Enthalpy Diagrams
Enthalpy Profile Diagram
This is the second set of enthalpy profile diagrams, these include the
activation energy.
Definition
Activation energy (Ea)
The minimum energy required for a reaction to occur.
Factors that affect the rate of reaction
1. Catalyst
2. Temperature
3. Pressure for gases
4. Concentration for liquids
5. Surface area for solids
Effect of an increased concentration
Leads to an increased number of particles within in the given area, leading
to a greater number of collisions, therefore an increased number of collisions
with the required activation energy, so the rate of reaction increases.
Effect of an increased pressure
Leads to an increased number of particles within in the given area, leading
to a greater number of collisions, therefore an increased number of collisions
with the required activation energy, so the rate of reaction increases.
Effect of an increased surface area when a solid reacts with either a
liquid or gas
Leads to an increased number of available particles to react with the
liquid or gas leading to a greater number of collisions so the rate of
reaction increases.
Maxwell-Boltzmann distribution of molecular energies
This is an alternate method to represent activation energy for molecules.
Some points of note
· The area under the curve is the total number of molecules in
the sample
· The curve doesn’t start at zero, because there are no particles
with no energy
· There is no maximum energy so the curve never touches the x-axis
· The highest point is the ‘average’ energy
At higher temperatures the distribution moves so that more particles have an energy greater than the activation energy. This is because the particles have a greater kinetic energy so there will be more collisions and therefore more successful collisions.
Some points of note
· The cold sample has a higher peak
· The cold sample has a more narrow peak
· The total number of particles remains constant