Question

in section 5.6, we learned that double bonds are stronger and shorter than single bonds. for example, a c - c single bond has an average bond energy of 347 kj/mole, while a c = c double bond has an average bond energy of 611 kj/mole. use valence bond theory to explain why a double bond is not simply twice as strong as a single bond
the c = c double bond is an exception. according to valence bond theory, a double bond is generally twice as strong as a single bond
because according to valence bond theory, a double bond is the sharing of two electron pairs.
because according to valence bond theory, a double bond is actually composed of two different kinds of bonds, one σ and one π.

Explanation:

According to valence bond theory, a single bond is a σ bond. A double bond consists of one σ bond and one π bond. The σ bond is formed by head - on overlap of atomic orbitals, which is a more effective overlap. The π bond is formed by side - on overlap of p orbitals, which is a less effective overlap compared to the head - on overlap of the σ bond. So, the strength of a double bond (σ + π) is not simply twice the strength of a single (σ) bond.

Answer:

C. Because according to valence bond theory, a double bond is actually composed of two different kinds of bonds, one σ and one π.