Basic Concepts of Chemical Bonding

Strengths of Covalent Bonds

Strengths of Covalent Bonds

The stability of a molecule is a function of the strength of the covalent bonds holding the atoms together.

How do we measure the strength of a covalent bond?

Bond-dissociation energy (i.e. "bond energy")

Bond energy is the enthalpy change (DH, heat input) required to break a bond (in 1 mole of a gaseous substance)

What about when we have a compound which is not a diatomic molecule? Consider the dissociation of methane:

There are four equivalent C-H bonds, thus we can that the dissociation energy for a single C-H bond would be:

D(C-H) = (1660/4) kJ/mol = 415 kJ/mol

Note:

• The bond energy for a given bond is influenced by the rest of the molecule.
• However, this is a relatively small effect (suggesting that bonding electrons are localized between the bonding atoms).
• Thus, the bond energy for most bonds varies little from the average bonding energy for that type of bond

Bond energy is always a positive value - it takes energy to break a covalent bond (conversely energy is released during bond formation)

Average bond energies:

 Bond (kJ/mol) C-F 485 C-Cl 328 C-Br 276 C-I 240 C-C 348 C-N 293 C-O 358 C-F 485 C-C 348 C=C 614 C=C 839

The more stable a molecule (i.e. the stronger the bonds) the less likely the molecule is to undergo a chemical reaction

Bond Energies and the Enthalpy of Reactions

If we know which bonds are broken and which bonds are made during a chemical reaction, we can estimate the enthalpy change of the reaction (DHrxn) even if we don't know the enthalpies of formation (DHf°)for the reactants and products:

DH = S(bond energies of broken bonds) - S(bond energies of formed bonds)

Example: The reaction between 1 mol of chlorine and 1 mol methane

Bonds broken: 1 mol of Cl-Cl bonds, 1 mol of C-H bonds

Bonds formed: 1 mol of H-Cl bonds, 1 mol of C-Cl bonds

DH = [D(Cl-Cl) + D(C-H)] - [D(H-Cl)+D(C-Cl)]

[242 kJ + 413 kJ] - [431 kJ + 328 kJ]

-104 kJ

Thus, the reaction is exothermic (because the bonds in the products are stronger than the bonds in the reactants)

Example: The combustion of 1 mol of ethane

bonds broken: 6 moles C-H bonds, 1 mol C-C bonds, 7/2 moles of O=O bonds

bonds formed: 4 moles C=O bonds, 6 moles O-H bonds

DH = [(6*413) + (348) + (7/2*495)] - [(4*799) + (6*463)]

= 4558 - 5974

= -1416 kJ (the reaction is exothermic)

Bond strength and bond length

 Bond Bond Energy (kJ/mol) Bond Length (Å) C-C 348 1.54 C=C 614 1.34 C=C 839 1.20

As the number of bonds between two atoms increases, the bond grows shorter and stronger

1996 Michael Blaber