3.7 Energy Distribution
1. Distribution Graphs
LO: I understand what is meant by an energy distribution diagram.
A Distribution Graph:
The activation energy Ea is the minimum kinetic energy of collision
for a chemical change to take place.
The Ea is a point on the x-axis.
Collisions on the right of Ea (shaded area) have enough energy to
form an activated complex.
Collisions on the left of Ea do not have enough energy to form an
activated complex and the molecules will not react.
The larger the shaded area in the diagram, the greater the rate of
reaction.
In some reactions the shaded area can be increased using light as
light energy is absorbed by molecules giving them greater kinetic
energy.
2. Concentration and Temperature
LO: I understand how increasing the concentration or increasing the
temperature affects an energy distribution diagram.
Concentration:
The shaded area shows the number of molecules which have enough
energy to form an activated complex.
With increased concentration the shaded area has increased in
size.
 there are more molecules present
 more molecules have energy greater than Ea
 the rate of the reaction is faster
Temperature:
Increasing the temperature increases the kinetic energy of the
particles.
As a result the distribution graph spreads to the right.
The height of the graph decreases as the total number of particles
stays the same but the range is wider.
The result is a much larger shaded area
 more molecules have energy greater than Ea
 the rate of reaction is greater
3. Catalytic Effect
LO: I understand how a catalyst speeds up a reaction without
having any effect on the energy distribution graph.
Adding a catalyst to a reaction lowers the activation energy.
The shape and position of the distribution graph does not change.
The Ea is lower and so shifts to the left.
 the shaded area is bigger
 more molecules have energy greater than Ea
 the rate of the reaction is faster.