Collision theory qualitatively explains how chemical reactions occur
and why reaction rates differ for different reactions.
LEARNING OBJECTIVE [ edit ]
Discuss the role of activation energy, collisions, and molecular orientation in collision theory
KEY POINTS [ edit ]
Molecules must collide in order to react.
In order to effectively initiate a reaction, collisions must be sufficiently energetic (kinetic energy)
to break chemical bonds; this energy is known as the activation energy.
As the temperature rises, molecules move faster and collide more vigorously, greatly increasing
the likelihood of bond breakage upon collision.
TERMS [ edit ]
activation energy
The minimum energy with which reactants must collide in order for a reaction to occur.
collision theory
Qualitatively explains how chemical reactions occur and whyreaction rates differ for different
reactions.
Give us feedback on this content: FULL TEXT [edit ]
Collision theoryqualitatively explains how chemical reactions occur and why reaction rates
differ for different reactions. A basic principal of collision theory is that, in order to react,
molecules must collide. This fundamental rule guides any analysis of an ordinary reaction
mechanism.
Consider the elementary bimolecular reaction: A + B
→
prod ucts
If the two molecules A and B are to react,
they must come into contact with
sufficient force so that chemical
bonds break. We call such an encounter a
collision. If both A and B are gases,
thefrequency of collisions between A and
B will be proportional to
the concentration of each gas. If we double
the concentration of A, the frequency of AB collisions will double, and doubling the
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concentration of B will have the same effect. Therefore, according to collision theory, the rate
at which molecules collide will have an impact on the overall reaction rate.
Molecular collisions
The more molecules present, the more collisions will happen.
Activation Energy and Temperature
When two billiard balls collide, they simply bounce off of one other. This is also the most
likely outcome when two molecules, A and B, come into contact: they bounce off one another,
completely unchanged and unaffected. In order for a collision to be successful by resulting in
a chemical reaction, A and B must collide with sufficient energy to break chemical
bonds. This is because in any chemical reaction, chemical bonds in the reactants are broken,
and new bonds in the products are formed. Therefore, in order to effectively initiate a
reaction, the reactants must be moving fast enough (with enough kinetic energy) so that they
collide with sufficient force for bonds to break. This minimum energy with which molecules
must be moving in order for a collision to result in a chemical reaction is known as
the activation energy.
As we know from the kinetic theory of gases, the kinetic energy of a gas is directly
proportional to temperature. As temperature increases, molecules gain energy and move
faster and faster. Therefore, the greater the temperature, the higher theprobability that
molecules will be moving with the necessary activation energy for a reaction to occur upon
collision.
Molecular Orientation and Effective Collisions
Even if two molecules collide with sufficient activation energy, there is no guarantee that the
collision will be successful. In fact, the collision theory says that not every collision is
successful, even if molecules are moving with enough energy. The reason for this is because
molecules also need to collide with the rightorientation, so that the proper atoms line up with
one another, and bonds can break and re-form in the necessary fashion. For example, in the
gas-phase reaction of dinitrogen oxide with nitric oxide, the oxygen end of N2O must hit
the nitrogen end of NO; if either molecule is not lined up correctly, no reaction will occur
upon their collision, regardless of how much energy they have. However, because molecules
in the liquid and gas phase are in constant, random motion, there is always the probability
that two molecules will collide in just the right way for them to react.
Of course, the more critical this orientational requirement is, like it is for larger or more
complex molecules, the fewer collisions there will be that will be effective. An effective
collision is defined as one in which molecules collide with sufficient energy and proper
orientation, so that a reaction occurs.
Conclusion
According to the collision theory, the following criteria must be met in order for a chemical
reaction to occur:
1. Molecules must collide with sufficient energy, known as the activation energy, so that
chemical bonds can break.
2. Molecules must collide with the proper orientation.
3. A collision that meets these two criteria, and that results in a chemical reaction, is known
as a successful collision or an effective collision.
Collision theory explanation
Collision theory provides an explanation for how particles interact to cause a reaction and the formation
of new products.