Bond Energy and Reaction Energy

Units for Measuring Heat The Joule is the SI system unit for measuring heat: The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree

- Reaction Energy = Bond Breaking always requires energy Bond Making always releases energy Energy needed to break bonds Energy released by bonds forming H Reaction Energy = -

Table of Bond Energies

Example – Formation of Water 2H2 + O2  2H2O  Bonds broken Bonds formed 2 H-H 2(432 kJ) 4 O-H 4(467 kJ) 1 O=O 1(498 kJ) 1362 kJ invested 1868 kJ produced H = 1362 kJ – 1868 kJ = -506 kJ

Exothermic Processes Processes in which energy is released as it proceeds, and surroundings become warmer Reactants  Products + energy

Endothermic Processes Processes in which energy is absorbed as it proceeds, and surroundings become colder Reactants + energy  Products

Calorimetry – Measuring Energy Change The amount of heat absorbed or released during a physical or chemical change can be measured, usually by the change in temperature of a known quantity of water in a calorimeter.