The Boltzmann distribution

• Why are collisions between gas molecules and each other/ walls of the container said to be elastic?

  The molecules do not slow down as a result of a collision and no energy is lost
• In a gas liquid or solution, some molecules move slowly with low energy, some move fast with high energy and most move close to the average speed and have close to the average energy.
• What is the spread of molecular energies in gases known as?

  The Boltzmann distribution
• What is the Boltzmann distribution?

  The spread of molecular energies in gases
• What are the features of the Boltzmann distribution?

  1. no molecules have zero energy - the curve starts at the origin
  2. the area under the curve is equal to the total number of molecules
  3. there is no maximum energy for a molecule - the curve does not meet the x-axis at high energy. The curve would need to reach infinite energy to meet the x-axis.
• What is the effect of temperature on a Boltzmann distribution?

  As the temperature increases, the average energy of the molecules also increases. A small proportion of molecules will still have low energy, but more will have higher energy and so the graph is spread over a greater range of energy values.
• How does the Boltzmann distribution graph change with temperature?

  The peak of the graph is lower on the y axis and further along the x axis - the peak is at higher energy. The number of molecules is the same and so the area under the curve remains the same
• At a higher temperature:

  1. more molecules ( a greater proportion of molecules) have an energy greater than or equal to the activation energy
  2. therefore a greater proportion of collisions will lead to a reaction, increasing the rate of reaction
  3. collisions will also be more frequent, as the molecules have more kinetic energy and so are moving faster, but the increased energy of the molecules is much more important than the increased frequency of collisions
• The Boltzmann distribution and catalysts:

  A catalyst provides an alternative reaction route with lower activation energy. Compared to the Ea, a greater proportion of molecules now have an energy equal to or greater than the lower activation energy, Ec. On collision, more molecules will react to form products - the result is an increase in the rate of reaction