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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