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 -----> 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

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