this video is brought to you by fiverr.com click on the link in the description below and make any purchase

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

for more tutorials support me on patreon so I can keep making content and as

always feel free to email me professor Dave explains at gmail.com