Equilibrium and Reaction Rates
Factors That Affect Reaction Rates
For the SAT II Chemistry test, you’ll have to be familiar
with certain aspects of chemical reactions, such as equilibrium
and reaction rate. The reaction rate is a measure of
the change in the concentration of reactants or products over time
in a chemical reaction. Four main external conditions affect reaction
rate. The first is the concentration of reactants. Generally speaking,
if we increase the concentration of one or more reactants, the reaction will
go more quickly. This is simple because the more molecules, the
more collisions between molecules, and the faster the reaction will
go.
The second factor that influences reaction rate is temperature.
The higher the temperature of the reaction, the more quickly it
will proceed. At higher temperatures, the molecules are moving around
more quickly (they have more kinetic energy); this means they will
collide with each other with more energy, and it’s more likely that
they will overcome the activation energy needed to start the reaction.
It’s a general rule of thumb that a 10˚C increase in temperature
will double the reaction rate.
The addition of a catalyst will also speed up a chemical
reaction. A catalyst speeds up the rate of reaction
by lowering the activation energy. Biological catalysts are known
as enzymes. The only other important thing you need to remember
about catalysts is that they are not consumed in the course of the
reaction.
The final factor that affects certain reactions is the
physical state of the reactants. For example, if you mix two gases
or two liquids, this represents a homogenous reaction, but if reactants
are in different phases, for example, if one is a gas and one is
a liquid, then the reaction area is limited to the area where they
touch each other, and the larger this area, the faster the reaction
will proceed. For example, consider a teaspoon of salt dissolving
in water. If you were to dump the salt into the beaker of water
and let it float to the bottom without stirring it, it would take
much longer for it to dissolve than if you stirred the solution.
Now let’s quickly go through those factors that influence
reaction rate again:
-
Concentration of the reactants
- Temperature
- Presence
of a catalyst
- Physical
state of the reactants
