How Concentration Affects the Rate of a Reaction

Question Which reaction mixture will have the fastest initial rate The mixture in (c) is fastest because it has the highest concentration of reactants and therefore the highest rate of collisions. 

The rate of a chemical reaction generally increases with increasing concentration of the reactants. 

The exact relationship between increases in concentration and increases in reaction rate varies for different reactions and is beyond the scope of this text. For our purposes, it will suffice to know that for most reactions the reaction rate increases with increasing reactant concentration. 

Armed with this knowledge, what can we say about the rate of a reaction as the reaction proceeds Since reactants turn into products in the course of a reaction, their concentration decreases. Consequently, the reaction rate decreases as well. In other words, as a reaction proceeds, there are fewer reactant molecules (because they have turned into products), and the reaction slows down. 

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Here are experiments that can be used to illustrate how concentration affects the rate of a reaction. 

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