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C5 B) Bond Energies – Part 1
C5 B) Bond Energies – Part 1
Click here for a printable PDF of the equations and tables in this section and the next section.
In the previous section, we learnt that there are two different types of reactions; endothermic and exothermic reactions.
The reaction profiles for both endothermic and exothermic reactions are shown below.
In the previous section, we learnt that there are two different types of reactions; endothermic and exothermic reactions.
- Endothermic reactions take in energy from their surroundings. They usually take in energy in the form of heat, which means that the temperature of the surroundings will decrease after an endothermic reaction has taken place. Photosynthesis is an example of an endothermic reaction.
- Exothermic reactions give out energy to their surroundings. The energy given out is usually heat, which means that the temperature of the surroundings will increase after an exothermic reaction has taken place. Respiration or combustion are examples of exothermic reactions.
The reaction profiles for both endothermic and exothermic reactions are shown below.
The overall energy change is the difference between the energy in the products and the energy in the reactants. In this section we are going to look at working out the overall energy change from a reaction using bond energy calculations.
Breaking and Creating Bonds
During a reaction, the bonds in the reactants are broken and the bonds in the products are created.
From the energy needed to break the bonds and the energy released when new bonds are created, we are able to work out the overall energy change during a reaction by using the following formula:
During a reaction, the bonds in the reactants are broken and the bonds in the products are created.
- The breaking of bonds in the reactants requires energy – bonds breaking is endothermic (this is why all reactions require an activation energy in order for a reaction to start).
- The creation of bonds in the products releases energy – bond creation is exothermic (this is why all of the reaction profiles go down after the peak part of the reaction profile).
From the energy needed to break the bonds and the energy released when new bonds are created, we are able to work out the overall energy change during a reaction by using the following formula:
The value for overall energy change informs us as to whether a reaction is endothermic or exothermic.
Let’s now have a look at a few examples.
- A positive value for overall energy change means that more energy is required to break the bonds in the reactants than the amount of energy released when the bonds in the products are created. This means that the reaction is an endothermic reaction as the energy in the products is greater than the energy in the reactants.
- A negative value for overall energy change means that more energy is given out by the creation of bonds in the products than the energy that is required to break the bonds in the reactants. This means that the reaction will be exothermic as the energy in the products is less than the energy in the reactants.
Let’s now have a look at a few examples.
Example 1
Hydrogen and chlorine react to produce hydrogen chlorine gas. The balanced symbol equation is:
Hydrogen and chlorine react to produce hydrogen chlorine gas. The balanced symbol equation is:
The bond energies are shown in the table below.
Find the overall energy change and say whether the reaction is exothermic or endothermic?
The first step is to calculate the energy that is required to break the bonds. We work this out by adding up the energies for all of the bonds in the reactants (the left side of the reaction). For the reactants, there is 1 H-H bond and 1 Cl-Cl bond. The working for adding up the bond energies for the reactants is shown below.
The first step is to calculate the energy that is required to break the bonds. We work this out by adding up the energies for all of the bonds in the reactants (the left side of the reaction). For the reactants, there is 1 H-H bond and 1 Cl-Cl bond. The working for adding up the bond energies for the reactants is shown below.
The energy required to break the bonds is 679 KJ/mol.
The next step is to work out the amount of energy that is released by the creation of new bonds in the products. We work this out by adding up the energies for all of the bonds in the products. The products of this reaction are HCl and there are 2 of them (2HCl). In each of the two HCl’s there is one H-Cl bond. Therefore, for all of the products, there are 2 H-Cl bonds. The working for finding the energy released when the bonds are created is shown below.
The next step is to work out the amount of energy that is released by the creation of new bonds in the products. We work this out by adding up the energies for all of the bonds in the products. The products of this reaction are HCl and there are 2 of them (2HCl). In each of the two HCl’s there is one H-Cl bond. Therefore, for all of the products, there are 2 H-Cl bonds. The working for finding the energy released when the bonds are created is shown below.
The energy released when the bonds are created is 864 KJ/mol.
We now have everything we need to work out the overall energy change; the amount of energy required to break the bonds is 679 KJ/mol, and the amount of energy released when the bonds are created is 864 KJ/mol. We sub these values into the overall energy change formula.
We now have everything we need to work out the overall energy change; the amount of energy required to break the bonds is 679 KJ/mol, and the amount of energy released when the bonds are created is 864 KJ/mol. We sub these values into the overall energy change formula.
This tells us that the overall energy change is -185 KJ/mol. This is a negative value, which tells us that energy is given out during the reaction. Therefore, the reaction is exothermic.
The Reaction Profile
I am now going to draw the reaction profile for this reaction.
The activation energy is the amount of energy that is required to start a reaction, and the activation energy is used to break the bonds in the reactants. From our working, we found that the activation energy (energy needed to break bonds) is 679 KJ/mol. On the reaction profile, the activation energy is the difference between the energy in the reactants and the highest energy point on the reaction profile.
After the bonds have been broken, new bonds in the products are created. The creation of bonds releases energy. From our working, we found that the amount of energy released when the bonds are created is 864 KJ/mol. On the reaction profile, the energy released from bond creation goes from the highest energy point on the reaction profile to the products.
A greater amount of energy was released when the bonds were created (864 KJ/mol) compared to the amount of energy required to break the bonds (679 KJ/mol). This means that the overall energy change is negative; the products have less energy than the reactants. From our working, we found that the overall energy change is -185 KJ/mol.
The reaction profile with the activation energy, energy released from bond creation and overall energy change is shown below.
I am now going to draw the reaction profile for this reaction.
The activation energy is the amount of energy that is required to start a reaction, and the activation energy is used to break the bonds in the reactants. From our working, we found that the activation energy (energy needed to break bonds) is 679 KJ/mol. On the reaction profile, the activation energy is the difference between the energy in the reactants and the highest energy point on the reaction profile.
After the bonds have been broken, new bonds in the products are created. The creation of bonds releases energy. From our working, we found that the amount of energy released when the bonds are created is 864 KJ/mol. On the reaction profile, the energy released from bond creation goes from the highest energy point on the reaction profile to the products.
A greater amount of energy was released when the bonds were created (864 KJ/mol) compared to the amount of energy required to break the bonds (679 KJ/mol). This means that the overall energy change is negative; the products have less energy than the reactants. From our working, we found that the overall energy change is -185 KJ/mol.
The reaction profile with the activation energy, energy released from bond creation and overall energy change is shown below.
Usually we wouldn’t label the amount of energy released from bond creation. So, here is the reaction profile with just the activation energy and the overall energy change.
Example 2
The reaction below shows hydrogen bromine reacting to produce hydrogen and bromine.
The reaction below shows hydrogen bromine reacting to produce hydrogen and bromine.
The bond energies are shown in the table below.
Is the reaction endothermic or exothermic?
The first step is to work out the amount of energy that is needed to break the bonds. We do this by adding up all of the bond energies in the reactants. For the reactants, there are 2 HBr molecules, and each of these HBr’s contain 1 H-Br bond. The working for finding the bond energies in the reactants is shown below.
The first step is to work out the amount of energy that is needed to break the bonds. We do this by adding up all of the bond energies in the reactants. For the reactants, there are 2 HBr molecules, and each of these HBr’s contain 1 H-Br bond. The working for finding the bond energies in the reactants is shown below.
The amount of energy required to break the bonds is 732 KJ/mol.
The next step is to work out the amount of energy that is released by the creation of new bonds in the products. We work this out by adding up the energies for all of the bonds in the products. The working is shown below.
The next step is to work out the amount of energy that is released by the creation of new bonds in the products. We work this out by adding up the energies for all of the bonds in the products. The working is shown below.
The energy released when the bonds are created is 629 KJ/mol.
We now have everything we need to work out the overall energy change; the energy needed to break the bonds is 732 KJ/mol, and the energy released when the bonds are created is 629 KJ/mol. We sub these values into the overall energy change formula.
We now have everything we need to work out the overall energy change; the energy needed to break the bonds is 732 KJ/mol, and the energy released when the bonds are created is 629 KJ/mol. We sub these values into the overall energy change formula.
The overall energy change is 103 KJ/mol. This is a positive value, which means that energy is taken in during this reaction. Therefore, this reaction is an endothermic reaction.