Two molecules of hydrogen and one molecule of water are required to join together in order to form water. For this reaction, we may require heat or a specific environment, and, of course, we may require energy. In this article we will be focussing on the bond energy of the reactant, products and the total energy change.
For new molecules to be formed, bonds need to be broken and bonds need to be formed. These can be classified as exothermic and endothermic processes. But how much energy is required for this? This is what we will be exploring today.
Fig. 1: Hydrogen bonds in water. https://commons.wikimedia.org
- We will explore what forming or making a bond is.
- What breaking a bond is.
- How to calculate bond energy.
- The units of bond energy.
- Finally, we explore some examples of bond energy calculations.
This topic is required for those sitting the higher tier.
Making and breaking bonds
In a reaction, the first thing that needs to take place, is bonds need to be broken. For this, energy is required so energy is absorbed from the surroundings. This is an example of an endothermic reaction. After the reactants have broken down, we then move on to bond-forming, whereby the newly split molecules form bonds with new molecules. However, this reaction actually releases energy into the surroundings. This is an example of an exothermic reaction. During this period temperature can increase if the bond-forming energy is higher than the bond-breaking energy.
Exothermic reaction: A reaction where energy is transferred from the reaction to the surrounding.
Endothermic reaction: A reaction where energy is absorbed from the surroundings to the reaction.
Although it is described as two steps within a reaction, we must note that in a reaction bond-forming may not happen straight after bond-breaking. Instead, they can happen at the same time. Nevertheless, we can use the total bond-breaking energy and total bond-forming energy to explore energy changes and then further class a reaction as either exothermic or endothermic.
Bond breaking absorbs energy and bond forming releases energy.
In the beginning of this article we looked at water. If we investigate this further we can identify that in order for water to be formed hydrogen (H2) and oxygen (O2) need to be broken down. The splitting of hydrogen and oxygen is classed as bond-breaking. The process can then move on to bond-forming, where two H atoms bond with a single O atom, thus producing water.
Fig. 2: Woolsey Fire, https://commons.wikimedia.org
We can also explore bond-breaking and forming when exploring combustion. In this type of reaction, bonds are broken between hydrocarbons as well as bonds breaking between oxygen molecules. Both of these are classed as bond-breaking. They absorb energy and so would be endothermic. To produce the final products of combustion, carbon dioxide and water, bonds need to form between carbon and oxygen as well as bonds forming between hydrogen and oxygen. This is known as bond-forming, whereby energy is released and therefore, classed as exothermic.
Bond energy equation
One of the main things we need to be able to calculate is the overall energy change of a reaction. To determine this we need knowledge of different bond energies.
Bond energy: The energy needs to break the bond between two atoms. It is measured in kJ/mol.
To calculate the change in energy we need two things:
- The energy required to break bonds from the reactants
- The energy released when bonds are formed to produce products
Together we can form this equation to calculate the overall energy change:
Energy change = total bond energy of the reactant – total bond energy of the products
Units of bond energy
Energy is generally measured in joules (J). But sometimes the number may be very large so can be divided by 1000 to show the equivalent in kilo-joules (kJ). We use kilo-joules when exploring the units of bond energy. In addition to this, we use the bond energy per mole. Therefore, the total units of bond energy are kilo-joules per mole which can be written like this: kJ/mol.
The bond energy of one H-H bond is 440,000 joules. To make this easier when using the value to work out bond energy we can divide this to form kilo-joules.
440,000 J : 1000 = 440kJ
This means 440,000J is the same/equals 440kJ.
Bond energy practice problems with answers
Now that we have explored how to calculate energy change, let us go through some examples. We will explore each problem step by step and use the average bond energy below. Please note that these bond energies are not accurate.
Average Bond Energies of Various Bonds in kJ |
|---|
| Bond | Bond Energy (kJ/mol) |
| H-H | 440 |
| F-F | 160 |
| H-F | 570 |
| C-H | 420 |
| O=O | 500 |
| C=O | 750 |
| H-O | 470 |
| N≡N | 940 |
| N-H | 400 |
For this example, let us use the Haber Process, where nitrogen and hydrogen react to form ammonia. This reaction can go forwards and backwards, but we will be calculating the forwards reaction.
Chemical equation:
$$N_{2(g)} + 3H_{2 (g)}\rightleftharpoons 2NH_{3(g)}$$
1. First we need calculate the bond energy between the reactants. For this reaction it is one mole of nitrogen to nitrogen (triple bond) and three moles of hydrogen to hydrogen (single bond).
1 mol · N≡N (940 kJ/mol) = 940 kJ
3 mol · H-H (440 kJ/mol) = 1600 kJ
So the total bond breaking energy is: 940 kJ + 1320 kJ = 2260 kJ
2. Now we need to calculate the bond energy between the products. For this reaction it is two moles of nitrogen to hydrogen single bonds. For this bond, each nitrogen is bond to three hydrogens so it is 6 bonds.
6 mol x N-H (400 kJ/mol) = 2400 kJ
3. Finally, we will calculate the energy change.
Energy change = 2260 kJ - 2400 kJ = -140 kJ/mol per reaction
Overall energy change = -140 kJ/mol
For our second example we will be exploring the reaction between hydrogen and fluorine.
Chemical equation: H2(g) + F2(g) → 2HF(g)
1. First we need calculate the bond energy between the reactants, so for this reaction it is the one mole of hydrogen to hydrogen in a single bond and one mole of fluorine to fluorine also in a single bond.
1 mol · H-H (440 kJ/mol) = 440 kJ
1 mol · F-F (160 kJ/mol) = 160 kJ
So the total bond breaking energy is: 440 kJ + 160 kJ = 600 kJ
2. Now we need to calculate the bond energy between the products. For this reaction it is two moles of hydrogen to fluorine in a single bond.
2 mol · H-F (570 kJ/mol) = 1140 kJ
So the total bond breaking energy is: 1140 kJ
3. Finally, we will calculate the energy change.
Energy change = 600 - 1140 = -540 kJ for each reaction
Overall energy change = -540 kJ for each reaction
For our final example let us explore the reacting between oxygen and methane.
Chemical equation: CH4(g) + 2O2 (g) → CO2 (g) + 2H2O
1. First we need calculate the bond energy between the reactants, so for this reaction it is 4 single bonds between carbon and hydrogen and 2 moles of double bonded oxygen.
4 mol x C-H (420 kJ/mol) = 1680 kJ
2 mol x O=O (500 kJ/mol) = 1000 kJ
So the total bond breaking energy is: 1680 + 1000 = 2680 kJ
2. Now we need to calculate the bond energy between the products. For this reaction it is two moles of water and one mole of carbon dioxide.
4 mol · H-O (470 kJ/mol) = 1880 kJ
2 mol · C=O (800 kJ/mol) = 1600 kJ
So the total bond breaking energy is: 1880 + 1600 = 3480 kJ
3. Finally, we will calculate the energy change.
Energy change = 2680 - 3480 = -800 kJ
Overall energy change = -800 kJ
The figures in these examples are purely to show how to work out energy changes.
Reaction profiles
Reaction profiles are another way in which we can explore exothermic and endothermic reactions.
Fig. 3: Exothermic Reaction, https://commons.wikimedia.org
This is an example of an exothermic reaction profile. Where the energy of the products (C) is less than the reactants (A+B). This is the opposite for endothermic reaction profiles, where the energy of the products is higher than the reactants. You can explore this further in another article.
Bond energy calculations - Key takeaways
- In a reaction bonds need to broken and bonds need to be formed.
- Making bonds is an example of an endothermic reaction.
- Breaking bonds is an example of an exothermic reaction.
- To calculate energy we need the average bond energies of the reactants and products.
- Energy change = total bond energy of the reactant – total bond energy of the products.
- The units of bond energy are kilo-joules per mole which can be written like this kJ/mol.
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