Feb.16--Endothermic and Exothermic Reactions

All chemical reactions involve changes in energy.

Endothermic Reaction: reactions that release energy to the surroundings

Exothermic Reaction: reactions that absorb energy

Ex. 1. Instant ice packs absorb energy and are endothermic reactions.

       2. Explosions release energy and are exothermic reactions.

Molecules are held together by chemical bonds.

- to break bonds: add energy

- to join together: give off energy

* Exothermic: if a reaction takes more energy to break bonds than it gives off to form bonds

* Endothermic: if a reaction takes less energy to break bonds than it gives off to form bonds

ENERGY DIAGRAMS

- shows the potential energy of the chemicals as they change from reactants to products

Reactants start with a certain amount of energy. Energy is added to start the reaction and then is released as the reaction proceeds. The relative amounts of energy determines if a reaction is endothermic or exothermic.

Potential Energy Diagram #1 (Endothermic)

Potential Energy Diagram #2 (Exothermic)

1) energy of reactants = total potential energy of all reactants in the reaction

2) energy of products = total potential energy of all products in the reaction

3) energy of the activated complex = potential energy of the “transition state” between

  reactants and products

4) activation energy = the energy that must be added to get the reaction to progress

5) △H (change in enthalpy ) = the change in potential energy during the reaction

                        = energy of products－energy of reactants

The energy absorption or release can be placed directly in the equation.

Ex. CH₄ + 2O₂ --> CO₂ + 2H₂O + 812 kJ

     higher energy    lower energy

Exothermic reactions have the energy term on the right hand side and a negative △H.

Endothermic reactions have the energy term on the left hand side and a positive △H.