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In This Video :
4. The Catalysts
To increase or decrease the speed of the chemical reactions, it is necessary to add certain chemical substances to them that do not affect the nature of the products, these chemical substances are known as catalysts.
A catalyst
It is a chemical substance which changes the rate of the chemical reaction without changing or being used up.

Chemical reactions in which catalysts are used are known as catalytic reactions, which are divided into two types according to the role of the catalyst as in the following diagram :

► The following activity shows the effect of manganese dioxide as a catalyst on the speed of decomposition of hydrogen peroxide.
Activity (4): The effect of the catalyst on the speed of the chemical reaction:
Observation:
Increasing the amount of the evolved bubbles on adding manganese dioxide powder to hydrogen peroxide.
Explanation:
Manganese dioxide is a catalyst that increases the speed of decomposition of hydrogen peroxide into water and oxygen gas which evolves as bubbles.
Conclusion:
The speed of the chemical reaction increases by adding a catalyst.

Enzymes
Enzymes
They are chemical substances produced by the body of a living organism, act as catalysts that increase the speed of biological reactions.


•The biological reactions occur in the presence of enzymes are more rapid than that without the enzymes, by thousands or even by millions times.

•Each enzyme has a specific function, a molecule of one enzyme can do its function million
times per one minute.

•Without enzymes, man can never breathe, move or even digest food.


► The following activity shows the effect of oxidase enzyme which exists in sweet potato on the speed of decomposition of hydrogen peroxide.

Activity (5): The effect of enzymes on the speed of the chemical reaction:
Observation:
Increasing the number of the evolved bubbles on adding a piece of sweet potato to hydrogen peroxide.
Explanation:
Oxidase enzyme in sweet potato acts as a catalyst which increases the rate of decomposition of H2O2 into water and oxygen.
Conclusion:
The speed of the chemical reaction increases by adding an enzyme.

Catalytic converter
It is a metallic can that exists in most modern cars to treat the harmful gases emitted from the engine.

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