Catalysts

A catalyst is a substance that speeds up chemical reactions without being used up during the reaction.

The catalyst can be recovered chemically unchanged and be reused at the end of the reaction.

We can look at an example of a reaction where a catalyst has been used to speed up the reaction. The example we will use is the decomposition (breaking down) of hydrogen peroxide.

The effect of Catalysts on the rate of a Chemical Reaction

Background:

Hydrogen peroxide is a compound that very slowly decomposes to make oxygen gas (and water). This (and every other chemical reaction) can be represented using equations. Notice there is an arrow in chemical equations and never an equal sign (=).

Word equation: hydrogen peroxide → water + oxygen

Formula Equation: H₂O₂ → H₂O + O₂

This decomposition reaction is shown in the video below. (Note: It is dangerous to drink hydrogen peroxide.)

In this reaction the black powder (manganese dioxide) is used as a catalyst and speeds up the decomposition of hydrogen peroxide. The managanese dioxide is not unchanged at the end of the reaction and has not been used up.

This could be proven by weighing the dry any manganese dioxide at the beginning and end of the experiment.

A catalyst increases the rate of a reaction but does not change how much product is made. A catalyst can allow reactions to take place at lower temperatures.

In the diagram below (source) you can see that in the presence of more catalyst the reaction takes place more quickly. With no cataslyst present the reaction still continues but at a very slow rate (line 1).

Enzymes

Enzymes are biological catalysts. They are protein molecules that speed up chemical reactions in living cells.

For example:

The enzyme catalase speeds up the decomposition of hydrogen peroxide into water and oxygen.

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