CHAPTER
14
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. Kinetics is the study of the speed of reactions and is largely an
experimental science. Some general qualitative ideas about reaction speed may be
developed, but accurate quantitative relationships require 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 chemi-
cal bonds and reform product ones. If the collision doesn’t transfer 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 a chemical reaction:- 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. - 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. - 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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