Download Endothermic And Exothermic Reactions

Endothermic And Exothermic
Reactions
Chemical bonds and Energy
Chemical energy is the energy stored in
the chemical bonds of a substance.
Energy changes in chemical reactions are
determined by changes that occur in
chemical bonding.
Chemical reactions involve the breaking of
bonds in the reactants and the formation
of bonds in the products.
Breaking bonds
For example, take the reaction of
propane that occurs in gas grills
C3H8 + 5O2  3CO2 + 4H2O + heat
– In order for the reaction to occur, the bonds
between the carbon, hydrogen, and oxygen
molecules must be broken.
– Breaking the bonds requires energy.
– This energy comes from igniting the gas
grill. The energy needed to break the bonds
is supplied when the igniter sparks.
Forming bonds
In order to form the carbon dioxide and
water, new bonds must be created.
The formation of chemical bonds releases
energy.
The heat that comes from the gas grill
comes from the formation of the new
chemical bonds.
Exothermic reactions
A chemical reaction
that releases energy to
its surroundings.
The energy released
as the products form is
greater than the energy
needed to break the
bonds in the reactants.
An exothermic
chemical equation is
written with the heat as
one of the products.
A + B  C + heat
This is a typical graph of an
exothermic reaction with the
products at a lower energy
than the reactants.
Endothermic reactions
A chemical reaction
that absorbs energy
from its surroundings.
More energy is
required to break the
bonds in the reactants
than is released by
the formation of
bonds in the products.
In these reactions,
heat is shown as one This is a typical graph of an
of the reactants
endothermic reaction with the
A + B + heat  C products at a higher energy
than the reactants.
Law of Conservation of Energy
This law states that during any chemical
reaction, energy is neither created nor
destroyed, it is only changed form one
form to another.
In both endothermic and exothermic
reactions, the total amount of energy
before and after the reaction is the same
Homework Questions
1.
2.
3.
4.
5.
What happens to chemical bonds as a
chemical reaction occurs?
Is the combustion of propane an endothermic
or an exothermic reaction?
How do chemical reactions involve energy?
Is energy created during an exothermic
reaction?
For the following reaction, what bonds are
broken and what bonds are formed during the
reaction? CH4 + 2O2  CO2 + 2H2O