Enthalpy

• Bond Energy
  bond dissociation energy
  ·a measure of the stability of a molecule
  ·energy supplied to a molecule to separate two atoms
  ·measure of the strength of a chemical bond
• Endothermic Process
  ·absorbing energy to break bonds
• Exothermic Process
  ·giving out energy to make bonds
• Enthalpy
  ·total amount of energy of the reactants or products (H)
• Exothermic Reaction
  ·type of reaction that gives out heat
  (ΔH < 0)
• Endothermic Reaction
  ·type of reaction that takes in heat
  (ΔH > 0)
• Activation Energy
  ·minimum amount of energy needed for a reaction to begin

Each bond is valued at a certain amount of energy. This is usually provided in a table.

Is the following reaction exothermic or endothermic?

Draw out the bonds, showing the energy values

Calculations

Try and remember that the LHS is the endothermic process
The RHS is the exothermic process.
Then the entire reaction is classified as an endothermic reaction or exothermic reaction depending on which is larger!

In this example the RHS (exothermic process) is larger, hence the reaction is exothermic.

To calculate the value with the sign, balance the energies.

Balance the energies by adjusting only the Right Hand Side.
2254 = 2340 - 86

Since the "adjusting energy" is negative
(ΔH = -86)
Reaction is EXOTHERMIC.