It appears obvious to say that different substances are likely to react at different rates. Moreover, for most reactions, the rate of the reaction is dependent on one or more reactant concentrations:
rate = k[A]m[B]n…
Here, k is the rate constant, a proportionality constant independent of reactant concentrations that is specific for a particular reaction at a particular temperature. (We will revisit this equation and discuss the implication of the exponents.) Because k is independent of reactant concentrations, it is a useful value when comparing rates of different reactions.
Just like the equilibrium constant, the rate “constant” is only constant at a given temperature. (Note that the equilibrium constant is a upper-case K, while the rate constant is a lower-case k.) The value of the rate constant is determined by three main factors, the first two of which depend on temperature:
- To react, molecules must come close enough to each other to exchange energy and perhaps to break and form bonds; that is, molecules must collide. The rate constant is proportional to the rate of collisions:
- For a reaction to occur, there must be sufficient energy in the reactant molecule or molecules to allow electrons to rearrange to break and/or form bonds. The smaller the energy required, the larger the rate constant is.
- The reacting molecules must collide in an orientation that allows the reaction to proceed; that is, even though the molecules have enough energy, a collision is more likely to result in reaction when the molecules are oriented in certain ways with respect to each other.
Let’s consider these factors in more detail from a molecular perspective.