Video: Bond Energy Changes in the Reactions of Acetylene

Acetylene (C₂H₄) can react with a wide variety of compounds to produce useful organic compounds containing carbon-carbon double bonds. The average energies of selected bonds are listed in the table. By calculating the total changes in bond energy per mole of acetylene reacted, place the following reactions in order from the most exothermic to the least.

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Video Transcript

Acetylene, C2H4, can react with a wide variety of compounds to produce useful organic compounds containing carbon carbon double bonds. The average energies of selected bonds are listed in the table. By calculating the total changes in bond energy per mole of acetylene reacted, place the following reactions in order of most exothermic to least.

During a chemical reaction, some or all of the bonds are broken. This process requires energy. We call the energy that’s needed to break a chemical bond the bond energy, which is given per mole of bonds. The bond energy is equivalent to the enthalpy. Since it takes energy to break bonds, the process of breaking bonds in a chemical reaction is endothermic. So its enthalpy change would be positive. When new bonds are formed, energy is released. When energy is released, we have an exothermic process. So the enthalpy change for this would be negative.

The difference in enthalpy between the breaking and the making of bonds will be the change in enthalpy for the reaction. So we can find the total enthalpy change of the reaction and therefore determine if the reaction is endothermic or exothermic. By finding the enthalpy change for the bonds that are broken and adding it to the enthalpy change for the bonds that are formed during their reaction.

Again, since the bond energy is the energy that’s needed to break a chemical bond. And so the negative of the bond energy will be the energy that’s released when a bond is formed. We can find the change in enthalpy for a reaction by taking the sum of the bond energies of the bonds that are broken and subtracting the sum of the bond energy for the bonds that are formed.

Since the bonds that are broken are in the reactants and the bonds that are formed are in the products. We could also think of this as taking the sum of the bond energies for the reactants and subtracting the sum of the bond energies for the products.

Now let’s find the total change in bond energy for each of the reactions to figure out which one is the most exothermic.

In the first reaction, the CC triple bond in acetylene is broken as well as the HCl single bond. We form a CH single bond, a CC double bond, and a CCl single bond. So to find the change in enthalpy for this reaction, we’ll add the bond energies of the bonds that are broken and subtract the bond energy of the bonds that are formed. If we plug in all the bond energies from the table, we’ll find that the enthalpy change for the first chemical reaction is negative 77 kilojoules per mole.

Now let’s look at the second chemical reaction. In the second chemical reaction, we again break the CC triple bond in acetylene. We break the HH single bond, and we form two CH single bonds and a CC double bond. We’ll calculate the enthalpy of the reaction by again summing the bond energies of the reactants and subtracting the bond energies of the products. We’ll want to make sure we multiply the bond energy of the CH bond by two since we form two of them in the products. If we plug in everything from the table, we’ll find that the enthalpy change for this reaction is 157 kilojoules per mole.

Finally, let’s have a look at our third reaction. In this reaction, we break the CC triple bond and the HBr single bond. And we form the CH single bond, the CC double bond, and the CBr single bond. We’ll calculate the change in the enthalpy for the reaction the same way we did for the other two reactions. Plugging everything in from the table, we’ll get negative 101 kilojoules per mole as the enthalpy change for this reaction.

Now let’s list our reactions in order of most exothermic to least. Since exothermic is negative enthalpy change, the most exothermic reaction is (2) followed by (3) and then (1).

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