Untitled - Kelly Walsh High School

188 CHEMISTRY FOR THE UTTERLY CONFUSED
Get Started
Our goal in this chapter is to help you learn about kinetics—those factors that
affect the speed of reactions. We will be discussing the concept of half-lives; you
will see this concept again in Chapter 20 on Nuclear Chemistry. It will be necessary
in some of the problems to solve for an exponential quantity along with
the use of the ln and the e x functions, so you might want to refresh yourself with
your calculator manual. And don’t forget!—Practice, Practice, Practice.
13-1 Reaction Rates
Many times, we can use thermodynamics to predict whether a reaction will
occur spontaneously, but it gives very little information about the speed at
which a reaction occurs. Kinetics is the study of the speed of reactions. It is
largely an experimental science. Some general qualitative ideas about reaction
speed may be applied, but accurate quantitative relationships require that we
collect experimental data.
In order for a chemical reaction to occur, there must be a collision between the
reactants at the correct place on the molecule, the reactive site. That collision is
necessary to transfer kinetic energy in order to break old chemical bonds and
reform new ones. If the collision doesn’t transfer enough energy, no reaction
will occur.
In general, five factors can affect the rates of chemical reaction:
1. Nature of the reactants—Large, slow-moving complex molecules will tend
to react slower than smaller ones because there is a greater chance of collisions
occurring somewhere else on the molecule rather than the reactive
site. Also, if the molecules are slow moving, the number of collisions will be
smaller.
2. The temperature—Increasing the temperature normally increases the reaction
rate since each species has a higher kinetic energy and the number of
collisions is increased. This increases the chance that enough energy will be
transferred during collisions to cause the reaction.
3. The concentration of reactants—Increasing the concentration of the reactants
(or pressure, if gases are involved) normally increases the reaction
rate due to the increased number of collisions.