2. Chemical Changes & Energy All chemical changes also involve some sort of energy change. Energy is either taken in or given off as the chemical change takes place. Energy is the capacity to do work. Work is done whenever something is moved.

3. Chemical Changes & Energy Chemical reactions that give off heat energy are called exothermic reactions. Chemical reactions that absorb heat energy are called endothermic reactions.

4. Physical Changes & Energy Freezing, condensation and deposition are exothermic. Melting, evaporation and sublimation are endothermic.

5. Assessment Questions State whether each of the following is an endothermic or exothermic process. Question 1. melting of ice 2. combustion of gasoline endothermic exothermic

6. Assessment Questions Endothermic or exothermic? 3. Natural gas is burned in a furnace. 4. When solid potassium bromide is dissolved in water, the solution gets colder. exothermic endothermic

7. Assessment Questions Endothermic or exothermic? exothermic 5. When concentrated sulfuric acid is added to water, the solution gets very hot.

8. Graph of an Endothermic Reaction

9. In an endothermic graph, the products have greater energy than the reactants. Reactants The change in energy is a positive value. Products

10. Graph of an Exothermic Reaction

11. In an exothermic graph, the reactants have greater energy than the products. Reactants The change in energy is a negative value. Products

12. Conservation of Energy Energy can be neither created nor destroyed in ordinary changes (not nuclear); it can only change form.