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
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
