when you come in… 1.get a mini whiteboard, pen and eraser 2.check that your pen works 3.review...
TRANSCRIPT
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…
exothermic reaction
endothermic reaction
activation energy
Enthalpy profile diagram
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
There are no molecules with zero potential energy There is no maximum
energy for a molecule- the curve does not touch the energy axis
As in a histogram, the area under the distribution is equal to the total number of molecules in the sample
Boltzmann distributions
There are no molecules with zero potential energy There is no maximum
energy for a molecule- the curve does not touch the energy axis
As in a histogram, the area under the distribution is equal to the total number of molecules in the sample
Ea
Molecules which have energy greater than the activation energy will react when they collide.
Molecules which have energy less than the activation energy will not react when they collide.
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
Ea
T1>T2
As the temperature is increased, a greater proportion of molecules move faster. The kinetic energy of the molecules increases.
A greater proportion of molecules have energy greater than the activation energy.
More collisions will lead to a chemical reaction.
The rate of reaction will increase.
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
Num
ber
of
mole
cule
s
Emp Ea Energy
The Boltzmann distribution of
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.
Consider the following changes:
i. The number of molecules is increased at constant temperature.
ii. The temperature is decreased without changing the number of molecules.
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:
1. the value of the most probable energy, Emp
2. the number of molecules with the most probable energy, Emp
3. the area under the molecular energy distribution curve
4. the number of molecules with energy greater than the activation energy, Ea
Plenary- Questions
Num
ber
of
mole
cule
s
Emp Ea Energy
The Boltzmann distribution of
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.
Consider the following changes:
(i) The number of molecules is increased at constant temperature.
For each change state how, if at all, the following would vary:
1. the value of the most probable energy, Emp NO CHANGE
2. the number of molecules with the most probable energy, Emp INCREASE
3. the area under the molecular energy distribution curve INCREASE
4. the number of molecules with energy greater than the activation energy, Ea
INCREASE
Plenary- Answers
Num
ber
of
mole
cule
s
Emp Ea Energy
The Boltzmann distribution of
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.
Consider the following changes:
(ii) The temperature is decreased without changing the number of molecules.
For each change state how, if at all, the following would vary:
1. the value of the most probable energy, Emp DECREASE
2. the number of molecules with the most probable energy, Emp INCREASE
3. the area under the molecular energy distribution curve NO CHANGE
4. the number of molecules with energy greater than the activation energy, Ea
DECREASE
Plenary- Answers
Num
ber
of
mole
cule
s
Emp Ea Energy
The Boltzmann distribution of
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.
Consider the following changes:
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:
1. the value of the most probable energy, Emp NO CHANGE
2. the number of molecules with the most probable energy, Emp NO CHANGE
3. the area under the molecular energy distribution curve NO CHANGE
4. the number of molecules with energy greater than the activation energy, Ea
INCREASE
Plenary- Answers