let's check out a problem regarding resonance energy so this asks
the standard molar enthalpies of formation of cyclohexane liquid and
benzene liquid at 25 degrees Celsius are negative 156 and 49 kilojoules per mole
respectively the standard enthalpy of hydrogenation of cyclohexene liquid at
25 degrees Celsius is negative 119 kilojoules per mole
using this data estimate the magnitude of the resonance energy of benzene so we
know that benzene is aromatic we know that aromatisse 'ti brings a system to a
lower energy and so we want to estimate that energy how much lower does benzene
go on an energy diagram then we might expect for a system with 3 PI bonds so
this is going to take a little bit of critical thinking it might not be
immediately obvious but we have some data here so try to think critically and
see if you can figure this out
so let's take a look at what we have first let's
look at the reaction whereby we perform hydrogenation on cyclohexene to get
cyclohexane and we know that the standard enthalpy of hydrogenation for
cyclohexane is negative 119 kilojoules per mole so that is how much energy is
released when a single pi bond on cyclohexane is hydrogenated therefore
what we might expect from an analytical standpoint is if we are to perform
hydrogenation on benzene we could triple this value and get negative 357
kilojoules per mole so that is what we might assume given the fact that benzene
has 3 PI bonds we could simply triple this previous value and
this might be what we would expect to get in terms of the energy released when
benzene is fully hydrogenated to get cyclohexane now in actuality if we use
the standard molar enthalpies of formation we know that we can do
products minus reactants we know that the enthalpy of formation of cyclohexane
is negative 156 and the enthalpy of formation of benzene is 49 negative one
56-49 is negative 205 kilojoules per mole so this is the actual
experimentally determined value for the enthalpy change when benzene is
completely hydrogenated to give cyclohexane and so this is giving us
some information about the resonance energy of benzene benzene is sitting at
a lower energy than we would expect if we ignored the effects of resonance if
we simply considered three isolated PI bonds in this system the resonance
associated with this aromatic system brings it to a lower energy such that
only 205 kilojoules per mole can be released when benzene is fully
hydrogenated because it is sitting at a lower energy to begin with and so to
find that resonance energy we're going to take our expected value minus the
actual value for these Delta H's that's negative 357 minus negative 205 and
that means that negative 152 kilojoules per mole is that resonance energy
benzene will sit 152 kilojoules lower on an energy diagram then we would expect
for a system with 3 isolated PI bonds so that was a very interesting exercise in
terms of looking at some expected enthalpy data versus some actual
enthalpy data and using that to discern some information about a phenomenon like resonance.