Chemistry Chapter 6

Endothermic reaction

*Heat is absorbed *^H is positive *The bonds broken in the reactants are stronger than the bonds formed in the products. *The products are higher in energy than the reactants

Exothermic reaction

*Heat is released *^H is negative *The bonds formed in products are stronger than the bonds broken in the reactants. *The products are lower in energy than the reactants.

Caloric value

The predictable amount of energy from proteins, carbs and fats, upon metabolism.

Energy released

the reaction is said to be exothermic and the heat of reaction is negative. *When ^H is negative, more energy is released in forming bonds than is needed to break bonds. The bonds formed in the products are stronger than the bonds broken in the reactants. The reaction is exothermic.

Reactions that form products having a lower potential energy than the reactants are favored.

A compound with lower potential energy is more stable than a compound with higher potential energy.

Bond breaking is endothermic

Energy must be added: H-H --> H+H ^H= +104 kcal/mol Exothermic energy is released: H+H --> H-H ^H = -104 kcal/mol

Bond dissociation energies for common bonds

Bond kcal/mol H-H +104 F-F +38 Cl-Cl +58 Br-Br +46 I-I +36 H-OH +119 H-F +136 H-Cl +103 H-Br +88 H-I +71

Bond dissociation energy

Because bond breaking requires energy, bond dissociation energies are ALWAYS positive numbers. *Breaking a covalent bond into atoms is always endothermic. *The stronger the bond, the higher its bond dissociation energy. *In comparing bonds formed from elements in the same group of the periodic table, bond dissociation energies generally decrease going down the column.

Law of Conservation of Energy

The total energy in the universe does not change. Energy cannot be created or destroyed.

Breaking a bond

always requires an input of energy and bond formation always releases energy. *The energy absorbed or released in ANY reaction is called the "heat of reaction" or the "enthalpy change"

Units of energy:

calories (cal) and joules (J). *A calorie is the amount of energy needed to raise the temperature of 1g of water 1 degree C. 1 cal=4.184 J 1 kcal = 1000 cal 1 kJ = 1000 J 1kcal = 4.184 kJ

Potential energy

is stored energy. Ex: energy stored in chemical bonds-both ionic and covalent-is a form of potential energy.

Kinetic energy

is the energy of motion.

Energy absorbed

the reaction is said to be endothermic and the heat of reaction is positive. *When ^H is positive, more energy is needed to break bonds than is released in forming bonds. The bonds broken in the reactants are stronger than the bonds formed in the product. The reaction is endothermic.