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).
