Concept Development Studies in Chemistry by John Hutchinson & John Hutchinson

Author:John Hutchinson & John Hutchinson [Hutchinson, John]
Language: eng
Format: epub

This reaction is exothermic with . In this case, we must break an H 2 bond, with energy , and a Br2 bond, with energy . Since two HBr molecules are formed, we must form two HBr bonds, each with bond energy . In total, then, breaking the bonds in the reactants requires , and forming the new bonds releases , for a net release of . This calculation reveals that the reaction is exothermic because, although we must break one very strong bond and one weaker bond, we form two strong bonds.

There are two items worth reflection in these examples. First, energy is released in a chemical reaction due to the formation of strong bonds. Breaking a bond, on the other hand, always requires the input of energy. Second, Equation 10.8 does not actually proceed by the two-step process of breaking both reactant bonds, thus forming four free atoms, followed by making two new bonds. The actual process of the reaction is significantly more complicated. The details of this process are irrelevant to the energetics of the reaction, however, since, as we have shown, the heat of reaction ΔH does not depend on the path of the reaction. This is another example of the utility of Hess' law.

We now proceed to apply this bond energy analysis to the energetics of reactions involving polyatomic molecules. A simple example is the combustion of hydrogen gas discussed previously here. This is an explosive reaction, producing 483.6kJ per mole of oxygen. Calculating the heat of reaction from bond energies requires us to know the bond energies in H 2 O . In this case, we must break not one but two bonds:

(10.9)

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