Kinetics is the study of the rate of reaction and the factors that affect it.
In order for a reaction to occur, the molecules must collide in the correct orientation with the minimum energy needed for the transition from the reactants to the products (the activation energy). Only a very few collisions meet these requirements and result in a reaction.
Reaction Coordinate Diagrams
Reaction coordinate diagrams show the energy of the reactants, the activation energy up to the activated complex, or transition state (the in-between state between the reactants and the products), and the energy of the products. The overall energy change of the reaction is also shown.
Factors affecting rate
Increasing the temperature increases the number of collisions, and also the number of collisions with the needed energy. Therefore, increasing temperature increases the rate of reaction. Increasing the concentration or the surface area also increases the number of collisions, therefore increasing the chance that a successful collision will occur—which increases rate. Adding a catalyst, a species that increases the rate of reaction without being used up in the reaction, also increases the rate.
Reaction mechanisms are a set of elementary steps. Each elementary steps show which molecules must collide at one time in order to produce a reaction. The elementary steps add up to the overall chemical reaction. The slowest elementary step is the rate determining step. The reaction rate law can be written from the correct rate determining elementary step—but it cannot be written from the overall chemical reaction. One way of evaluating the possibility of a proposed reaction mechanism is to see if it matches the experimentally found rate law.
Differential rate laws relate the rate of reaction to the concentration of the reactants. Each reactant’s concentration is taken to a power, or “order”, that corresponds to the number of that species that must collide in the rate determining step. The rate law has a rate law constant that is different for each reaction at each temperature. Integrated rate laws relate the concentration of a species over time. If one rate law is known, the other rate law can be found—they come in “matched” sets. The half-life (time that it takes for half of the reactants to react away) can be found using the integrated rate law and setting the [A] at t1/2 to ½[A]0.
Rate law constants and activation energy
The higher the activation energy, the less often a collision will result in a successful reaction. Therefore, the higher the activation energy the lower the temperature. The Arrhenius equation relates the rate law constant to the activation energy at a given temperature.