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In chemical substances the atoms are held together by chemical bonds. Breaking these bonds requires energy, it is an endothermic process. When bonds are formed the opposite takes place and there is a release of energy, an exothermic reaction. |
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Average bond enthalpy
The average bond enthalpy term is the average amount of energy needed to break a specific type of bond, measured over a wide variety of different gaseous molecules. It is essentially the average of all of the bond dissociation enthalpies for a specific type of bond.
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Table of bond enthalpies / kJ mol-1
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C-C
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348 |
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C-H
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412 |
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C-O
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360 |
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C=C
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612 |
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C=O
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743 |
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Cl-Cl
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242 |
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O-H
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463 |
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Cl-H
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431 |
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C-Cl
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338 |
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O=O
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496 |
| Ref: CRC Handbook of chemistry and physics - Edition 44 | |
Bond enthalpies are measured per mole of bonds. The units of the bond enthalpy term is kJ mol-1
Reaction enthalpy from bond energy terms
For any chemical reaction:
Reactants  products |
There is a rearrangement of the bonding between the component atoms of the reactants and products. Thanks to Hess' law, we can follow an alternative route from the reactants to the products by breaking all of the bonds and then reforming the new bonds.
Breaking the reactant's bonds is an endothermic process, it requires energy. Making the product's bonds is an exothermic process. Using the alternative route via the individual atoms we can state that the enthalpy change of the reaction is the SUM of these two processes.
| Enthalpy change = step 1 (bond breaking) + step 2 (bond formation) |
However, forming the products releases energy, i.e. delta H is negative, giving:
| Σ the reactant bond enthalpies - Σ the product bond enthalpies. |
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ΔH(reaction)= ΣΔH
 (bond
enthalpy reactants) - ΣΔH(bond
enthalpy products) |
In reality, all of the bonds in the reactants are not broken before reforming them into the products, this is another example Hess' law; the route may be different, but the final answer must be the same.
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Example: Use the bond enthalpy terms from the table above to find the enthalpy change of the following reaction: CH4 + 2O2 Sum of the bond enthalpy terms of the reactants:
Total = 2640 kJ Sum of the bond enthalpy terms of the products
Total = 3338 kJ Reaction enthalpy change = 2640 - 3338 = -698 kJ |
