Comprehensive notes on energy changes

ENERGY CHANGES


Enthalpy
 The total amount of energy of the reactants or products
 Cannot be measuredonly change in enthalpy (H)
 When a chemical reaction takes place, and reactants form
products, energy is usually given out or taken inthis energy is
called heat of reaction / change in enthalpy
 Heat of reaction = energy of products - energy of reactants
 H = Hproducts - Hreactants
 Unit = kJmol-1

Bond Energy
 The amount of energy required to break a particular bond
 The higher the energy, the stronger the bond
 The bond energy required to break a bond = the energy released
to form the same bond

Breaking and forming of bonds
 During a chemical reaction, existing bonds between the reactant
atoms / ions are broken and the free atoms / ions are then
rearranged and new bond form to yield new products
 Energy is required to break existing bonds in the reacting
substancesthe breaking of bonds is an endothermic process
 Energy is released again when new bonds formformation of bonds
is an exothermic process
 Net energy change for a reaction can be endothermic or
exothermic

Endothermic Reactions
 Reactions that absorb energy (H > 0)
 More energy is absorbed than is being released
 New bonds are weaker than bonds being broken
 Reaction vessel feels cold, temperature of reaction decreases
Exothermic Reactions
 Reactions that release energy (H < 0)
 More energy is released than is being absorbed
 New bonds are stronger than the bonds being broken
 Reaction vessel feels hot, temperature of reaction increases