Collision Theory

Section 6.1 (continued)

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

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

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

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.

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

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

Concentration/Pressure

Increase the concentration of the reactants and more collisions will occur

Increasing the pressure for reacting gases is like increasing the concentration

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

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

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

More

The average energy of the particles is proportional to the temperature in Kelvin

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

Maxwell-Boltzman curve with and without a catalyst.

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

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