What Factors
Affect the Speed of Chemical Reactions?
Kinetics is the study of the speed of a chemical reaction.
Some chemical reactions are fast; others are slow. Sometimes chemists want to
speed the slow ones up and slow the fast ones down.
There are several factors that affect the speed of a
reaction:
- Nature
of the reactants
- Particle
size of the reactants
- Concentration
of the reactants
- Pressure
of gaseous reactants
- Temperature
- Catalysts
If you want to produce as much of a product as
possible as fast as possible in a chemical reaction, you must consider
the kinetics of the reaction.
Nature of chemical reactants
In order for a reaction to occur, there must be a
collision between the reactants at the reactive site of the molecule. The
larger and more complex the reactant molecules, the less chance there is of a
collision at the reactive site.
Sometimes, in very complex molecules, the reactive
site is totally blocked off by other parts of the molecule, so no reaction
occurs. There may be a lot of collisions, but only the ones that occur at the
reactive site have any chance of leading to chemical reaction.
In general, the reaction rate is slower when the
reactants are large and complex molecules.
Particle size of chemical reactants
Reaction depends on collisions. The more surface area
on which collisions can occur, the faster the reaction. You can hold a burning
match to a large chunk of coal and nothing will happen. But if you take that
same piece of coal, grind it up very, very fine, throw it up into the air, and
strike a match, you’ll get an explosion because of the increased surface area
of the coal.
Concentration of chemical reactants
Increasing the number of collisions speeds up the
reaction rate. The more reactant molecules there are colliding, the faster the
reaction will be. For example, a wood splint burns okay in air (20 percent
oxygen), but it burns much faster in pure oxygen.
In most simple cases, increasing the concentration of
the reactants increases the speed of the reaction. However, if the reaction is
complex and has a complex mechanism (series of steps in the reaction),
this may not be the case. Determining the concentration effect on the rate of
reaction can give you clues as to which reactant is involved in the
rate-determining step of the mechanism.
You can do this by running the reaction at several
different concentrations and observing the effect on the rate of reaction. If,
for example, changing the concentration of one reactant has no effect on the
rate of reaction, then you know that reactant is not involved in the slowest
step (the rate-determining step) in the mechanism.
Pressure of gaseous reactants
The pressure of gaseous reactants has basically the
same effect as concentration. The higher the reactant pressure, the faster the
reaction rate. This is due to the increased number of collisions. But if
there’s a complex mechanism involved, changing the pressure may not have the
expected result.
How temperature affects chemical
reaction rate
Increasing the temperature causes molecules to move
faster, so there’s an increased chance of them colliding with each other and
reacting. But increasing the temperature also increases the average kinetic
energy of the molecules.
The following figure shows an example of how
increasing the temperature affects the kinetic energy of the reactants and
increases the reaction rate.
The effect
of temperature on the kinetic energy of reactants.
At a given temperature, not all the molecules are
moving with the same kinetic energy. A small number of molecules are moving
very slow (low kinetic energy), while a few are moving very fast (high kinetic
energy). A vast majority of the molecules are somewhere in between these two
extremes.
In fact, temperature is a measure of the average
kinetic energy of the molecules. As you can see in the figure, increasing the temperature
increases the average kinetic energy of the reactants, essentially shifting the
curve to the right toward higher kinetic energies.
But also notice the minimum amount of kinetic energy
needed by the reactants to provide the activation energy (the energy required
to get a reaction going) during collision. The reactants have to collide at the
reactive site, but they also have to transfer enough energy to break
bonds so that new bonds can be formed. If the reactants don’t have enough
energy, a reaction won’t occur even if the reactants do collide at the reactive
site.
Notice that at the lower temperature, very few of the
reactant molecules have the minimum amount of kinetic energy needed to provide
the activation energy. At the higher temperature, many more molecules possess
the minimum amount of kinetic energy needed, which means a lot more collisions
will be energetic enough to lead to reaction.
Increasing the temperature not only increases the
number of collisions but also increases the number of collisions that are
effective — that transfer enough energy to cause a reaction to take place.
How catalysts increase chemical
reaction rate
Catalysts are substances that increase the reaction rate
without themselves being changed at the end of the reaction. They increase the
reaction rate by lowering the activation energy for the reaction.
In the preceding figure, if you shift to the left that
dotted line representing the minimum amount of kinetic energy needed to provide
the activation energy, then many more molecules will have the minimum energy
needed, and the reaction will be faster.
Catalysts lower the activation energy of a reaction in
one of two ways:
- Providing
a surface and orientation
- Providing
an alternative mechanism (series of steps for the reaction to go through)
with a lower activation energy
