1. Principles to Production: Chemical Energy
Bond Enthalpies

2. Breaking bonds
If we want to break a covalent bond between two atoms, we need to to overcome the attractive force.
put energy in
put energy in C C
© Nueyer
Bond breaking is an endothermic process.

3. 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 2H
∆H = 432 kJ 2H H
∆H = –432 kJ

4. 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 bond in ethane (C2H6) will be different to the energy needed to break a bond in decane (C10H22) C

5. 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.

6. 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.

7. This will require energy to be put in.
C2H2 (g) + 2H2 (g)
C2H2 (g) + 2H2 (g) C2H6 (g)
C2H6 (g)
In this reaction, we must first break all the bonds inside the reactant molecules.
This will require energy to be put in.
Next, new bonds must be formed between the atoms in the product molecule.
This releases energy.
Potential energy
Reaction pathway

8. What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g) 2 C H H C 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.

9. What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g) + C H H C H C C H
Bond Breaking C
1 x C
1 x C H
2 x C H
2 x H
2 x
Step Two:
Make a list of all the bonds being broken in the reactants

10. What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g) + C 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.

11. What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g) + C H C H C
Bond Breaking
Bond Making
1 x C C
= 835 C
1 x
= 346
2 x C H
= 2 x 414
= 828 C H
6 x
= 6 x 414
= 2484
2 x H H
= 2 x 432
= 864
Step Four:
Repeat this process for the Bond Making steps.

12. What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g) + C H
Bond Breaking
Bond Making
1 x C C
= 835 C
1 x
= 346
2 x C H
= 2 x 414
= 828 C H
6 x
= 6 x 414
= 2484
2 x H H
= 2 x 432
= 864
Total put in = 2527 kJ
Step Five:
Calculate the total energy put in breaking bonds and total energy given out making new bonds.
Total given out = –2830 kJ
Remember: Bond breaking is an endothermic process
Remember: Bond making is an exothermic process

13. What is the enthalpy change when hydrogen is added to ethyne, producing ethane?
C2H2 (g) + 2H2 (g) C2H6 (g) + C H
Bond Breaking
Bond Making
1 x C C
= 835 C
1 x
= 346
2 x C H
= 2 x 414
= 828 C H
6 x
= 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:
Calculate the enthalpy change for the reaction. =
2527 –
2380 =
– 303 kJ mol-1

14. 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

15. 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).

16. Example 2 C H C (s) Calculate the enthalpy of formation of propane. H+
C (s) H H
3C (s) + 4H2 (g) C3H8 (g)

17. Example 2 Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g) C3H8 (g) C H
C (s)
C (s) H H H
C (s) +
C (s) H H
Bond Breaking 3
3 x
C(s) C(g)
= 3 x 715
= 2145
4 x 4 H H
= 4 x 432
= 1728
Total put in = 3873 kJ

18. Example 2 Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g) C3H8 (g) C H
C (s) H H
C (s) +
C (s) 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

19. Example 2 Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g) C3H8 (g) C H C H C H C H C H
C (s) H H
C (s) + C H C C C H C C H C H C H
C (s) H H
Bond Breaking
Bond Making
3 x
C(s) C(g)
= 2145 C
2 x 2
= 2 x 346
= 692
4 x H H
= 1728 8 C H
8 x
= 8 x 414
= 3312
Total put in = 3873 kJ
Total given out = –4004 kJ

20. Example 2 Calculate the enthalpy of formation of propane.
3C (s) + 4H2 (g) C3H8 (g) C H
C (s) H H
C (s) +
C (s) H H
Bond Breaking
Bond Making
3 x
C(s) C(g)
= 2145 C
2 x
= 2 x 346
= 692
4 x H H
= 1728 C H
8 x
= 8 x 414
= 3312
Total put in = 3873 kJ
Total given out = –4004 kJ ∆H =
3873 +
(–4004) =
3873 –
4004 =
– 131 kJ mol-1