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TRANSCRIPT
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Energy Changes in Reactions���(Chp 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 -----> 3CO2 + 4H2O + Heat
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
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
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
Endothermic Reactions • A 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
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