Rate Constant and Factors Affecting Reaction Rate

Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach you what you need to know about rate constant and factors affecting reaction rate of kinetics. Watch this video to get all the mcat study tips you need to do well on this section of the exam!
Chemical reactions occur more quickly when lots of molecules of reactant are colliding frequently and with sufficient energy to form the transition state of a reaction. A reaction rate can be experimentally determined by measuring the change in moles of reactants or products over time, but reaction rate can be thought of more rigorously. The rate constant and the Arrhenius equation are tools for quantitatively understanding the factors impacting how fast a reaction will occur.
Rate constant
The rate constant “k” is the part of a rate law which describes the dependency of a reaction on temperature and activation energy. The Arrhenius equation describes the factors contributing to the rate constant.
For the purposes of the MCAT, A is a constant which can be disregarded. Ea is activation energy, or the energy required to overcome the greatest energetic barrier of a reaction. R is the gas constant and T is temperature. The math here is not important, but you should understand the relationship between the rate constant, activation energy, and temperature.
Relationships in the Arrhenius equation
From the Arrhenius equation, note that the rate constant is directly proportional to temperature and inversely related to activation energy. If temperature increases, kinetic energy increases, and so collisions between molecules will occur more frequently and with greater energy. Chemical reactions depend on molecules colliding in the proper orientation with sufficient energy, therefore, increasing temperature will increase reaction rate.
We can see visually on a reaction coordinate diagram that the higher in energy a transition state is, the more difficult it is to overcome its energetic barrier. Quantitatively, it makes sense that the rate constant is inversely proportional to activation energy (Ea). Conversely, adding a catalyst reduces this energetic barrier, thus reducing the numeric value of Ea and increasing the value of the rate constant k.
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