3.7C: Use Bond Energies to Calculate the Enthalpy Change during a Chemical Reaction

BOND ENERGY: Energy required to break a bond

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METHOD:

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• Add all bond energies for all the bonds in reactant - this is the ‘energy in’
• Add all bond energies for all the bonds in products - this is the ‘energy out’
• Calculate the energy change: energy in - energy out

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EQUATION:

Energy Change     =     Energy In     -     Energy Out

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Example: An Exothermic Reaction

Hydrogen and Chlorine reacts to form Hydrogen Chloride Gas:

H - H   +   Cl - Cl    →    2    x    (H - Cl)

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Table below shows the bond energies relevant to this reaction:

BOND	BOND ENERGY ( KJ / MOLE )
H - H	436
Cl - Cl	243
H - Cl	432

Energy In    =     436     +     243    =    679 KJ / Mole Energy Out     =    2    x    243    =    864 KJ / Mole Energy Change    =    679    -    864    =    -185 KJ / Mole *Energy change is negative, therefore showing that more energy is given out to the surroundings than taken in, indicating that this is an Exothermic Reaction

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Example: An Endothermic Reaction

Hydrogen Bromide Decomposes to Form Hydrogen and Bromine:

2   x   ( H - Br )    →    H - H    +    Br - Br

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Table below shows the bond energies relevant to this reaction:

BOND	BOND ENERGY ( KJ / MOLE )
H - Br	366
H - H	436
Br - Br	193

Energy In    =     2     x     366    =    732 KJ / Mole Energy Out     =    436    +    193    =    629 KJ / Mole Energy Change    =    732    -    629    =    +103 KJ / Mole *Energy change is positive, therefore showing that more energy is taken in from the surroundings than is given out, indicating that this is an Endothermic Reaction