Chemical Energetics (3) The enthalpy
of combustion of alcohols and bond energies.
Let’s
examine in this blog the implications that follow from measuring the enthalpy
of combustion of the alcohols
Here is the
data we might have collected:
Name of
Alcohol
|
Formula of
alcohol
|
Enthalpy
of combustion
ΔHoc
kJ/mol
|
Methanol
|
CH3OH
|
–726
|
Ethanol
|
CH3CH2OH
|
–1367.3
|
Propan-1-ol
|
CH3CH2CH2OH
|
–2021.0
|
Butan-1-ol
|
CH3CH2CH2CH2OH
|
–2675.6
|
Pentan-1-ol
|
CH3(CH2)4OH
|
–3328.7
|
Hexan-1-ol
|
CH3(CH2)5OH
|
–3983.8
|
Let’s plot
this data in the form of standard enthalpy of combustion vs number of carbon
atoms per alcohol.
As you can
see the plot is linear.
Why?
It seems
that adding a —CH2— group to each alcohol increases the enthalpy of
combustion by the same amount.
This feature
suggests that the additional bonds contribute a specific amount of energy to
the overall combustion of an alcohol.
In fact, we
could probably predict the enthalpy of combustion of heptan-1-ol from this
data.
The
implication is that bonds require a specific energy value to break a mole of
them.
So what do
we mean by bond energy?
Sometimes bond energy is termed bond enthalpy or bond strength or even bond
dissociation enthalpy!! (I’m making a collection of these terms !!!)
All four
terms though refer to the same thing.
Bond
enthalpy is not merely the energy required to break one mole of covalent bonds.
You need to
define the states of the particles and
the type of species involved
Here is a
definition from a well-known book of data:
“E(X—Y) Bond Energy defined
a) for X2 molecules as the
molar enthalpy change for the process X2(g) =
2X(g)
b) for XYn molecules as the
molar enthalpy change for the process
1/n
XYn(g) = 1/n X(g)
+ Y(g)
Both processes are at 298K with
individual species pressures of 1 atm.
A looser definition is here:
The bond dissociation
enthalpy is the energy needed to break one mole of the bond to give
separated atoms - everything being in the gas state.
Or this from Avogadro.com:
The Bond
Enthalpy is the energy required to break a chemical bond. It is
usually expressed in units of kJ mol-1, measured at 298 K. The exact
bond enthalpy of a particular chemical bond depends upon the
molecular environment in which the bond exists.
Because bonds
only exist in elements or compounds the context or chemical environment affects
the bond energy.
The energy of a
mole of C—H bonds in methane is slightly different then from that of a mole of
the same bonds in alcohol.
So tables of
bond energies usually caution you to remember that these are AVERAGE values
taken over several molecular environments.
Breaking bonds
requires energy to pull the atoms apart so all bond enthalpies are endothermic values.
Now the point of bond enthalpies is to use them to calculate enthalpy changes
in different reactions especially in what are called Hess Cycles.
I will blog
about Hess Cycles here later.