
During a chemical reaction heat can be
either absorbed or released.
› As the reactants become the products
energy can be released  exothermic
› As the reactants become the products
energy can be absorbed  endothermic

The energy levels of the reactants and
the products will be different.

ΔH = Enthaply  The difference in the
energy levels of a reaction.
ΔHreaction = ΔHproducts – ΔHreactants
ΔH is negative for exothermic reactions
ΔH is positive for endothermic reactions

Many reactions require an input of
energy for them to start, this is known as
Activation Energy.

The ΔH of the reactants, products and
activation energy can be graphed.

The variables of energy
and reaction
path/progress are
graphed.

The amount of energy
required to push the
reactants over the edge,
to start the reaction, is the
activation energy.

Once at this point the
reaction takes place and
products are formed.

The difference in the
energy of the reactants
and products is the ΔH

In exothermic
reactions energy of
the products is less
than the reactants.

Energy is released
(heat is given off).

ΔH is negative.

Endothermic reactions
usually have a steep
activation energy, because
the energy of the products
is MUCH higher than the
reactants (LOTS of energy
input).

ΔH is positive (energy
absorbed).
Another factor influencing reactions is
known as Entropy (ΔS).
 Entropy is a measure of the random
disorder in a reaction.
 With respect to molecules in phases:

Phase
Amount of Entropy
Solid
Low
Liquid
Medium
Gas
High
Enthalpy and Entropy both influence
reaction rate.
 Free energy is a measure of the
available energy to do work.

ΔG = ΔH - T ΔS
ΔG = Free energy
ΔH = Enthalpy
ΔS = Entropy
T = Temperature
Spontaneous reactions occur on their
own, without any outside energy input.
 This depends upon ΔS, ΔH and ΔG

ΔG
Reaction Behavior
Negative
Spontaneous
Zero
Equilibrium
Positive
Will not proceed on its own