1. Section 7–3: Energy Changes in Reactions
Physical Science
Coach Kelsoe
Pages 206–209

2. Objectives
Interpret chemical equations in terms of reactants, products, and conservation of mass.
Balance chemical equations by manipulating coefficients.
Convert between moles and mass of a substance using molar mass.
Calculate amounts of reactants or products by using molar mass, mole ratios, and balanced chemical equations.

3. Chemical Bonds and Energy
Chemical energy is the energy stored in the chemical bonds of a substance.
Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products.
In a compound, chemical energy is contained in the bonds.
For the chemical reaction C2H kJ  C2H4 + H2, the chemical energy of the ___. A: Products is greater than the chemical energy of the reactant.

4. Breaking Bonds Breaking chemical bonds requires energy.
Most of the time, the energy required to break these bonds are provided by heat.

5. Forming Bonds The formation of chemical bonds releases energy.
The heat and light given off by a flame result from the formation of new chemical bonds.
Which of the following statements is true about what happens during a chemical reaction? A: Bonds of the reactants are broken, and bonds of the products are formed.

6. Exothermic and Endothermic Reactions
Physical changes can release or absorb energy.
During a chemical reaction, energy is either released or absorbed.

7. Exothermic Reactions
A chemical reaction that releases energy to its surroundings is called an exothermic reaction.
In exothermic reactions, the energy released as the products form is greater than the energy required to break the bonds in the reactants.
An exothermic reaction will always have an energy quantity in the products. For example:
C3H8 + 5O2  3CO2 + 4H2O kJ

8. Exothermic Reactions
During a chemical reaction, energy is released during the formation of chemical bonds.
For a certain chemical reaction, the reactants contain 385 kJ of chemical energy, and the products contain 366 kJ of chemical energy. In order for energy to be conserved, 19 kJ of energy must be released.

9. Endothermic Reactions
A chemical reaction that absorbs energy from its surroundings is called an endothermic reaction.
In an endothermic reaction, more energy is required to break the bonds in the reactants than is released by the formation of the products.
An endothermic reaction will always have an energy quantity in the reactants. For example:
2HgO kJ  2Hg + O2

10. Endothermic Reactions
In terms of energy, the general chemical equation AB + CD + energy  AD + CB represents an endothermic reaction.
Cooking requires continuous addition of energy to the chemical reactions that are taking place. The chemical reactions involved in cooking can be described as endothermic.

11. Conservation of Energy
The amount of energy before a reaction takes place is equal to the amount of energy after the reaction takes place. This principle is known as the law of conservation of energy.
The total amount of energy before and after a chemical reaction is the same. Thus, energy is _______. A: conserved

12. Vocabulary
Chemical energy
Exothermic reaction
Endothermic reaction

13. Study Guide
#17-20, should be completed.