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C7 H) Bond Energies – Part 2
C7 H) Bond Energies – Part 2
Click here for a printable PDF of the equations and tables in this section and the previous section.
The content in this section builds on the content that was discussed in the previous section. Therefore, make sure that you have covered the content in the previous section before working through this section (click here to be taken to the previous section).
The content in this section builds on the content that was discussed in the previous section. Therefore, make sure that you have covered the content in the previous section before working through this section (click here to be taken to the previous section).
Example 1
The balanced reaction for the burning of propane in air is shown below.
The balanced reaction for the burning of propane in air is shown below.
The bond energies are shown in the table below.
Work out the overall energy change for the reaction, and comment on whether the reaction is endothermic or exothermic?
Like with the two examples in the previous section, we need to work out the energy needed to break the bonds and the energy released when the new bonds are created. This question is a little bit trickier than the previous 2 because there are more of each of the molecules. However, you should be okay providing that you are careful.
Breaking Bonds
Like with the two examples in the previous section, we need to work out the energy needed to break the bonds and the energy released when the new bonds are created. This question is a little bit trickier than the previous 2 because there are more of each of the molecules. However, you should be okay providing that you are careful.
Breaking Bonds
Let’s start by working out the energy that is needed to break the bonds. In C3H8, there are 8 C-H bonds and 2 C-C bonds. Each of the 8 C-H bonds requires 413 KJ/mol to break, and each of the C-C bonds requires 348 KJ/mol to break. The working for the amount of energy required to break the bonds in C3H8 is shown below.
We now need to find the amount of energy required to break the bonds in 5 O2. In each of the 5 O2, there is 1 O=O bond, which requires 495 KJ/mol to break. The working for the amount of energy required to break the bonds in 5 O2 is shown below.
We now add the energies for each of these together.
This tells us that the energy required to break the bonds in the reactants is 6475 KJ/mol.
Creating Bonds
We work out the amount of energy released when the bonds are created by adding up all of the bond energies in the products. For the products, there are quite a few of each of the molecules; there are 3 carbon dioxides and 4 waters. The easiest way to work out the bond energies for the carbon dioxides and waters is to work out the amount of energy in one of the molecules, and then multiply by the number of molecules that we have.
Let’s do this for the carbon dioxide first. In each of the carbon dioxides, there are 2 C=O bonds. There are 3 carbon dioxides in total, so we multiply the energy in 1 carbon dioxide molecule by 3.
Creating Bonds
We work out the amount of energy released when the bonds are created by adding up all of the bond energies in the products. For the products, there are quite a few of each of the molecules; there are 3 carbon dioxides and 4 waters. The easiest way to work out the bond energies for the carbon dioxides and waters is to work out the amount of energy in one of the molecules, and then multiply by the number of molecules that we have.
Let’s do this for the carbon dioxide first. In each of the carbon dioxides, there are 2 C=O bonds. There are 3 carbon dioxides in total, so we multiply the energy in 1 carbon dioxide molecule by 3.
We now do the same for water. In each of the waters, there are 2 H-O bonds. There are 4 waters in total, so we multiply the energy in 1 water molecule by 4.
We then add the energies from all of the carbon dioxides and waters together.
The amount of energy released in the creation of bonds in the products is 8498 KJ/mol.
Energy Change
We now have everything that we need to work out the overall energy change; the energy needed to break the bonds is 6475 KJ/mol and the energy released when the bonds are created is 8498 KJ/mol. We sub these values into the formula below.
Energy Change
We now have everything that we need to work out the overall energy change; the energy needed to break the bonds is 6475 KJ/mol and the energy released when the bonds are created is 8498 KJ/mol. We sub these values into the formula below.
The overall energy change is -2023 KJ/mol. This is a negative value, which means that energy has been given out during the reaction. Therefore, this reaction is exothermic.