Transcript 7.3 energy changes in reactions

Energy Changes
in Reactions
(7.3)
Chemical Energy
Is the energy stored in the chemical
bonds of a substance
Energy changes are determined by
the changes that occur in chemical
bonding
Chemical reactions involve the
breaking of chemical bonds in the
reactants and the formation of
chemical bonds in the products
Chemical Bonds and
Energy
Heat is a form of energy that can be released
from a chemical reaction
In a propane grill, a chemical reaction takes
place that produces the heat that cooks your
food
The chemical equation for the combustion of
propane shows heat as one of the products of
the equation
C3H8 + 5O2 ----->
3CO
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Breaking Bonds
Breaking bonds requires energy
Propane grills have an igniter which
produces the spark that provides enough
energy to break the bonds of the reacting
molecules.
The spark gets the reaction going
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Forming Bonds
The formation of chemical bonds releases
energy
For each molecule of propane burned, 3
molecules of carbon dioxide and 4 molecules of
water are formed.
So six C=O double bonds and eight O-H single
bonds form
The formation of these new bonds give off the
heat and light
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Exothermic & Endothermic
Reactions
During a chemical reaction, energy is
either released or absorbed
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Exothermic Reactions
A chemical reaction that releases energy
to its surroundings
The energy that is released as the
products form is greater than the energy
required to break the bonds in the
reactants
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Endothermic
AReactions
chemical reaction that absorbs more energy
from its surroundings
More energy is required to break the bonds in
the reactions than is released by the formation
of the products
Mercury (II) oxide + 181.7 kJ (energy) --> 2 mercury +
O2
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Conservation of
Energy
The total amount of energy before and
after the reaction is the same
Exothermic
Chemical energy of reactants = heat + chemical energy of
products
Endothermic
Heat + chemical energy of reactants = chemical energy of
products