The Boltzmann distribution

Question 1

Define Boltzmann distribution

Answer 1

The Boltzmann distribution is the distribution of energies of molecules at a particular temperature, often shown as a graph.

Question 2

In a sample of gas or liquid describe the movmement of the molecules and their energy.

Answer 2

In a sample of gas or liquid:

• Some molecules move fast and have high energy
• Some molecules move slowly and have low energy
• The majority of the molecules have an average energy.

Question 3

Draw a Boltzmann distribution graph, showing the part of the graph were the molecules have enough energy to react.

Answer 3

Question 4

What are the important features of the Boltzmann distribution?

Answer 4

• The area under the curve is equal to the total number of molecules in the sample. The area does not change with conditions
• There are no molecules in the system with zero energy- the curve starts from the origin.
• There is no maximum energy for a molecule- the curve gets close, but does not touch or cross the energy axis.
• Only molecules with an energy greater than the activation energy, Ea, are able to react.

Question 5

Draw the Boltzmann distribution when the temperature is increased.

Answer 5

• At higher temperatures, the kinetic energy of all the molecules increases. The Boltzmann distribution flattens and shifts to the right. The number of molecules in the system does not change, so the area under the curve remains the same.

Question 6

Explain why when the temperature of a reaction increases the rate of reaction increases.

Answer 6

The rate increases because:

• More collisions take place in a certain length of time-the molecules are moving faster and have more kinetic energy.
• A higher proportion of molecules have an energy that is greater than the activation energy - more collisions will lead to a chemical reaction.

Thus, on collision, more molecules in the system will overcome the activation energy of the reaction. There will be more succesful collisions in a certain length of time and the rate of reaction will increase.

Question 7

Draw the Boltzmann distribution with a catalyst added.

Answer 7

Question 8

Explain why adding a catalyst increases the rate of reaction.

Answer 8

On, collision more molecules in the system will overcome the new lower activation energy of the reaction. There will be more succesful collisions in a certain length of time and the rate of reaction will increase.