In this post
The amount of energy required to break one mole of a particular covalent bond in a substance is known as the bond energy. Different covalent bonds have different bond energies depending upon the identities of the atoms which are bonding together and the compound that the bond is found in.
The amount of energy needed to break a particular covalent bond is numerically the same as the amount of energy released when that bond is formed. For example, two hydrogen atoms share one pair of electrons to form a single covalent bond. It takes 436kJ/mol of energy to break this covalent bond so the bond energy for a H-H bond is +436kJ/mol. When the H-H bond is formed 436kJ/mol of energy is released.
All bond energy values are positive. The plus sign in front of the bond energy value tells you that bond-breaking requires an input of energy and is therefore an endothermic process. The bond energies for some covalent bonds are shown in the table below:
| Covalent bond | Bond energy (kJ/mol) |
| H-H | 436 |
| C-C | 347 |
| C-H | 413 |
| O-H | 464 |
| C=C | 612 |
| C-O | 336 |
| O=O | 498 |
| C=O | 740 |
You do not have to remember these values as they will be provided during the exam. However, you must be able to use bond energies to calculate the enthalpy change (△H) that occurs during a chemical reaction.
To calculate the overall enthalpy change (△H) for a reaction, you must work out the total amount of energy required to break all of the bonds in the reactants and the total amount of energy release when the new bonds are formed to make the products. The enthalpy change for the reaction is then calculated as the difference between the two values. The expression used to calculate the overall enthalpy change for a reaction is:
△H = energy needed to break bonds – energy produced from making bonds
△H = ∑ bonds broken – ∑ bonds formed
(where ∑ means the sum of)
To calculate the overall enthalpy, change of a reaction, we use the following method:
- Write the balanced equation for the reaction.
- List the bonds to be broken in the reactants along with their bond energies.
- List the bonds made in the products along with their bond energies.
- Calculate the sum of the total bond energies in the reactants.
- Calculate the sum of the total bond energies in the products.
- Calculate the overall enthalpy change of the reaction using the equation:
△H = ∑ bonds broken – ∑ bonds formed
(where ∑ means the sum of)
Example
Calculate the enthalpy change for the combustion of methane using the bond energy values provided in the table.
- Write the balanced equation for the reaction:
![\[CH_{4(g)} +2O_{2(g)} \rightarrow CO_{2(g)} + 2H_2O_{(l)}\]](https://env-onlinelearningcollege-gpclone.kinsta.cloud/wp-content/ql-cache/quicklatex.com-d61a132aff32a9f58e8b601f5274f2e3_l3.png "Rendered by QuickLaTeX.com")
- List the bonds to be broken in the reactant molecules with their bond enthalpy values:
C-H 4 413kJmol-14
O=O 2 498kJmol-12
- List the bonds made in the product molecules with their bond enthalpy values:
C=O 2 = 740kJmol-12
O-H 4 = 464kJmol-14
- Sum of bond enthalpies = 1652 +996 = 2648kJmol-1
- Sum of bond enthalpies = 1480 +1856 = 3336kJmol-1
