explain how a catalyst can affect the rate of reaction but not be in the overall equation.?
a catalyst is a substance that speeds up the rate of reactions that would already happen (the reactions are spontaneous) but would take a long long time to occur. Every reaction proceeds from a level of high energy to a lower level of energy, but in order to start going downhill you need to get up, in affect getting more energy than the reactants have now. This is because the transition state, or what the reactnat/product is in the middle of the reaction, is less stable and requires more energy than the reactants. Catalysts lower the extra energy needed (called activation energy) to a level that the reactants already have, and the reaction occurs.
Simply, the catalyst provides a better way for the reaction to occur (some won't work without the catalyst) and usually a favorable reaction will run faster if there is a catalyst. There are many ways for this to happen. The catalyst can provide more surface area for a reaction to occur, it can do an adsorption process where one of the reactants sticks to the surface and exposes a portion of the molecule which is more favorable to the reaction. It can lower the energy required for the reaction to occur (same effect as increasing temperature) by favoring an intermediate step in the reaction. Lots of different ways, some not fuly identified or understood. The catalyst people who work with the platinum metals groups are notorious for keeping their mixtures secret. A better gasoline catalyst for refineries is worth billions in profits. It is not considered in the equation because you get back what you start with even if one of the intermediate steps involves changing the chemical composition of the catalyst and then has it returning to its original state with the formation of the product.
Catalysts act to increase the rate of reaction, for example by providing an alternative reaction pathway which lowers the activation energy of the reaction i.e. increasing the likelihood of successful collisions between the reactants. However, they are not used up during the course of the reaction, and at the end you have exactly the same mass of catalyst as you started with, unlike the reactant(s), which will be used up to form product(s). So, the overall equation of the reaction does not include the catalyst because it only needs to show the substances which are used up or formed during the course of the reaction i.e. the reactants and products.
