Collision Theory Questions
CHE 40S
Part I. Collision Theory
1. What 2 criteria must be met in order for a reaction to proceed towards the formation of
products?
Particles must collide (collision frequency) with the appropriate geometry
and sufficient energy to produce an activated complex (fraction of effective
collisions).
2.
Explain using collision theory how each of the following observations affect the rate of a
chemical reaction. Use diagrams where necessary to support your reasoning.
a) The concentration of a reactant molecule is increased.
More molecules mean that more collisions will occur in the same given
space, increasing the number of successful collisions. There will be a
greater number of molecules with sufficient activation energy to form an
activated complex. Reaction rate will increase.
A
A
A
A
B
B
1 possible collision
5 possible collisions
b) A catalyst is added.
Reaction rate will increase because a catalyst provides an alternate, lowerenergy pathway for the formation of a different activated complex.
Potential
Energy
Uncatalyzed reaction
Catalyzed reaction
Reaction Pathway
c) The surface area of a solid reactant is decreased.
A lower surface area means that less reactant molecules are available to
react, therefore, decreasing the frequency of collisions and lowering the
reaction rate.
3.
a) Why must a match be struck in order for it to burn, and
A match will not burn unless the activation energy barrier is overcome as
occurs when a match is struck.
b) why does it burn all by itself after being struck?
A match burns to completion after being struck because the molecules of
wood have sufficient stored or potential energy to keep the combustion
reaction going.
Part II. Kinetics Terms
4. Define or explain the following terms.
a) activated complex
An activated complex is a high-energy, short-lived, intermediate molecule
formed at the peak of the activation energy barrier.
b) change in enthalpy
Is the difference in energy between the reactant particles and the product
particles in a chemical reaction. The change in enthalpy can be negative or
exothermic, indicating heat is released during the reaction, or the change
can be positive, indicating an endothermic reaction requiring a net input of
energy.