Exothermic and Endothermic Reactions

• All chemical reactions require changes in energy. Some reactions release energy (exothermic) and others absorb energy (endothermic)
• Molecules are held together by chemical bonds; you add energy to break bonds and give off energy to join bonds.
• Reaction takes more energy to break bonds than it gives off to form bonds= endothermic; reaction takes less energy to break bonds than it gives off to form bonds = exothermic.
• Enthalpy, H, is heat contained in the system.

ENERGY DIAGRAMS

• We chart potential energy of chemicals as they change from reactants to products
• Reactants start with certain amount of energy, energy is added to start the reaction and then energy is either released or absorbed as reaction proceeds.
• Relative amounts of energy determine if the reaction is endothermic or exothermic

ENDOTHERMIC REACTION

EXOTHERMIC REACTION

Definitions:

1. Energy of reactants: The total potential energy of all reactants in a reaction
2. Energy of the activated complex: Potential energy of the "transistion state" between reactants and products.
3. Change in enthalpy (deltaH): change in potential energy during a reaction. It is: energy of products minus energy of reactants.