TPR gen chem thermodynamics question

[HealthHare]

Reaction 1: CH4 + OH --> CH3 + H2O
Reaction (1) is exothermic because:

A.
the C–H σ bond is stronger than the O–H σ bond.

B.
the C–H σ bond is weaker than the O–H σ bond.

Correct Answer

C.
the C–H σ bond is stronger than the O–H π bond.

D.
the C–H σ bond is weaker than the O–H π bond.
Explanation:
B. There is a single sigma bond between the H and O (choices C and D are eliminated). Recall that ΔH = Hproducts – Hreactants. In order for the reaction to be exothermic (negative ΔH), Hreactants must be greater than Hproducts. The H of a molecule is the sum of the bond energies. Watch out, though, because you have to remember that bond strength is inversely related to bond energy! So the bond strength of the reactants have to be less than that of the products (choice B).

I don't understand why they said bond strength is inversely related to bond energy. I thought it was a direct relationship. Can someone please explain?

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[Schenker]

B is the correct answer, but their explanation is incorrect. Bond strength and bond energy are directly related. Also, this:

The H of a molecule is the sum of the bond energies.

is not correct.

Here is a correct explanation:

The deltaH of a reaction can be calculated from this equation*:

deltaH = sum of the bond energies of the bonds that are broken - sum of the bond energies of the bonds that are formed

So in this case,

deltaH = D(C-H) - D(O-H)

and, since deltaH is negative (as the reaction is exothermic), D(C-H) < D(O-H), implying that C-H is a weaker bond than O-H.

*Note: you could have also used this equation:

deltaH = sum of the bond energies of the reactants - sum of the bond energies of the products