If we want to break a covalent bond between two atoms, we need to put energy into overcome the attractive force.
Bond breaking is an endothermic process.
The bond enthalpy is the energy required to break one mole of H-H bonds
The product is 2 H atoms in the gaseous state.
This is equal and opposite to the energy given out when 2 H atoms in the gaseous state form 1 mole of H-H bonds.
For a diatomic molecule, the bond enthalpy values can be measured directly
Mean bond enthalpies have to be calculated as the value of a C-H bond enthalpy will vary according to the environment it is in.
Energy used to break a bond (endothermic) will equal that released when the bond is made (exothermic).
Bond enthalpies can be used to estimate the enthalpy change occurring for a gas phase reaction, by calculating the energy required to break bonds in the reactants and the energy released when new bonds are formed in the products.
ΔH can be calculated from bond enthalpies using the equation:
ΔH = Σ ΔH bonds broken + Σ ΔH bonds made
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 ΔH = 432kJ
2H → H-H ΔH = -432kJ
The values for bond enthalpies are found on page 10 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)
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.
What is the enthalpy change when hydrogen is added to ethyne to produce ethane?
C2H2 + 2H2 → C2H6
Draw the full structural formulae of all the molecules from the equation. This will show exactly what bonds are involved.
Make a list of all the bonds being broken in the reactants
Bond Breaking
1 x C≡C
2 x C-H
2 x H-H
Bond Breaking
1 x C≡C = 838
2 x C-H = 2 x 412 = 824
2 x H-H = 2 x 436 = 872
Repeat this process for the Bond Making steps.
Bond Making
1 x C-C = 348
6 x C-H = 6 x 412 = 2472
Step 5
Calculate the total energy put in breaking bonds and total energy given out making new bonds.
Remember: Bond breaking is an endothermic process and bond making is an exothermic process.
Bond Breaking
1 x C≡C = 838
2 x C-H = 2 x 412 = 824
2 x H-H = 2 x 436 = 872
Total put in = 2534 kJ
Bond Making
1 x C-C = 348
6 x C-H = 6 x 412 = 2472
Total given out = -2820 kJ
Calculate the enthalpy change for the reaction.
ΔH = 2534 + (-2820)
ΔH = 2534 - 2820
ΔH = -286 kJmol-1