1. Energy Changes in Reactions (7.3)

2. 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

3. 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 + 5O > 3CO2 + 4H2O + Heat

4. 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

5. 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

6. Exothermic & Endothermic Reactions
During a chemical reaction, energy is either released or absorbed

7. 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

8. 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 kJ (energy) --> 2 mercury + O2

9. 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