How a catalyst increases the rate of a reaction?

There are two ways in which catalysts work. You already know that when two different molecules bump into each other, they might react to make new chemicals. We usually talk about "collisions" between molecules, it would be much simpler to say that the molecules bumped into each other. How fast a chemical reaction is depends upon how frequently the molecules collide. You have probably been told about the "kinetic theory" which is all about heat and how fast molecules move around. What catalysts are doing when they make a chemical reaction go faster is to increase the chance of molecules colliding. The first method is by "adsorption", the second method is by the formation of intermediate compounds.

Adsorption This occurs when a molecule sticks onto the surface of a catalyst. Make sure that you spell this Word correctly; it is not the same as absorption. Here is an example: it is possible to use Platinum as a catalyst to make sulphur Trioxide from Sulphur Dioxide and Oxygen. Sulphur Trioxide is very important because it is used to make Sulphuric acid which is needed for car batteries. The molecules of the two gases (Sulphur Dioxide and Oxygen) get adsorbed (stuck onto) the surface of a Platinum catalyst. Because the two molecules are held so close together, it is more likely that they will collide and therefore react with each other. The Sulphur Trioxide easily falls off the catalyst leaving space for more Sulphur Trioxide and Oxygen.

Intermediate Compounds Many catalysts, including all enzymes" work by forming intermediate compounds. What happens is very simple: the chemicals involved in the reaction combine with the catalyst making an intermediate compound, but this new compound is very unstable. When the intermediate compound breaks down it releases the new compounds and the original catalyst.