The factors that affect the rate of reaction
“A change in one or more of these factors may alter the rate of a
Surface area is the exposed matter of a solid substance.
Surface area is the exposed matter of a solid. The rate
of reaction of a solid substance is related to its surface are. In a reaction
between a solid and an aqueous /liquid/gas species, increasing the
surface area of the solid phase reactant increases the number of
collisions per seconds and therefore increases the reaction rate. In a
reaction between magnesium metal and hydrochloric acid, magnesium
atoms must collide with hydrogen ions.
Below in diagrams changes are shown.
The number of collisions per second between magnesium and hydrogen
is higher, and the rate of reaction is faster.
Increasing the surface area of a solid reactant increases the reaction
By increasing surface area, there are more collisions per unit of time.
That's why many solids are powdered using a mortar and pestle before
being used in a reaction.
Examples of other reactions where surface area is important are:
active metals with acids, e.g. HCl with zinc
coal dust with oxygen gas
grain dust with oxygen gas
The concentration of a substance can be expressed in a variety of ways
depending on the nature of a substance. Aqueous solutions typically
have their concentrations expressed in mol/L. For example, a solution
made by dissolving sodium hydroxide in water has its concentration
expressed as moles of NaOH per litre of solution. Gases can also have
their concentrations expressed in mol/L. In terms of the collision theory,
increasing the concentration of a reactant increases in the number of
collisions between the reacting species per second and therefore
increases the reaction rate.
Consider the reaction between hydrochloric acid and
In one beaker, 6.00 mol/L HCl is reacted with 2.00 g
In another, 1.00 mol/L HCl is reacted with 2.00 g of Zn.
Which reaction should occur at the faster rate?
In terms of the collision theory, collisions between zinc atoms and
hydrochloric acid are more frequent in the beaker containing 6.0 M HCl
- there is more acid per unit of volume.
You can change the concentration of an aqueous species by simply
adding more solute (to make it more concentrated) or adding more
solvent (to make it more dilute).
The concentration of a gas is a function of the pressure on the gas.
Increasing the pressure of a gas is exactly the same as increasing its
concentration. If you have a certain number of gas molecules, you can
increase the pressure by forcing them into a smaller volume.
Under higher pressure or at a higher concentration, gas molecules collide
more frequently and react at a faster rate. Conversely, increasing the
volume of a gas decreases pressure which in turn decreases the collision
frequency and thus reduces the reaction rate.
Under higher pressure or at a higher concentration,
gas molecules collide more frequently and react at a
faster rate. Conversely, increasing the volume of a
gas decreases pressure which in turn decreases the
collision frequency and thus reduces the reaction rate.
It is important to note however that there are
reactions involving gases in which a pressure change
does not affect the reaction rate. For this reason, the
rates of reactions involving gases have to be
determined by experiment.
Also note that solids and liquids are not affected by pressure changes.
With the exception of some precipitation reactions involving ionic
compounds in solution, just about all chemical reactions take place at a
faster rate at higher temperatures. The question is
Temperature (in Kelvin degrees) is proportional to
the kinetic energy of the particles in a substance. For
example, if the Kelvin temperature of a substance is
doubled, then the average kinetic energy of the
particles in that substance is doubled.
At higher temperatures, particles collide more frequently and with
Increasing the temperature by say 10°C causes some of the intermediate
speed molecules to move faster. The result is more molecules with
sufficient kinetic energy to form an activated complex upon collision!
Now consider the relationship between threshold kinetic energy and
activation energy. Threshold kinetic energy is the minimum amount of
energy required for colliding particles to react - it is the equivalent of
activation energy or the minimum potential energy gain required to form
an activated complex.
As you can see on the graph, a small increase in temperature can double
the number of molecules with the threshold kinetic energy.
Thus there are two effects of increasing temperature: greater collision
intensity and more frequent collisions.
A general rule is that a 10°C temperature increase can double a reaction
rate. It turns out that the increase in the reaction rate is mainly a function
of the more intense collisions. Increased collision frequency is not as
significant a factor.
A catalyst is a species that speeds up a chemical reaction without being
chemically changed upon completion of the reaction. In other words, the
mass of a catalyst is the same before and after a reaction occurs.
Common examples of catalysts include:
MnO2 in the decomposition of H2O2
Fe in the manufacture of NH3
Pt in the conversion of NO and CO to N2 and CO2
The collisions only result in reactions if the particles collide with enough
energy to get the reactions started (i.e. to overcome the activation energy
Also recall that activation energy corresponds to threshold energy.
Only collisions involving particles with sufficient kinetic energy result
in the formation of an activated complex. Particles possessing less than
the threshold energy simply bounce apart upon collision.
The number of successful collisions per unit of time be increased by
lowering the threshold energy (or in terms of potential energy, lowering
the activation energy).
Adding the appropriate catalyst to a chemical system has exactly this
effect on threshold/activation energy.
A catalyst provides an alternative pathway for the reaction - a pathway
that has a lower activation energy. Be careful how you say it.
The catalyzed pathway (shown as a dotted green line above)
has lower activation energy.
Relating this back to the kinetic energy diagram, you see that more
particles will have sufficient kinetic energy to react. In other words, the
addition of the catalyst increases the reaction rate.
Real Life Applications
Many things in the real world are a result of the transfer of
heat. From the simple things such as putting ice into your
glass of water to the common such as burning fuel for a
car, thermochemistry pervades our lives. When one
exercises, the body naturally cools down due to sweating.
That is because our bodies supply the heat necessary to
evaporate the water. Perhaps after exercising, one uses a
hot or cold pack. The manufacturing of these little
godsends to athletes relies on the endothermic properties
of some compounds such as ammonium nitrate to create
the cold sensation. Conversely, calcium chloride or
magnesium sulfate are compounds that dissolve in water
exothermically, releasing that warm, soothing sensation
after the blistering cold.
Power plants, from the ranging from the tried and
true steam turbines to the dangerous yet high-yield nuclear
power plants, all require some method of cooling its
machinery. For this task, engineers and chemists once more
look towards the fundamentals of thermochemistry for the
answer. By utilizing water’s high specific heat capacity,
power plants continuously pump water and into their
machinery to keep them from overheating. Conveniently,
the resulting steam can be collected and reused. This
efficient method of cooling machinery has spread to all
forms of manufacturing.