[floridagirl]

As lattice energy increases, does solubility decrease or increase?

I have an absent-minded chemistry professor who is always saying things backwards and then correcting it later on in the semester, so I have in my notes that she said (and I quote), "As lattice energy increases, solubility INCREASES, and so does the melting point."

That just doesn't make sense to me - if the lattice energy essentially measures the strength of the ionic bond, you would think an increased lattice energy value would mean a decrease in solubility (and an increase in melting point), since the oppositely charged ions would be that much more difficult to pull apart.


For reference, this is the problem I'm working on:

Which is the least soluble compound?

LiI - lattice energy of -758kj/mol
NaI - lattice energy of -704kj/mol
KI - lattice energy of -658kj/mol
RBI - lattice energy of -629kj/mol
CsI - lattice energy of -603kj/mol

(I'm also kind of confused about the negative symbol - I'm assuming that means the formation of the ionic bond between Li and I releases 758kj/mol of energy and is exothermic, which means it would take 758kj/mol of lattice energy to pull them apart...correct?)

[washey]

As lattice energy increases, solubility will decrease. Lattice energy is the amount of energy required to seperate the crystals into gaseous ions. Energy is required to break apart the solid, so the lattice energy will always be positive. In your question I do not think the dash is a negative symbol. I think it is just seperating the word from the value.

When an ionic solid is formed, the Lattice energy will be negative.

Hope this helps.

[macman104]

Yes, energy is released when the ionic compound forms, so the negative sign is probably the release of energy.  And yes, the compound that requires the least amount of energy to break apart will be the most soluble compound assuming all other things equal.

[washey]

Either way, negative or positive, it should affect the outcome of the ccorrect answer. If it is a negative, they are indicating the energy required to form the ion, since energy will be released in the formation. The positive number will indicate the energy required to break apart the solid. So, for LiI the lattice energy would be +/- 758 KJ/mol depending on if it forming (-) or breaking apart(+). Refer to the Born-Harbor cycle.

[macman104]

Either way, negative or positive, it should affect the outcome of the ccorrect answer. If it is a negative, they are indicating the energy required to form the ion, since energy will be released in the formation. The positive number will indicate the energy required to break apart the solid. So, for LiI the lattice energy would be +/- 758 KJ/mol depending on if it forming (-) or breaking apart(+). Refer to the Born-Harbor cycle.

You are correct, I was responding to the posters question about the meaning of the negative value at the bottom of their post.

[kashikarvedant26]

As lattice energy increases, does solubility decrease or increase?

I have an absent-minded chemistry professor who is always saying things backwards and then correcting it later on in the semester, so I have in my notes that she said (and I quote), "As lattice energy increases, solubility INCREASES, and so does the melting point."

That just doesn't make sense to me - if the lattice energy essentially measures the strength of the ionic bond, you would think an increased lattice energy value would mean a decrease in solubility (and an increase in melting point), since the oppositely charged ions would be that much more difficult to pull apart.


For reference, this is the problem I'm working on:

Which is the least soluble compound?

LiI - lattice energy of -758kj/mol
NaI - lattice energy of -704kj/mol
KI - lattice energy of -658kj/mol
RBI - lattice energy of -629kj/mol
CsI - lattice energy of -603kj/mol

(I'm also kind of confused about the negative symbol - I'm assuming that means the formation of the ionic bond between Li and I releases 758kj/mol of energy and is exothermic, which means it would take 758kj/mol of lattice energy to pull them apart...correct?)

in cases where bigger anions are present(Like Br-,I-,(SO3)2-) LE = f(k/(r+ + r-) = f(k/(r-)(since, cation is very small)
and hydration energy  which is f(k/r+) + f(k/r-) = f(k/r+) as r- is very large.
solubility is proportional to HE- LE
therefore down a grp (here grp 1), HE decreases as r+ increases and LE remains almost unaffected.
hence, solubility order is :
LiI>NaI>KI>RbI>CsI

[mjc123]

See for example this thread: http://www.chemicalforums.com/index.php?topic=81896.msg297925#msg297925. It's a complex balance - usually a small difference between large quantities. And for "solubility" you have to consider entropy as well as enthalpy!

[Enthalpy]

[...] hence, solubility order is : LiI>NaI>KI>RbI>CsI

Unfortunately, Nature isn't educated enough to understand your logic. Solubilities at Wiki for instance, in varying units:
https://en.wikipedia.org/wiki/Lithium_iodide
https://en.wikipedia.org/wiki/Sodium_iodide
https://en.wikipedia.org/wiki/Potassium_iodide
https://en.wikipedia.org/wiki/Rubidium_iodide
https://en.wikipedia.org/wiki/Caesium_iodide

KI has the highest melting point, and despite it's neither the first nor last to melt, NaI is the most soluble.