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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 EaEa 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 EaEa T 1 >T 2 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

Number of molecules E mp E a 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 E mp and the activation energy by E a. 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, E mp 2.the number of molecules with the most probable energy, E mp 3.the area under the molecular energy distribution curve 4.the number of molecules with energy greater than the activation energy, E a Plenary- Questions

Number of molecules E mp E a 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 E mp and the activation energy by E a. 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, E mp NO CHANGE 2.the number of molecules with the most probable energy, E mp INCREASE 3.the area under the molecular energy distribution curve INCREASE 4.the number of molecules with energy greater than the activation energy, E a INCREASE Plenary- Answers

Number of molecules E mp E a 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 E mp and the activation energy by E a. 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, E mp DECREASE 2.the number of molecules with the most probable energy, E mp INCREASE 3.the area under the molecular energy distribution curve NO CHANGE 4.the number of molecules with energy greater than the activation energy, E a DECREASE Plenary- Answers

Number of molecules E mp E a 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 E mp and the activation energy by E a. 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, E mp NO CHANGE 2.the number of molecules with the most probable energy, E mp 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, E a INCREASE Plenary- Answers