Bond Dissociation Energy
Heat of a Reaction
An exothermic reaction
…
…
N
N
Bond
energy
LAW OF CONSERVATION OF ENERGY
Molecular
fragments (g)
Molecule (g) + energy
Heat of a Reaction
Reaction Energy
ENDOTHERMIC REACTIONS –
An endothermic reaction
∆H is + ive
EXOTHERMIC REACTIONS –
∆H is – ive
So What is Going On?
How to Calculate Heat of Reaction?
A large bond dissociation energy
corresponds to a strong covalent bond.
Redistribution of ‘bonds’ means redistribution of energies
How to Calculate Heat of Reaction?
As you can see different covalent bonds
have different amounts
of energy needed to
break apart. A large
dissociation energy
(see N triple N) is a
strong bond. A low
dissociation energy
(see N-O) is a weak
bond.
How to Calculate Heat of Reaction?
Enthalpy: A measure of the heat content of a substance at constant
pressure
you cannot measure the actual enthalpy of a substance
you can measure an enthalpy CHANGE
written as the symbol ∆H , “delta H ”
Enthalpy change (∆H) = Enthalpy of products - Enthalpy of reactants
Energy required
= 610 kJ for 1 mol of C=C bonds + 436 kJ for 1 mol of H―H bond
= 1046 kJ/mol
Energy evolevd
= 346 kJ for C―C bond + 2 mol Χ 413 kJ for C―H bonds
= 1172 kJ/mol
∆H = 1046 kJ/mol – 1172 kJ/mol = -126 kJ/mol