Why do chemical reactions tend to speed up when the concentration of reactants is increased?

Answer by  labgirl (333)

Bonds cannot form if the atoms are too far apart. Increasing the concentration increases the probability two atoms will encounter one another and react. If you increase the number of people walking around a room blindfolded, they are more likely to run into one another. This is similar.

Answer by  willard (874)

Chemical reactions occur when the reactant molecules "collide" with one another. When the concentration of reactants is increased, these collisions become more frequent.