Group Three
Activation Energy and the Activation Complex
Another principle of the collision theory is related to
the behaviour of the reactant particles as they collide.
According to collision theory, when particles collide
at the proper angle with the proper amount of energy,
they form particles with structures unlike the
structures of either the reactants or products. These
intermediate particles are unstable and thus exist for
very short periods of time. It is during this time the
atoms rearrange themselves by producing a grouping
of molecules considered to have bonds that are
simultaneously beings formed and being broken. Each
intermediate particle is called an activated complex.
To illustrate the formation of an activated complex,
consider the reaction between a molecule of hydrogen
(H2) and a molecule of iodine (I2) to produce HI
(Figure 1). For a reaction to occur, H2 and I2 must
collide. If they approach from the wrong angle or
have too little kinetic energy, they will rebound from
each other without forming the product HI. If,
however, the angle of approach is right and the
particles have sufficient energy, an activated complex
Figure 1
containing all for atoms (H2I2) is temporarily formed.
Shortly after the activated complex is formed, it may
break apart to re-form the original molecules of H2 and I2 or to form the product HI. The
minimum amount of energy needed to form the activated complex is called the
activation energy.
Potential Energy Diagrams
Why is there an activation energy for a chemical reaction to take place? Why don’t all
collisions result in the formation of new products? To answer these questions consider
the analogy illustrated in Figure 2. Imagine you are trying to roll a bowling ball up a very
steep hill. On most tries the bowling ball slows down and stops before it gets to the top of
the hill. The kinetic energy of the bowling ball is converted to potential energy as the ball
slows down. Then it rolls back down on the same side of the hill. The hill acts as a
barrier. Only occasionally does the bowler give the ball enough kinetic energy so that it
gets to the top of the hill and rolls down the other side. Once one the downhill slope, the
potential energy of the bowling ball gets converted back to kinetic energy, causing the
ball to pick up speed.
Figure 2
Picture a similar situation for molecules in a chemical reaction. During molecular
collisions, atoms take up new bonding arrangements that have more potential energy than
either the reactants or products. These atomic arrangements (i.e. the activated complex)
have high potential energy like the bowling ball at the top of the hill. There is a minimum
potential energy that must be achieved by colliding reactants before they can convert to
some other form. This minimum potential energy is the activation energy for a given
reaction.
The relationship between the activation energy and the energy absorbed or given off in a
reaction (i.e. and endothermic or exothermic reaction respectively) can be shown
graphically on a potential energy diagram (Figure 3).

Transition State
Figure 3
It shows how the energy of the reacting system changes as the reaction proceeds. The
potential energy of each chemical (reactants, the activated complex, and products) is
indicated along the vertical axis. The progress of the reaction is plotted along the
horizontal axis or reaction pathway. All potential energy diagrams have a similar
profile. The reaction passes through energy maximum, known as the transition state. It
is at this highest energy point that the activated complex is formed.
The difference between the energy of the reactants in the beginning and when they are in
the transition state is the activation energy or Ea. The activation energy, you will recall, is
the minimum energy required for the reaction to occur and corresponds to the energy
necessary to reach the transition state and form the activated complex. Most reactions
have activation energies in the range of 100 to 200 KJ for every mole of reactant present.
Figure 4a shows that if the products of the reaction are at a lower energy than the
reactants, the reaction is exothermic. This type of reaction gives off energy, usually in the
form of heat, and this indicated by a negative enthalpy (H) value. The change in
enthalpy or H is the heat released or absorbed during a reaction, under constant
pressure. If the products are of higher energy than the reactants (Figure 4b), we are
dealing with an endothermic reaction. In either case the activation energy is always the
difference between the energy of reactants initially and the highest point on the reaction
pathway.
Figure 4
(Exothermic)
(Endothermic)
The activation energy for a reverse reaction is not the same as the activation energy for its
forward reaction. For example, nitrogen monoxide will react with ozone to produce
nitrogen dioxide and oxygen:
NO (g) + O3 (g)  2 NO2 (g) + O2 (g)
For this reaction, the activation energy, Ea, is +10 KJ/mol, while the enthalpy change,
H, is –200 KJ/mol (see Figure 5). We find that collisions between nitrogen dioxide and
oxygen sometimes lead to the formation of nitrogen monoxide and ozone.
2 NO2 (g) + O2 (g)  NO (g) + O3 (g)
This reverse reaction will have an enthalpy change of +200 KJ/mol. As a result, the
activation energy of the reverse reaction (E’a) will be 210 KJ/mol (10 KJ/mol + 200
KJ/mol). The activation energy of the reverse reaction is very high, indicating that the
reaction will be very slow.
Figure 5
Demonstration 1:
Demonstration 1:
Background: The wax of a candle burns readily in the presence of oxygen.
-
Place a candle on the demonstration table and ask students questions like:
o Why is the candle, in the presence of oxygen, not reacting?
o How can we get this reaction to occur?
o What is the function of the match in causing the reaction to occur?
Demonstration 2:
- Create a way to demonstrate activation energies: a suggestion would be rolling a ball up
a hill, very similar to the diagram under the potential energy section in these notes.
Sample Questions
1. Define the terms ‘activation energy’ and ‘activated complex’.
2. Differentiate between activated complex and transition state.
3. What is a potential energy diagram? What does it show?
4. The potential energy diagram shown below is for the hypothetical reaction C  A + B.
(a) Is the reaction exothermic or endothermic?
(b) Select the number on the diagram that indicates the
(i) activation energy for the forward reaction
(ii) activation energy for the reverse reaction
(iii) potential energy of the reactant
(iv) potential energy of the activated complex
(v) the enthalpy of the reaction
5. In the hypothetical reaction, X + Y  Z, the enthalpy of the forward reaction is
H = -36 KJ/mol. the activation energy for the forward reaction is 73 KJ/mol
(a) Draw a potential energy level diagram for this reaction. Be sure to label all the
necessary parts.
(b) What is the activation energy for the reverse reaction?