1. Endothermic And Exothermic Reactions

2. Chemical bonds and Energy Chemical energy is the energy stored in the chemical bonds of a substance. Energy changes in chemical reactions are determined by changes that occur in chemical bonding. Chemical reactions involve the breaking of bonds in the reactants and the formation of bonds in the products.

3. Breaking bonds For example, take the reaction of propane that occurs in gas grills C 3 H 8 + 5O 2  3CO 2 + 4H 2 O + heat –In order for the reaction to occur, the bonds between the carbon, hydrogen, and oxygen molecules must be broken. –Breaking the bonds requires energy. –This energy comes from igniting the gas grill. The energy needed to break the bonds is supplied when the igniter sparks.

4. Forming bonds In order to form the carbon dioxide and water, new bonds must be created. The formation of chemical bonds releases energy. The heat that comes from the gas grill comes from the formation of the new chemical bonds.

5. Exothermic reactions A chemical reaction that releases energy to its surroundings. The energy released as the products form is greater than the energy needed to break the bonds in the reactants. An exothermic chemical equation is written with the heat as one of the products. A + B  C + heat This is a typical graph of an exothermic reaction with the products at a lower energy than the reactants.

6. Endothermic reactions A chemical reaction that absorbs energy from its surroundings. More energy is required to break the bonds in the reactants than is released by the formation of bonds in the products. In these reactions, heat is shown as one of the reactants A + B + heat  C This is a typical graph of an endothermic reaction with the products at a higher energy than the reactants.

7. Law of Conservation of Energy This law states that during any chemical reaction, energy is neither created nor destroyed, it is only changed form one form to another. In both endothermic and exothermic reactions, the total amount of energy before and after the reaction is the same

8. Homework Questions 1. What happens to chemical bonds as a chemical reaction occurs? 2. Is the combustion of propane an endothermic or an exothermic reaction? 3. How do chemical reactions involve energy? 4. Is energy created during an exothermic reaction? 5. For the following reaction, what bonds are broken and what bonds are formed during the reaction? CH 4 + 2O 2  CO 2 + 2H 2 O