Practice Problem: Lattice Energy and Ionic Bond Strength


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to support this channel. 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 watching guys subscribe to my channel
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always feel free to email me professor Dave explains at gmail.com

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