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so let's check
out a problem regarding lattice energy the question reads for each pair of
compounds state which one has the greater lattice energy so we have al 2 O
3 versus al 2 se 3 ZnO versus NaCl m GF 2 vs. mg i2 and LIF vs. mg o so think a
little bit about what goes into lattice energy what would make a particular
ionic compound have a greater or lesser lattice energy think about that and give
this question a try so to start let's just talk a little bit about lattice
energy lattice energy is calculated this way we have a constant C and then we
have Z plus and Z minus those are the charges on the ions so if we have plus 1
and minus 1 plus 2 and minus 2 etc that's where the charges go and then we
have this R value that represents inter ionic distance or how far apart the
centres of each ion are away from each other and that has to do with the size
of the ions because the bigger they are the farther apart the centres will be so
in order to get a greater lattice energy in order for that value to be bigger we
can have greater charges or larger values we can have larger z values right
because if those z values get bigger the fraction gets bigger because those are
in the numerator so greater values for those charges will result in a greater
lattice energy or a smaller in tirana distance will also result in a greater
lattice energy because if we have smaller ions and we have a smaller in
tirana distance then that value in the denominator will get smaller and as a
result the whole fraction will become larger so the two ways to get a greater
lattice energy are greater charges and a smaller inter ionic distance or smaller
ions so as we go through these just to remind ourselves greater charges or
smaller ions equals more energy so let's get our first example al 2 O 3 versus Al
2 S III so we have aluminum in each that's not going to change we have
aluminum and aluminum so let's compare what's different we have oh 3 and we
have s III what do we know that is different about these elements
well they're in the same group so they have the same common charge so the
charges are the same here that's not going to help however oxygen is smaller
than selenium so that means al 2 O 3 will have the smaller in tirana distance
because it has smaller ions and therefore that compound has a greater
lattice energy now for the second example we have zinc oxide versus sodium
chloride well here we do have different charges zinc oxide we know that oxides
are two minus so zinc must be two plus so we have we have a two plus cation and
a two minus anion and then sodium chloride we know that those are going to
be plus one and minus one so here we are looking at the magnitudes of the charges
if zinc oxide has two plus and 2 minus in that fraction we're gonna have two
and two up top versus one and one that's going to be a factor of four so without
even really having to look at inter onic distance we know that zinc oxide is
going to have a greater lattice energy now looking at magnesium fluoride versus
magnesium iodide once again magnesium is the same so let's look at fluorine and
iodine those are in the same group so they're gonna have the same common
charge they will both become minus one to become fluoride and iodide so it's
not about charge but we know that iodide is much larger than fluoride so because
fluoride is so much smaller than iodide magnesium fluoride has a smaller in
tirana distance than magnesium iodide so
smaller ions smaller intronic distance magnesium fluoride is going to have a
stronger or a greater lattice energy lastly looking at lithium fluoride
versus magnesium oxide let's look at the charges lithium fluoride those are gonna
commonly make plus 1 and minus 1 charges and magnesium oxide we've got 2 plus and
2 minus so because magnesium oxide has greater charges that is going to have
the greater lattice energy so that is the answer for these 4 thanks for
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