Kinetics Concepts

Chemical reactions are dynamic processes in which matter and energy change continuously.  Up until now we have used a balanced equation and stoichiometry to determine which products will form and how much of each product will form.  Chemical kinetics is the study of the rate at which reactions occurs.  Will products form quickly or slowly?  Will it take 10 years for a reaction to occur in the troposhpere or does that reaction occur instantaneously?  These are the questions that chemical kinetics can answer.   The focus of study in chemical kinetics is on:
 

• reaction rates the rate at which the reactant becomes a product

• reaction mechanisms , the stepwise molecular events through which a reacton occurs.

Each reaction has its own rate although the rate can be affected by factors the influence the frequency of collisions, and energy of collisions between reactant molecules.  In the section on the ideal gas law  we saw that the kinetic theory of gases predicts that increasing the temperature of a gas sample both increases the number of collisions between gas particles and increases the average energy involved in those collisions. It is easy to see how increases in sample temperature would also increase the rate of a reaction.  The energy that must be present during a collision to produce a change from reactants to products is called the activation energy, E_a .

Another way to increase the reaction rate would be to increasing the gas concentration.  With a larger number of gas particles present in a gas sample the probability of a collision occuring would also increase.  Adding catalyst , a substance that, when added to reactants, reduces the activiation energy during a reaction by providing an alternate pathway can also be used to accelarate reaction rates.  Catalysts are regenerated at the end of the reaction,
 

Temperature Effects

Catalyst Effects

Here is a link to a kinetics simulation . This simulation considers the reversible reaction between two reactants. The reactants and products are represented by differently colored balls. When a collision with the appropriate energy occurs, two products are produced as follows:
 

The simulation allows you to explore the some of the factors that can alter the reaction rate.  You can vary the number of particles however, very large numbers of particles will slow down your computer without adding much new information.  The reaction rate constant can also be varied in this simulation.  The reaction rate constant is the constant the describes the probability of a reaction occuring per collision for the conditions set in the simulation.  The reaction rate constants for the forward reaction k_f, and the reverse reaction k_r, can be set independantly.  To start or stop the simulation click anywhere on the image.  You can add a strip chart that graphically records the number of product particles.  Activating the stripchart slows the simulation down so you should use it sparingly.