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Collision Theory
Section 6.1 (continued)
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Collisions
Vital for chemical change Provides the energy required for a
particle to change Brings the reactants into contact As particles approach each other,
there is a repulsion between the electron clouds of the particles
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Collisions
The collision must have enough kinetic energy to overcome the repulsion in order for a reaction to occur
Frequently energy is also required to break some of the bonds before a reaction can take place
Therefore, not all collisions result in a reaction
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Activation Energy
Activation energy is the minimum amount of energy required for a reaction
Ea has units of kJ mol-1
Varies greatly from reaction to reaction
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Requirements for a Reaction to Occur
The two particles involved must: Collide with each other The collision must be energetic
enough to overcome the activation energy of the reaction (KE>E
a)
The collision must overcome the steric factor, i.e. be in the correct orientation.
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Steric Factor/orientation
Molecules have parts that are arranged in a certain pattern
For a collision to occur that leads to a reaction, the parts have to “match” with the reactants
This contact must be in a particular way Very important factor for large organic
molecules
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Increase Collision Rate
Anything that increases the collision rate increases the rate of the reaction
Concentration/pressure Surface area Temperature Presence of a catalyst Light
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Concentration/Pressure
Increase the concentration of the reactants and more collisions will occur
Increasing the pressure for reacting gases is like increasing the concentration
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Surface Area
Especially important in a reaction that involves substances in phases that do not mix, like a solid with a liquid, or a gas with a liquid
An increase in the surface area in contact will increase the collision rate
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Maxwell-Boltzmann Energy Distribution Curve
A curve describing the distribution of velocities or kinetic energies among the atoms or molecules of an ideal gas
Often used to explain the effects of a temperature change or the presence of a catalyst on the rate of a chemical reaction
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More
The area under the curve represents the total number of particles, so in a closed system this area is constant
Increasing the temperature causes more collisions, but more importantly it increases the proportion of molecules with KE>E
a
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More
The average energy of the particles is proportional to the temperature in Kelvin
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Catalyst
Catalyst: a substance which, when present in relatively small amounts, increases the rate of a chemical reaction but which is not consumed during the reaction
A catalyst provides a new reaction pathway with a lower activation energy
This means that a greater number of collisions will have the required energy to react
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Maxwell-Boltzman curve with and without a catalyst.
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Efficiency of a Catalyst
The efficiency of a catalyst decreases with time
It becomes inactive due to: Impurities in the reaction mixture Side reactions Its surface becomes coated and unavailable
for activity
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Light
Some chemical reactions are brought about by exposure to light
The reactant particles absorb light energy that then initiates the reaction
Many chemicals are stored in brown glass containers so light can't come in contact with the molecules easily