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:

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