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Wize University Chemistry Textbook > Chemical Bonding

BDE Calculation

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Bond Energies

Enthalpy (H) is the energy stored in bonds.

Bond dissociation energy (BDE) is the energy needed to break a chemical bond.

ΔHrxn=[(∑nHbonds broken)−(∑nHbonds formed )]\Delta H_{rxn}=\left[\left(\sum_{ }^{ }nH_{bonds\ broken}\right)-\left(\sum_{_{ }}^{ }nH_{bonds\ formed\ }\right)\right]

Or (an easier way to think about it):

ΔHrxn=[(∑nBDEreactants)−[(∑nBDEproducts)]\boxed{\Delta Hrxn=[(\sum n BDE_{reactants})-[({\sum nBDE_{products})]}}

ΔHrxn=\Delta H_{rxn}= enthalpy change for the reaction (kJ/mol)
BDE = bond energy per mole of bonds (kJ/mol), always positive

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ΔHrxn=[(∑nBDEreactants)−[(∑nBDEproducts)]\boxed{\Delta Hrxn=[(\sum n BDE_{reactants})-[({\sum nBDE_{products})]}}


Photo by Rice University / CC BY

Practice: Bond Dissociation Energies

Predict the overall change in enthalpy (i.e. change in enthalpy, change in heat, ΔHrxn in kJ/mol) for the reaction:

N2 (g) + 3 H2 (g) → 2 NH3 (g).

BDE:
NN = 945 kJ/mol
H-H = 436 kJ/mol
N-H = 391 kJ/mol