KINETICS
Kinetics is concerned with the rate and mechanism of a chemical change. The rate of a
chemical reaction is expressed as the change in the concentration of a reactant or
product in unit time and usually has the unit mol.dm-3.s-1
Although a chemical equation shows a chemical change occurring in one step, most
reactions are more complex and proceed in a number of steps, which occur at different
rates. The slowest of these steps, irrespective of where it occurs, governs the overall
rate of the reaction and is known as the rate-determining step.
Collision Theory
Before two substances can react with each other, their particles, which may be atoms,
ions or molecules, must collide with each other. However, only a small proportion of the
collisions which occur are successful i.e. they lead to a reaction. This is because, for a
reaction to occur, particles must collide with sufficient energy and, in the case of more
complex molecules, with the correct orientation.
The activation energy is the minimum energy which colliding
molecules must have to undergo a reaction.
Maxwell-Boltzmann Distribution Curve
In a sample of a gas or a liquid the molecules are in constant motion. They undergo
elastic collisions with each other and with the walls of the container. An elastic
collision is one in which kinetic energy is not dissipated in any other form of energy.
Varying amounts of kinetic energy are transferred from one molecule to another during
these collisions. The result is that, at a given temperature, the molecules in a sample
have a range of energies. This distribution of energies is shown in a Maxwell-Boltzmann
distribution curve:
most common energy
mean energy
Number of
molecules
The area under the
curve represents the
total number of
molecules in the sample
Energy
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Points to notice:
 The curve starts at the origin, because there are no molecules with zero energy.
 Only a few particles have very high energies.
 At high energies, the curve is asymptotic, because there is no maximum energy
for molecules.
Factors Affecting Reaction Rate
The rate of a chemical reaction is affected by:
 The concentration of reagents in solution
 The pressure of gaseous reagents
 The surface area (particle size) of solid reagents
 The temperature of the reaction mixture
 The presence of a catalyst
1. Concentration of a Solution
Increasing the concentration of a reagent increases the number of particles in a given
volume, and therefore increases the probability of a successful collision.
The graph below shows the Maxwell-Boltzmann distribution curves for two samples at
the same temperature. The shape of the curves is the same, but the more concentrated
solution contains more particles, and therefore the area under the curve is greater.
The particles which can undergo a successful collision are those with energy greater
than or equal to the activation energy (E a). This number is greater for the more
concentrated solution, therefore the rate is faster.
Higher conc.
Lower conc.
Number of
molecules
Ea
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Energy
As a reaction proceeds, the reactants are used up and their concentration therefore
falls. This means that as time goes on, the chances of a successful collision diminish. In
other words, the rate of the reaction is at its greatest at the start of the reaction and
gradually falls as the reaction progresses.
Concentration
of reactant
Time
The rate of the reaction at any time is given by the gradient of this graph.
2. Pressure of a Gas
There is no fundamental difference between an increase in the concentration of a
solution and an increase in the pressure of a gas. Increasing the pressure forces the
gas molecules closer together. Since there are more molecules in a given volume, the
probability of a successful collision increases. All the arguments in section 1. are equally
valid here.
3. The Surface Area (Particle Size) of a Solid
Increasing the surface area of a solid increases the rate of reaction.
When a solid reagent reacts, the reaction usually takes place only on the exposed
surface of the solid, where the second reagent can collide with it. Therefore, if the solid
is ground up, increasing its surface area (decreasing its particle size), this effectively
increases its concentration and allows more successful collisions to occur with the
second reagent.
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4. Temperature
An increase in temperature always increases the rate of a chemical reaction. A useful
rule of thumb is that increasing the temperature by 10oC roughly doubles the rate.
When the temperature is increased, the average kinetic energy of the particles
increases. This has two effects:
 The particles move faster and so make more collisions in a given time: this is a
minor effect
 The proportion of particles with energy greater than or equal to the activation
energy increases, so a larger proportion of collisions is successful. This is the
major effect and is responsible for the rapid increase in rate as temperature
increases.
As temperature increases, the shape of the Maxwell-Boltzmann distribution curve
changes. Since the sample is the same, the area under the curve is constant, but the
curve broadens because there is a larger proportion of molecules with higher energies.
The most common energy and the mean energy both increase, but the height of the
maximum decreases.
T1
T2 > T1
Number of
molecules
T2
Ea
Energy
The shaded areas represent the numbers of molecules with energy greater than or
equal to the activation energy at T1 and T2. At T2 many more molecules have enough
energy to react and therefore the rate is greater.
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5. Addition of a Catalyst
A catalyst is a substance which alters the rate of a chemical reaction
without itself being consumed in the reaction.
Most catalysts are positive catalysts, which means that they increase the rate of a
reaction. However, negative catalysts are used to slow down the rate of unwanted
reactions. An example of a negative catalyst is ethanol, which is added to
trichloromethane to slow down its degradation in air.
A positive catalyst works by providing an alternative reaction path
which has a lower activation energy.
Ea
bonds
breaking
Chemical
Energy
bonds
forming
The catalyst does not affect H
for the reaction. For this reason,
it does not affect the position of
an equilibrium.
Ea cat
reactants
H
products
Reaction path
If the activation energy is reduced, more particles have energy greater than or equal to
the activation energy, so the rate increases. The shaded areas below correspond to the
numbers of particles which have enough energy to react in the catalysed and
uncatalysed reactions.
Number of
molecules
Ea cat
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Ea
Energy