Video Transcript
Ethene and iodine react to form
1,2-diiodoethane. The equation for this reaction is
shown. The total energy change per mole of
ethene reacted is negative 24 kilojoules per mole. The energies of selected bonds in
the reactants and products are given in the table. Calculate, to the nearest
kilojoules per mole, the bond energy of the CI bond.
Bond energy is the average amount
of energy required to break a bond in one mole of gaseous particles. To answer this question, we need to
determine the bond energy of a carbon-iodine single bond. We’ve been given a reaction
equation and the bond energies of several types of bonds involved in the
reaction. We’ve also been told that the total
energy change per mole of ethene reacted is negative 24 kilojoules per mole. This value is the change in bond
enthalpy represented with the symbol Δ𝐻. The negative sign tells us that
this reaction is exothermic, meaning that energy is released to the surroundings
during the reaction.
The change in bond enthalpy of a
reaction can be calculated by summing the bond energies of the bonds broken during
the reaction and subtracting the sum of the bond energies of the bonds that are
formed. If we look at the reaction
equation, we can see that a carbon-carbon double bond and an iodine-iodine single
bond are broken during the reaction. So the sum of the bond energies of
the bonds that are broken is 602 kilojoules per mole plus 148 kilojoules per
mole. This gives us a value of 750
kilojoules per mole. We can go ahead and substitute this
value along with the change in bond enthalpy into the equation.
During the reaction, one
carbon-carbon single bond and two carbon-iodine single bonds are formed. So the sum of the bond energies of
the bonds formed during the reaction is equal to 346 kilojoules per mole plus two
times the bond energy of the carbon-iodine single bond, which we need to
determine. We can substitute the sum of the
bond energies of the bonds formed into the equation. We can begin to solve for the bond
energy of the carbon-iodine bond by distributing the negative sign inside of the
parentheses. Then we can subtract 346 kilojoules
per mole from 750 kilojoules per mole, giving us 404 kilojoules per mole.
Subtracting 404 kilojoules per mole
from both sides of the equation gives us negative 428 kilojoules per mole equals
negative two times the carbon iodine bond energy. Dividing both sides of the equation
by negative two gives us the bond energy of the carbon-iodine bond, which is 214
kilojoules per mole. So to the nearest kilojoules per
mole, we have determined that the bond energy of the carbon-iodine single bond is
214 kilojoules per mole.
