When you come in…
1. Get a mini whiteboard, pen and eraser
2. Check that your pen works
3. Review material from the last lesson
Quietly!
Enthalpy profile diagrams and
Boltzmann distributions
What do you remember from GCSE chemistry…
activation
energy
Enthalpy
profile diagram
endothermic
reaction
exothermic
reaction
What do you remember from GCSE chemistry…
Exothermic and endothermic reaction
profiles
Enthalpy profile diagrams and
Boltzmann distributions
Learning Outcomes
1. Explain exothermic and endothermic reaction routes using
enthalpy profile diagrams.
2. Explain the Boltzmann distribution and its relationship with
activation energy.
3. Describe using the Boltzmann distribution the effect of
temperature change on:
–
The proportion of molecules exceeding the activation energy
–
The rate of reaction
Boltzmann distributions
1. Make a list of the factors that affect the rate of a
reaction?
2. For each effect, use collision theory to explain
why?
Boltzmann distributions
Let’s look at the molecules in of gas or liquid
in reactions in more detail.
Boltzmann distributions
Boltzmann distribution: The distribution of molecular
energies at a particular (constant) temperature.
Boltzmann distributions
As in a histogram, the
area under the
distribution is equal to the
total number of molecules
in the sample
There are no molecules
with zero potential energy
There is no maximum
energy for a molecule- the
curve does not touch the
energy axis
Boltzmann distributions
As in a histogram, the
area under the
distribution is equal to the
total number of molecules
in the sample
Molecules which have
energy less than the
activation energy will not
react when they collide.
Molecules which have
energy greater than the
activation energy will
react when they collide.
Ea
There are no molecules
with zero potential energy
There is no maximum
energy for a molecule- the
curve does not touch the
energy axis
Enthalpy profile diagrams and
Boltzmann distributions
Learning Outcomes
1. Explain exothermic and endothermic reaction routes using
enthalpy profile diagrams.
2. Explain the Boltzmann distribution and its relationship with
activation energy.
3. Describe using the Boltzmann distribution the effect of
temperature change on:
–
The proportion of molecules exceeding the activation energy
–
The rate of reaction
Boltzmann distributions
3. Describe using the Boltzmann distribution the effect of
temperature change on:
–
The proportion of molecules exceeding the activation energy
–
The rate of reaction
Boltzmann distributions
As the temperature is
increased, a greater
proportion of molecules move
faster. The kinetic energy of
the molecules increases.
T1>T2
A greater proportion of
molecules have energy
greater than the activation
energy.
The rate of reaction will
increase.
Ea
More collisions will lead to a
chemical reaction.
Enthalpy profile diagrams and
Boltzmann distributions
Learning Outcomes
1. Explain exothermic and endothermic reaction routes using
enthalpy profile diagrams.
2. Explain the Boltzmann distribution and its relationship with
activation energy.
3. Describe using the Boltzmann distribution the effect of
temperature change on:
–
The proportion of molecules exceeding the activation energy
–
The rate of reaction
Plenary- Questions
Consider the following changes:
The Boltzmann distribution of
i.
The number of molecules is increased at
constant temperature.
ii.
The temperature is decreased without
changing the number of molecules.
energies, at a constant temperature,
for a molecular gas is shown below.
The most probable molecular energy
at this temperature is indicated by
Emp and the activation energy by Ea.
iii. A catalyst is introduced without changing
the temperature or number of molecules.
For each change state how, if at all, the
following would vary:
Number of
molecules
1. the value of the most probable energy,
Emp
2. the number of molecules with the most
probable energy, Emp
Emp
Ea
Energy
3. the area under the molecular energy
distribution curve
4. the number of molecules with energy
greater than the activation energy, Ea
Plenary- Answers
The Boltzmann distribution of
Consider the following changes:
energies, at a constant temperature,
for a molecular gas is shown below.
(i) The number of molecules is increased at
constant temperature.
The most probable molecular energy
at this temperature is indicated by
Emp and the activation energy by Ea.
For each change state how, if at all, the
following would vary:
1. the value of the most probable energy,
Emp NO CHANGE
Number of
molecules
2. the number of molecules with the most
probable energy, Emp INCREASE
3. the area under the molecular energy
distribution curve INCREASE
Emp
Ea
Energy
4. the number of molecules with energy
greater than the activation energy, Ea
INCREASE
Plenary- Answers
The Boltzmann distribution of
Consider the following changes:
energies, at a constant temperature,
for a molecular gas is shown below.
(ii) The temperature is decreased without
changing the number of molecules.
The most probable molecular energy
at this temperature is indicated by
Emp and the activation energy by Ea.
For each change state how, if at all, the
following would vary:
1. the value of the most probable energy,
Emp DECREASE
Number of
molecules
2. the number of molecules with the most
probable energy, Emp INCREASE
3. the area under the molecular energy
distribution curve NO CHANGE
Emp
Ea
Energy
4. the number of molecules with energy
greater than the activation energy, Ea
DECREASE
Plenary- Answers
The Boltzmann distribution of
Consider the following changes:
energies, at a constant temperature,
iii. A catalyst is introduced without changing
the temperature or number of molecules.
for a molecular gas is shown below.
The most probable molecular energy
at this temperature is indicated by
Emp and the activation energy by Ea.
For each change state how, if at all, the
following would vary:
1. the value of the most probable energy,
Emp NO CHANGE
Number of
molecules
2. the number of molecules with the most
probable energy, Emp NO CHANGE
3. the area under the molecular energy
distribution curve NO CHANGE
Emp
Ea
Energy
4. the number of molecules with energy
greater than the activation energy, Ea
INCREASE