Bond Enthalpies 5.4

Chemical reactions involve the breaking and
making of bonds.

To understand the energy changes in a chemical
reaction, we need to look at the energy needed
to break the bonds that hold the atoms together
in the reactants and the energy released when
new bonds are formed in the products.
 Energy
is needed to separate the atoms in a
bond.
 The
bond enthalpy is the energy needed to
break one mole of bonds in gaseous
molecules under standard conditions.
 For
example, the energy change during the
formation of two moles of chlorine atoms
from one mole of chlorine molecules can be
represented as:
Cl2(g)  2Cl(g)
Δ Ho = +242 kJ/mol
 The
situation is complicated in molecules
which contain more than two atoms.
 For example, the C-H bond energies differ
slightly from compound to compound, as in
CH4 (435 kJ) and CH3F (452 kJ).
 However, their bond energies are sufficiently
constant to be useful.
 So, for polyatomic molecules, we speak of
average bond enthalpies.
 Multiple bonds generally have higher bond
enthalpies and are shorter than single bonds.
• Average bond enthalpies are positive, because bond
breaking is an endothermic process.
NOTE: These are
average bond
enthalpies; the
C—H bonds in
methane, CH4,
will be a bit
different than
the C—H bond in
chloroform,
CHCl3.
 The
same amount energy that is absorbed
when a bond is broken, is released when a
bond is made.
 For example:
Δ Ho = -242 kJ/mol
 We
can now understand how energy changes
occur in chemical reactions.
 Consider the complete combustion of
methane:
CH4(g) + 2O2(g)  CO2(g) and 2H2O(g)
1. Draw the Lewis structures for the molecules
involved.
+ 2O=O  O=C=O + 2H-O-H
2. Determine what type of bonds need to be
broken.
4 C-H and 2 O=O bonds need to be broken
3. Add up all the bond enthalpies to obtain an
estimate of how much energy is required to
break the bonds in the molecules. (obtain bond
enthalpies from the average bond enthalpies
table).
For C-H = 4 (412kJ/mol) =1648 kJ/mol
For O=O = 2 (496 kJ/mol)= 992 kJ/mol
Total energy required to break bonds = 2640
kJ/mol
4. Determine the bonds that will form during
the reaction with their corresponding energy
changes.
2 C=O and 4 O-H bonds will form
For C=O, 2 (743 kJ/mol) = 1486 kJ/mol
For O-H = 4 (463 kJ/mol) = 1852 kJ/mol
Total energy released by the formation of the
C=O and O-H bonds = 3338 kJ/mol
5. To estimate H for the reaction, simply compare
the bond enthalpies of bonds broken to the bond
enthalpies of the new bonds formed.
 Hrxn = (bond enthalpies of bonds broken)  (bond
enthalpies of bonds formed)
 So, Hrxn = 2640 kJ/mol- 3338 kJ/mol= -698 kJ/mol

This value should be compared with the experimental value of
-890 kJ/mol measured under standard conditions given in the
enthalpies of combustion table. The values are different
because the standard state of water is liquid and the bond
enthalpy calculation assumes that the reaction occurs in the
gaseous state.
 Comparing


Fuels
The enthalpy change of combustion of methane
(CH4 ) is -890 kJ/mol while that of methanol
(CH3OH) is -715 kJ/mol.
Methanol has a lower enthalpy of combustion,
because when it reacts only three O-H bonds are
formed, compared with the combustion of
methane, in which 4 O-H bonds are formed.
 Exercises:
1. Calculate the H for the reaction:
C2H4(g) + H2(g) C2H6(g)
 2. Calculate the H for the reaction:
2H2(g)+ O2(g)  2H2O(g)
