How does a catalyst affect the rate of a chemical reaction?

For a reaction to occur, two molecules have to bump into each other. What a catalyst does in a reaction is make them collide more often. This then leads the reaction to speed up faster. An example of a catalyst would be an enzyme(s). One way that enzymes work in the human body would be in the case of carbonic anhydrase (salt). What the enzyme does with the carbonic anhydrase, is speed up the transfer of carbon dioxide from cells to the blood. Another example of how an enzyme would be the enzyme peptidase. This enzyme breaks peptide bonds in proteins.

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Resources:
"Catalyst and Inhibitors." Chem4Kids.com. N.p, n.d. Web. 2 Mar. 2010. <http://www.chem4kids.com/files/react_catalyst.html>.
"The Central Role of Enzymes as Catalysts." The Cell. N.p, n.d. Web. 7 Mar. 2010. <http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cooper∂=A279>.

Image Resource:
http://www.cartage.org.lb/en/themes/sciences/lifescience/GeneralBiology/Biochemistry/Enzymes/OrganicCatalysts/OrganicCatalysts.htm