1. 1.3 Collision Theory Collisions and Concentrations
The relationship between concentration of particles colliding and rate is: OR
Rate Law
The relationship above is called a rate law. The proportionality constant in this case
is called a rate constant. Every reaction has its own unique rate law and its own unique
rate constant. For many reactions, the exponents, x and y, are each equal to 1. The values
of x and y are called reactant orders.
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2. 1.3 Collision Theory Distribution of Collisions
A Pool Table as a Model
The break at the start of the game is probably the best model of the distribution of
collision energies involved between the reacting particles in a chemical reaction (Figure
1.3.2). Some of the particles move quickly and collide directly with others to transfer a great deal of energy. Others barely move at all or collide with a glancing angle that transfers very little energy. In addition, we must remember that real reactant collisions occur in three dimensions, while our pool table model is good for two dimensions only (Figure 1.3.3).
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3. 1.3 Collision Theory Collision Theory
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4. 1.3 Collision Theory Effective Collisions
Requirements
There are a tremendous number of collisions in a reacting sample that results in a wide range of velocities and kinetic energies. Only a small percentage of the molecules in a given sample have sufficient kinetic energy to react.
Only collisions between particles having the threshold energy of Ea are energetic
enough to overcome the repulsive forces between the electron clouds of the reacting
molecules and to weaken or break bonds, resulting in a reaction.
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5. 1.3 Collision Theory Effective Collisions - 2
Temperature, Number of Molecules and Kinetic Energy
Note that in both cases, with and without a catalyst, the number of particles capable of colliding effectively (as represented by the shaded area under the curve) increases at the higher temperature.
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Ea1 with catalyst
Ea2 without catalyst

6. 1.3 Collision Theory Effective Collisions - 3
Shape and Orientation of Molecules
Particles having the activation energy associated with a given reaction don’t always react
when they undergo a collision. Even very high-energy collisions may not be effective
unless the molecules are properly oriented toward one another.
SUMMARY
Increased temperature, concentration, and surface area all increase the frequency of collisions leading to an increase in reaction rate. The presence of a catalyst and temperature both increase the fraction of collisions that are successful.
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