Concentration
A chemical reaction will occur only if the particles of the reacting substances are allowed to come into contact. This is based on the kinetic theory:
The rate of reaction would be expected to depend on the frequency with which the particles collide, which in turn will depend (among other factors) on their density, i.e. on their concentration. The more crowded the particles are, the more often we should expect them to bump into each other.
Surface Area
A solid in a solution can only react when particles collide with the surface. The bigger the area of the solid surface, the more particles can collide with it per second, and the faster the reaction rate is. You can increase the surface area of a solid by breaking it up into smaller pieces, therefore, a powder has the largest surface area and will have the fastest reaction rate. This is why catalysts are often used as powders.
The Addition of a Catalyst
A catalyst is a substance which alters the rate of a chemical reaction, but remains chemically unchanged at the end of a reaction. The catalyst itself does not take part in the reaction. It is not changed by the reaction, it is not used up during the reaction, and it is still there when the reaction is complete.
A catalyst is usually a transition metal, a transition metal oxide, or an enzyme in living cells. An exception is aluminium oxide, used in the Cracking of Hydrocarbons.
A catalyst works by providing a convenient surface for the reaction to occur. The reacting particles gather on the catalyst surface and collide more frequently with each other and more of the collisions result in a reaction between particles because the catalyst can lower the activation energy for the reaction. A catalyst is often used as a powder, so that it has a bigger surface area per gram.
An example of an experiment which uses a catalyst is:
Hydrogen peroxide oxygen + water
2H2O2(aq) O2(g) + 2H2O(l)
Hydrogen peroxide is stable at room temperature, the presence of a catalyst however, may cause it to decompose.
The catalyst used is Manganese(IV) oxide - MnO2(s). Using more catalyst will show an increase in reaction rate. This is because more catalysts will have a greater surface area for the reaction to take place. The reaction can be performed using (the same amount) of different catalysts, to compare how well each catalyst works for the same reaction.
As you can see from the graph, Manganese(IV) oxide - MnO2(s), is the best catalyst. The gradient of the plot is greater (steeper) than the other two. Copper(II) oxide - CuO(s), is not as good as manganese(IV) oxide but is better than zinc oxide - ZnO(s). However, without a catalyst, hydrogen peroxide will not decompose.