7.3 energy changes in reactions
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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