\text{O}(\text{l}) + \text{O}_{2}(\text{g}) + \text{catalyst}\)
\(\text{H}_{2}\text{O}_{2}\) before the addition of \(\text{MnO}_{2}\) (left) and after the addition of \(\text{MnO}_{2}\) (right).
Results
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Which chemical compounds are acting as catalysts in these reactions?
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What causes the bubbles that form in the reaction?
Conclusions
The bubbles that form are oxygen gas formed through the decomposition of hydrogen peroxide. This would happen over time without the presence of the catalyst. The manganese dioxide speeds up the reaction significantly. The yeast speeds up the reaction, but not as much as the manganese dioxide.
Optional Video: Hydrogen Peroxide + Catalyst
Optional Experiment: Catalysts and reaction rate
Aim
To determine the effect of a catalyst on the average rate of a reaction
Apparatus
Method
Warning:
Do not get hydrochloric acid (\(\text{HCl}\)) on your hands. We suggest you use gloves and safety glasses whenever handling acids. Be especially careful when removing the copper pieces from the test tube.
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Place a few of the zinc granules in the test tube, using tongs.
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Measure the mass of a few pieces of copper and, using tongs, keep them separate from the rest of the copper.
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Add \(\text{20}\) \(\text{cm$^{3}$}\) of \(\text{HCl}\) to the test tube. You will see that a gas is released. Take note of how quickly or slowly this gas is released (use a stopwatch or your cellphone to time this). Write a balanced chemical equation for the chemical reaction that takes place.
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Now add the copper pieces to the same test tube. What happens to the rate at which the gas is produced?
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Carefully remove the copper pieces from the test tube (use tongs), rinse them in water and alcohol and then weigh them again. Has the mass of the copper changed since the start of the experiment?
Results
During the reaction, the gas that is released is hydrogen. The rate at which the hydrogen is produced increases when the copper pieces (the catalyst) are added. The mass of the copper does not change during the reaction.
Conclusions
The copper acts as a catalyst during the reaction. It speeds up the average rate of the reaction, but is not changed itself in any way.
We will return to catalysts in more detail once we have explored the mechanism of reactions later on.
This lesson is part of:
Energy and Chemical Reactions