D24.1 Rate Constant

The rate of a chemical reaction is usually defined as the change in concentration of a reactant or a product per unit time. Different substances react at different rates. Moreover, for most reactions, the rate of the reaction is dependent on one or more reactant concentrations:

Here, k is the rate constant, a proportionality constant independent of reactant concentrations that is specific to a particular reaction at a particular temperature. (We will revisit this equation later 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“.) In our molecular-level exploration of chemical reactions, we have already discussed qualitatively several aspects of a reaction that would influence its rate. To summarize, there are three main factors that determines the value of k, and the first two depend on temperature:

1. 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 when all concentrations are 1 M:
   
2. 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. If all else are equal, a smaller energy requirement (smaller activation energy) leads to a larger rate constant.
3. The reacting molecules must collide in an orientation that allows the reaction to proceed; that is, even if 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. This is the steric factor .

Next, we will consider the relationship between rate constant and temperature quantitatively.