Collision Theory

In order for a reaction to
occur, the particles of the
reactant must have
enough energy, and must
collide at the correct
angles (proper
orientation).

The collision theory
explains the factors that
affect the rate of a
reaction.

The greater the rate of
effective collisions, the
greater the reaction rate is

The collision theory
explains the factors that
affect the rate of reaction
Factors that Affect the Rate of a
Chemical Reaction

The following factors can increase the rate of a
chemical reaction by increasing the number of
effective collisions that a occur at a given time
Concentration
 Temperature
 Surface Area
 Nature of Reactants
 Catalyst

Heat of Reaction (Enthalpy)

The amount of heat given off or absorbed in a
chemical reaction. Heat of reaction is the difference
in heat content of the products and reactants. Table
I lists common reactions, and there heat of reactions.
∆H = Hproducts - Hreactants

∆H = heat of reaction
Hproducts = potential energy of the product(s)
Hreactants = potential energy of the reactant(s)

∆H is measured in kJ (kilojoules)


Table I
Remember that a -∆H
does not mean negative
energy – it only implies
an exothermic reaction
has occurred.
Potential Energy Diagrams
Endothermic
Exothermic
Equilibrium

When the forward and reverse reactions occur
at the same rate.

The rates are equal, but the quantities
(amounts) of reactants and products are not
necessarily equal.
Types of Equilibrium:
 Phase (Dynamic)
 Solution
 Chemical
Spontaneous Reactions


A reaction that takes place under a specific set of
conditions spontaneously.
Occur in the direction of:
Less energy (lower enthalpy): favors
exothermic reactions.
 Greater entropy (disorder):

• Solids have the least entropy (most order), liquids
have more, and gases have the most entropy
(disorder).
• When a solid dissolves in water (salts or sugars),
entropy increases.
• At low temperature, energy is important; at high
energy, entropy is important.
Reactions Going to Completion

Some reactions go to completion; the reaction
goes in only one direction, the reactants form
products, products DO NOT form reactants.

Some indicators that a reaction has gone to
completion when the following are produced:
(1) a gas
(2) an essentially unionized product (like water)
(3) a precipitate is one of the products.

Remember that precipitates are insoluble - Table F
Le Chatelier’s Principle

Henry Louis Le Chatelier was a
French chemist who devised a
principle to predict the effect of
change in conditions on a
chemical equilibrium reaction

The principle states that if a
system at equilibrium is subjected
to stress, the equilibrium will shift
in the direction that relieves the
stress.

Types of stresses include:
concentration, temperature, and
pressure
CIA (concentration increase away)
TIA (temperature increase away)
PILM (pressure increase less moles
of gas)
CaCO3(s) + heat ↔ CaO(s) + CO2(g)
2SO3(g) + heat ↔ 2SO2(g) + O2(g)