hello and welcome to the chemistry
solution this tutorial is on lattice
energies the lattice energy is the
energy required to separate one mole of
a solid ionic compound into its gaseous
ions and it's directly related to the
force of attraction between oppositely
charged particles so it's directly
related to the force of attraction
between the negatively and positively
charged ions composing the ionic
compound so in this equation Q 1 and Q 2
are the charges on the ions and D is the
distance between the ions so what does
this mean well it means that the lattice
energy is directly related to the charge
of the ions so as the charge of the ions
increases the lattice energy will
increase and it's indirectly related to
the size of the ions or the distance
between them so as the ions get smaller
that also means that the lattice energy
will increase so lattice energy
increases upon increasing charge of the
ions and decreases upon increasing size
of the ions now while the distance
between the ions plays a role in the
lattice energy the size of ions doesn't
change significantly though if you are
comparing different compounds and trying
to determine the relative magnitude of
their lattice energies the charges of
the ions will have a greater effect on
the lattice energy than the size of the
ions or the distance between them
lattice energy cannot be calculated
directly and so how our lattice energy
is calculated and so they calculate
lattice energies by forming gaseous
neutral atoms forming ions from the
gaseous atoms and then combining the
ions to form an ionic solid you can then
use Hess's law to in directly calculate
the lattice energy by combining the
enthalpy changes from these three
separate
steps let's look at an example using
lattice energies rank the following
compounds in order of increasing lattice
energy first let's locate where these
elements are on the periodic table and
so what you'll notice is that because
lithium potassium and sodium are all in
Group one of the periodic table that
means they all form ions with the charge
of plus one
and likewise fluorine chlorine and
bromine are all in group seven of the
periodic table which means they all
forms ions with the charge of negative
one so in this example the charges of
our ions remain constant and so the
difference in lattice energies is going
to be due to the distance between the
ions forming the compounds and the
distance between the ions forming the
compounds is going to get bigger as the
size of the ions increases remember that
the size of ions increase as you go down
your periodic table and remember that
increasing size corresponds to a
decrease in lattice energy so KBR will
have the lowest lattice energy followed
by NaCl and LIF will have the greatest
lattice energy let's look at another
example rank the following compounds in
order of increasing lattice energy
magnesium oxide sodium chloride and
potassium chloride first let's locate
where these elements are in the periodic
table here we have magnesium oxide
sodium chloride and potassium chloride
so what if we're trying to compare
compounds differing in both ion charge
and size remember that the charge of
ions has more of an effect on the
lattice energy because the atomic radius
and distance between ions does not vary
significantly over a large range of ions
and so in this case potassium chloride
will have the lowest lattice energy as
both potassium chloride and sodium
chloride are composed of ions having
charges of plus 1 in mine
one and now the only difference between
potassium chloride and sodium chloride
is the presence of either the sodium or
the potassium ion because the potassium
ion is larger than the sodium ion when
comparing these two compounds that are
composed of ions with the same charge
potassium chloride and will have the
lowest lattice energy because potassium
is the larger ion when comparing sodium
chloride and magnesium oxide sodium
chloride is composed of ions with a
charge of plus one and minus one where
magnesium oxide is composed of ions
having charges of plus 2 and minus 2
because magnesium is on the second group
of the periodic table and those elements
form ions with the charge of plus 2 an
oxygen is in the sixth group of the
periodic table and those elements form
ions with a charge of minus two so in
this case magnesium oxide will have the
greatest latticed energy thanks for
watching the chemistry solution we hope
you enjoyed this tutorial
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