2.3 (a) Factors affecting the rate of reaction
Task 2.3a Complete and extend the table below:
| variable (factor) | value | high or low rate |
2.3 (b) Explanations using collision theory
The collision theory is used to explain changes in reaction
rate. In a reaction between 2 gaseous substances A and B, a molecule
of A must collide with a molecule of B before reaction can occur. The number
of collisions in a given time, the collision frequency, controls the rate
of reaction. The greater the collision frequency the greater the rate of
reaction. Not every collision leads to a reaction. A reaction
takes place during a collision if the molecules hit at the
correct angle or orientation and if they have enough energy. This
amount of energy per mol of molecules is called the activation energy. The theory is extended to cover liquids
and solids.
Temperature
Increasing the temperature increases
the speed of the reacting particles and faster particles collide more often
than slow ones. The increase in the number of collisions leads to an increase
in the collision frequency and rate of reaction. Increasing the temperature also gives the particles
more energy so that they collide with more violence. Energetic particles
have a better chance of their collisions leading to a reaction.
Concentration
The concentration of a substance,
normally a solution, is the amount in a given volume.
concentration = amount {units
= mol/dm3 or M}
volume
In a higher concentration solution
there are more particles to react therefore there are more collisions and a
higher collision frequency.
As a reaction depends on collisions happening, a higher collision frequency leads to
a faster reaction rate. If we were doing a reaction with acid and we double
the number of acid particles, we double the number of collisions and therefore
are likely to double the reaction rate.
Pressure
Increasing the pressure of a gas puts more gas molecules into a given
volume. There will be more collisions and a higher collision frequency
leading to a higher rate of reaction.
Surface area
Surface area is controlled by the particle size of a solid. A
powder has a higher surface area than lumps and therefore a powder has more
atoms or ions exposed on its surface in a position to react. More collisions
take place between the ions or molecules in the surrounding liquid. The
collision frequency in increased and so is the rate of reaction.
Task 2.3b Draw diagrams to show slow and fast
reactions caused by changing the factors above.
2.3 (c) Maxwell-Bolzmann distribution
As the temperature is raised the average energy of the
molecules increases. The proportion of molecules with the activation energy (see
section under graph) is greater at higher temperatures. A small increase in temperature
gives a large increase in reaction rate. Simulation
of change in temperature requires Microsoft Excel (source www.chemit.co.uk
)
2.3 (d) Activation energy
Activation energy is a measure of the energy needed, when molecules collide,
to lead to a reaction. The lower the activation energy the more molecules
at a particular temperature will have enough energy to react when they
collide. As the temperature increases more molecules will have an amount
of energy equal to or more than the activation energy. At a high
temperature more collisions therefore lead to reaction.
2.3 (e) Catalysts and activation energy
Catalysts alter the rate of a chemical reaction without
being consumed in the reaction. (e.g. the catalyst may end up in the oxidation
state it began with). A catalyst reduces the activation energy for a reaction
by providing an alternative mechanism for the reaction.
Homogeneous catalysts can form intermediates which contain
the catalyst but then decompose to form products. For the reaction
A -----> B + C
high activation energy
A + catalyst -----> A-catalyst
low activation energy
A-catalyst -----> B + C + catalyst
low activation energy
The catalyst may change oxidation state during the reaction
see
http://www.wbateman.demon.co.uk/newsums/sum5.2/sum5.2.htm
Heterogeneous catalysts such as surface catalysts involve
steps such as; diffusion to surface, adsorption on surface, reaction at
surface, deadsorption from surface, diffusion from surface.
If two reactant molecules collide they may react if they have
enough energy. If they are brought together on the surface of a catalyst
the activation energy may be lower so at a given temperature the reaction will
be faster.
The reaction profile for a catalysed and an uncatalysed reaction is shown below.
2.3 (f) Thermodynamic and kinetic stability
One system is thermodynamically stable with respect to
a second one if the first one is lower than the second on an enthalpy level
diagram.
e.g. Oxygen is energetically stable with respect to ozone.
ozone (unstable)
|
|
oxygen \/ (stable)
Even if a system is thermodynamically unstable and is
expected to react to form a stable one the system may not react.
The system will not react if it is kinetically stable. This means that
the reaction proceeds too slowly for any reaction to be seen. If kinetically
unstable, a reaction is fast and observations can be made. When a
system is thermodynamically unstable but kinetically stable, the reaction
is likely to be seen but only under favourable conditions. Sugar
and oxygen is a system like this with respect to carbon dioxide and water.
A bowl of sugar on the table does not react but if heated an exothermic reaction
takes place.