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Ch 9.4c Born Haber Cycle | General Chemistry | YouTubeToText
YouTube Transcript: Ch 9.4c Born Haber Cycle
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Core Theme
The Born-Haber cycle is a thermodynamic tool that uses Hess's Law to calculate lattice energies by breaking down the formation of an ionic solid into a series of known energy changes, allowing for the determination of the unknown lattice energy.
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so how do we calculate these lattice energies
energies
we can use hess's law and do a classic cycle
cycle
which we call the born haber cycle and
what this
is is a cyclic kind of series of reactions
reactions
that we know most of the energies that
are involved
except for some of the uh except for one
of the values
and we can calculate one of those values
uh in there so we are first going to
look at our compound
um thinking about the formation of
csf and what is the delta h
lattice of this reaction
so just remembering uh what the lattice
energy is
it should be csf um
forming as a solid to get cs
plus one in the gaseous state plus fluorine
fluorine
minus 1 in the gaseous state
so we can write a series of reactions
that look like
this and kind of thinking about the the
reactions that are
important to us um so we're going to
start off at the bottom with csf
and then you'll notice on the series of
reactions that one of them looks very familiar
familiar
that one thing that looks very familiar is
is
the formation of csf delta h
not formation and that delta h naught
formation if you look this up in the
backyard book
is minus 553.5
kilojoules and if we write that reaction
uh for it so remember we are going to
form a mole of that
we're going to start off with cesium solid
solid
plus one half of a molecule of fluorine
gaseous so that is already here we
already see that right here
so that is this reaction down here this delta
delta
h formation not however
we want to go the opposite direction we
want to go
this direction we want to stay cesium
fluoride and breaking it apart
to form cesium solid and fluorine gas
half a mole of fluorine gas
so that ends up being negative of our delta
delta
h not a formation and that's going to equal
equal
we're going to change the sign of that
553.5
kilojoules per mole of this reaction
and then we can look at these other
reactions and most of them can be found
in tables so this first one is the heat
of sublimation
we are going to look at the cesium solid
going to cesium
gas so it will sublime into a gas form
and how much energy does that take
the next thing we want to think about is
ionization energy
so remember our ionization energy uh
periodic trends it's the energy it takes
to remove
one electron from its outer core shell
so taking it moving it off
cesium has one that will remove and this
is the energy
to remove one of those electrons now we
have to think about taking fluorine gas
so we want to do fluorine gas
and we want to break this apart so we
want to break that apart
of that fluorine gas how much energy
does that take
and since we're only looking at half a
mole of fluorine gas
we're going to cut that dissociation
energy in half
so half of that dissociation energy equals
equals
79.4 kilojoules per mole
so we're at this point right now in our
born haber cycle
the next step uh is we're interested in forming
forming
the anion fluorine gas so there's an
electron affinity
and how much energy it takes to attract
an electron
so when we attract an electron we're
going to uh
drop in our energy so that's our
electron affinity
energy for that reaction on here
so uh then we're left with this reaction
and this cesium plus going to fluorine minus
minus
will form cesium f and that is actually the
the
opposite of this reaction over
here so our delta h lattice
oops different color
going from cesium uh over here to the
cesium plus and the fluorine gas
but what we actually want to calculate
is the opposite of this so that's our
negative delta h
of our lattice for that reaction
okay so that means on red we are going down
down
in energy so the born haber cycles means
we started at cesium fluoride remember
according to the state function if we start
start
and end at the same point we haven't
done anything
so the sum of all of our delta h's have
to equal
zero for our reactions
and when we have an l delta h equals
zero we can add all them
up our only unknown is this value over here
here
we are able to look up everything so
that means our plus
553.5 our delta h formation
plus our ionization energy
plus half of our dissociation energy
which is 79.4
328.2 kilojoules
our sorry minus our delta h lattice
so that is this value right here
we set this all equal to zero that means
our delta h lattice
equals 756.9
kilojoules per mole
of reaction and that is our reaction
over here
so this is our cesium f forms
cesium plus in the gaseous state
plus fluorine fluoride in the gaseous state
state
so that is how we calculate
one of the values using the born haber cycle
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