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Kinetics
IN THIS CHAPTERSummary:Thermodynamics often can be used to predict whether a reaction
will occur spontaneously, but it gives very little information about the speed
at which a reaction occurs. Kineticsis the study of the speed of reactions and
is largely an experimental science. Some general qualitative ideas about reac-
tion speed may be developed, but accurate quantitative relationships require
that experimental data to be collected.
For a chemical reaction to occur, there must be a collision between the
reactant particles. That collision is necessary to transfer kinetic energy, to break
reactant chemical bonds and reform product ones. If the collision doesn’t trans-
fer enough energy, no reaction will occur. And the collision must take place with
the proper orientation at the correct place on the molecule, the reactive site.
Five factors affect the rates of chemical reaction:
1 .Nature of the reactants—Large, complex molecules tend to react more
slowly than smaller ones because statistically there is a greater chance of
collisions occurring somewhere else on the molecule, rather than at the
reactive site.
2 .The temperature—Temperature is a measure of the average kinetic energy
of the molecules. The higher the temperature, the higher the kinetic energy
and the greater the chance that enough energy will be transferred to cause
the reaction. Also, the higher the temperature, the greater the number of
collisions and the greater the chance of a collision at the reactive site.
3 .The concentration of reactants—The higher the concentration of
reactants, the greater the chance of collision and (normally) the greater
the reaction rate. For gaseous reactants, the pressure is directly related to
the concentration; the greater the pressure, the greater the reaction rate.
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