A chemical reaction is characterized by the change of reactant chemicals (or simply reactants) into product chemicals (or products). The rate at which this change takes places is called the reaction rate. Some reactions occur slowly - like the rusting of the steel body of an automobile. This is a low rate of reaction. Other reactons - like the combustion (i.e., explosion) of hydrogen gas. Hydrogen combustion is a reaction that occurs at a high rate. 

Reaction rates can be altered and controlled by changing a few variables. Some variables affecting the rate of a reaction are concentration of aqueous-state reactants, temperature at which the reaction occurs, the exposed surface area of solid-state reactants, and whether or not a catalyst is used. 
 

The rate at which a reaction takes place depends on several variables. The use of a catalyst increases the rate at which the reaction takes place. By definition, a catalyst is a substance that increases the rate of a reaction without itself being used up.

The concentration-time plots shown here provide information about the rate of the reaction. The slope of these plots are equal to the reaction rate. Rates are greatert wherever the slope of the line is greatest. A catalyzed reaction would be one having a greater slope.