BOND ENERGY: Energy required to break a bond
sMETHOD:
s- 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
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 |
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