Fundamental Chemistry and Hydraulics - Mathematics and Statistics for the Environment - page 362

• Increasing temperature —Two molecules will only react if they have enough energy. By
heating the mixture, the energy levels of the molecules involved in the reaction are raised.
Raising the temperature means that the molecules move faster. This is kinetic theory.
• Increasing concentration of reactants —Increasing the concentration of the reactants
increases the frequency of collisions between the two reactants (collision theory again).
• Introducing catalysts —Catalysts speed up chemical reactions. Catalysts speed up reac-
tions by lowering the activation energy. Only very small quantities of the catalyst are
required to produce a dramatic change in the rate of the reaction. This is the case because
the reaction proceeds by a different pathway when the catalyst is present. Note that adding
more catalyst will make absolutely no difference.
• Increasing surface area —The larger the surface area of a solid, the faster the reaction will
be. Smaller particles have a bigger surface area than larger particles for the same mass of
solid. There is a simple way to visualize this so-called “bread and butter” theory. If we take
a loaf of bread and cut it into slices, we get an extra surface onto which we can spread butter.
The thinner we cut the slices, the more slices we get and the more butter we can spread on
them. Also, by chewing our food we increase the surface area so that digestion goes faster.
• Increasing the pressure on a gas to increase the frequency of collisions between them —
By increasing pressure, molecules are squeezed together so that the frequency of collisions
between them is increased.
Reaction rates are also affected by each reaction's activation energy—the energy the reactants
must reach before they can react.
Note: A catalyst may be recovered unaltered at the end of the reaction.
Forward and backward reactions can occur, each with a different reaction rate and associated
activation energy:
A + B ⇔ C + D
For example, in a dissociation reaction that occurs readily, initially the dissociation takes place at a
faster rate than recombination. Eventually, as the concentration of dissociated ions build up, the rate
of recombination catches up with the rate of dissociation. When the forward and backward reactions
eventually occur at the same rate, a state of equilibrium is reached. The apparent effect is no change,
even though both the forward and backward reactions are still occurring.
Key Point: Note that the point at which equilibrium is reached is not fixed, but it is also dependent
on such variables as temperature, reactant concentration, pressure, and reactant structure.
14.1.2.9.4 Types of Physical Reactions
Knowledge of the physical behavior of wastes and hazardous wastes has been used to develop vari-
ous unit processes for waste treatment that are based on physical reactions. These operations include
the following:
• Phase separation involves separation of components of a mixture that is already in two
different phases. Types of phase separation include filtration, settling, decanting, and
centrifugation.
• Phase transition is a physical reaction in which a material changes from one physical
phase to another. Types of phase transition include distillation, evaporation, precipitation,
and freeze drying (lyophilization).
• Phase transfer consists of the transfer of a solute in a mixture from one phase to another.
Two examples of phase transfer include extraction and sorption (i.e., transfer of a substance
from a solution to a solid phase).