Principles to Production: Chemical Energy
Bond Enthalpies
Breaking bonds
If we want to break a covalent bond between two atoms,
we need to put energy in to overcome the attractive
force.
C
C
© Nueyer
Bond breaking is an endothermic process.
Making bonds
The opposite is true if we want to make new bonds.
Energy is released when new chemical bonds are
formed.
Bond making is an exothermic process.
Breaking or making the same chemical bond will require
the same energy to be put in or released.
H H
2H
2H
H H
∆H = 432 kJ
∆H = –432 kJ
Bond enthalpy values
The values for bond enthalpies are found on page 9 of
the data book.
For some bonds, the mean bond enthalpy is quoted.
This is to give an average value to work from since the
precise enthalpy value for a bond may be different in
different molecules.
For example, the energy needed to break a C C bond
in ethane (C2H6) will be different to the energy needed
to break a C C bond in decane (C10H22)
Gaseous state
The bond enthalpies quoted in the data book are the
energies required to break 1 mole of a particular bond
between a pair of atoms in the gaseous state.
We can use these bond enthalpies to approximately
calculate the enthalpy change for a given reaction.
Example 1
What is the enthalpy change when hydrogen is added
to ethyne to produce ethane?
C2H2 (g) + 2H2 (g)
C2H6 (g)
To answer this we must look at what types of bonds
must be broken in the reactants and formed in the
products.
Potential energy
C2H2 (g) + 2H2 (g)
In this reaction, we must first
break all the bonds inside the
reactant molecules.
This will require energy to be
put in.
C2H6 (g)
Next, new bonds must be
formed between the atoms in
the product molecule.
This releases energy.
Reaction pathway
What is the enthalpy change when hydrogen is added
to ethyne, producing ethane?
C2H2 (g) + 2H
2 2 (g)
H
C
C
H
+
H H
H H
C2H6 (g)
H
H
H
C
C
H
H
H
To answer this question we can follow these steps.
Step One: Draw the full structural formulae of all the
molecules from the equation. This will show exactly
what bonds are involved.
What is the enthalpy change when hydrogen is added
to ethyne, producing ethane?
C2H2 (g) + 2H2 (g)
H
C
C
H
+
H H
H H
C2H6 (g)
H
H
H
C
C
H
H
H
Bond Breaking
1xC
2xC
2xH
C
H
H
Step Two:
Make a list of all the bonds being broken in the reactants
What is the enthalpy change when hydrogen is added
to ethyne, producing ethane?
C2H2 (g) + 2H2 (g)
H
C
C
H
+
H H
H H
C2H6 (g)
H
H
H
C
C
H
H
H
Bond Breaking
1 x C C = 835
2 x C H = 2 x 414 = 828
2 x H H = 2 x 432 = 864
Step Three:
Fill in the values for the bond enthalpies from Page 9 of
the data book.
What is the enthalpy change when hydrogen is added
to ethyne, producing ethane?
C2H2 (g) + 2H2 (g)
H
C
C
H
+
H H
H H
C2H6 (g)
H
H
H
C
C
H
H
Bond Breaking
Bond Making
1 x C C = 835
2 x C H = 2 x 414 = 828
2 x H H = 2 x 432 = 864
Step Four:
1xC
6xC
H
C = 346
H = 6 x 414 = 2484
Repeat this process for the Bond Making steps.
What is the enthalpy change when hydrogen is added
to ethyne, producing ethane?
C2H2 (g) + 2H2 (g)
H
C
C
Bond Breaking
H
+
H H
H H
C2H6 (g)
H
H
H
C
C
H
H
H
Bond Making
1 x C C = 835
1 x C C = 346
2 x C H = 2 x 414 = 828 6 x C H = 6 x 414 = 2484
2 x H H = 2 x 432 = 864
Total
in = 2527 kJ
Total given out = –2830 kJ
Stepput
Five:
Remember:
Remember:
Calculate the total energy put in breaking bonds and total
Bond breaking is an
Bond making is an
energy given out making new bonds.
endothermic process
exothermic process
What is the enthalpy change when hydrogen is added
to ethyne, producing ethane?
C2H2 (g) + 2H2 (g)
H
C
C
Bond Breaking
H
+
H H
H H
C2H6 (g)
H
H
H
C
C
H
H
H
Bond Making
1 x C C = 835
1 x C C = 346
2 x C H = 2 x 414 = 828 6 x C H = 6 x 414 = 2484
2 x H H = 2 x 432 = 864
Total put in = 2527 kJ
Total given out = –2830 kJ
∆H = 2527 + (–2380)
Step Six:
= 2527 – 2380
-1 reaction.
Calculate the enthalpy
= –change
303 kJ for
molthe
Enthalpy of formation
Bond enthalpies are also useful when calculating the
enthalpy change in making 1 mole of a compound from
its elements in their standard states.
For example, the enthalpy of formation of methane
can be represented by:
C (s) + 2H2 (g)
CH4 (g)
The data book quotes the value for this reaction as
∆Hformation = –75 kJ mol–1
Enthalpy of formation
To calculate the enthalpy of formation for methane using
bond enthalpies, we would need to change 1 mole of
solid carbon atoms into 1 mole of gaseous carbon
atoms.
This change, C (s)
C (g), is known as the
enthalpy of sublimation of carbon and has a value
of 715 kJ mol–1 (see page 9 of the data book).
Example 2
H H H
C
(s)
Calculate theHenthalpy
H H of
H formation of propane.
C (s) +
H C C C
H H H H
C (s)
H H
3C (s) + 4H2 (g)
C3H
H8 (g)
H
Example 2
Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g)
C3H8 (g)
H H H
C (s)
H H H H
C (s) +
H C C C
H H H H
C (s)
H H H
Bond Breaking
3 x C(s) C(g) = 3 x 715 = 2145
4 x H H = 4 x 432 = 1728
Total put in = 3873 kJ
H
Example 2
Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g)
C3H8 (g)
H H H
C (s)
H H H H
C (s) +
H C C C
H H H H
C (s)
H H H
Bond Breaking
3 x C(s) C(g) = 3 x 715 = 2145
4 x H H = 4 x 432 = 1728
Total put in = 3873 kJ
H
Example 2
Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g)
C3H8 (g)
H H H
C (s)
H H H H
C (s) +
H C C C H
H H H H
C (s)
H H H
Bond Breaking
Bond Making
3 x C(s) C(g) = 2145
2 x C C = 2 x 346 = 692
4xH
H = 1728
Total put in = 3873 kJ
8xC
H = 8 x 414 = 3312
Total given out = –4004 kJ
Example 2
Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g)
C3H8 (g)
H H H
C (s)
H H H H
C (s) +
H C C C H
H H H H
C (s)
H H H
Bond Breaking
Bond Making
3 x C(s) C(g) = 2145
2 x C C = 2 x 346 = 692
4xH
H = 1728
Total put in = 3873 kJ
8xC
H = 8 x 414 = 3312
Total given out = –4004 kJ
∆H = 3873 + (–4004)
= 3873 – 4004
= – 131 kJ mol-1