Covalent vs Ionic - Lithium Iodide

[betterfuture]

So the following reaction takes place

LiI + BaS --> no reaction

I would have thought LiI dissociates into Lithium and iodide ions and thus would undergo a double displacement reaction, but no. What I don't understand is how NaCl is an ionic compound and can dissociate but LiI is a molecule (shares a covalent bond). Doesn't Li fall under the alkali metal and Iodide falls under the halogens? They share a different type of bond. Seems strange.

Strong acids like HCl, HBr, and HI all dissociate, yet all are non-metals so you'd think they they'd form covalent bonds. But with H2S, its covalent.

I know this deals with electronegativity differences, but can someone explain how to figure this out on a test? Do we just memorize the exceptions or will they give us the Pauling scale. Periodic trends don't help much; there are exceptions.

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[aldol16]

How would you get a double displacement reaction? Whether LiI dissolves depends on the solvent but LiI is fairly soluble in water and alcohol solvents. But the problem is that BaS will not dissociate. Sulfide anions are not particularly stable - it's a really strong base.

 

[betterfuture]

Oh. So it's BaS that won't dissociate? I thought LiI was a molecule and joined by a covalent bond? I got mixed answers from google.

 

[aldol16]

Oh. So it's BaS that won't dissociate? I thought LiI was a molecule and joined by a covalent bond? I got mixed answers from google.

Depends on the solvent, but LiI should dissociate - you should be familiar with Li+ and I- ions in solution in the context of other salts. But S(2-) is an ion you usually don't see because it does not exist in solution freely - it's strongly basic and will react.

 

[betterfuture]

Thanks!

 

[deleted799625]

So there's no reaction because the products LiS and BaI would dissociate - are soluble in water?


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[aldol16]

In most cases, Li or Na -> ionic bond. Both form ionic bonds with even oxygen, from memory.

It is very interesting indeed, although this is due more to Li+ and Na+ functioning essentially as larger protons. So you get Li2O and Na2O.

What the question is really asking then, "is this fat halogen likely to pull an alkali earth metal off sulfur, leaving you with BaI+ and S-2" which is actually pretty easy.

That isn't very easy, which is why the reaction doesn't occur, as given in the question. First of all, S(2-) is not stable at all and therefore would not exist freely in solution. Therefore, this reaction must be thermodynamically uphill, which means it's not going to happen. Second of all, you wouldn't ever form BaI+ because that's also dangerously unstable. If this reaction occurs (which it doesn't), it would form BaI2.

 

[aldol16]

Isn't that exactly what I said?

What the question is really asking then, "is this fat halogen likely to pull an alkali earth metal off sulfur, leaving you with BaI+ and S-2"

No, what you said is that this is "pretty easy," which it isn't at all.

Except to get to BaI2 you have to violate conservation of mass, which is more egregious than a charge where it shouldn't be.

It's not a violation of conservation of mass. This problem is a mass balance problem which, I agree, any good chemistry student could answer. 2LiI + BaS ---> BaI2 + Li2S. Stoichiometry, my friend. Again, this reaction does not happen because BaS does not dissociate readily.

 

[aldol16]

Your reading comprehension is problematic:

"What the question is really asking then, "is this fat halogen likely to pull an alkali earth metal off sulfur, leaving you with BaI+ and S-2" which is actually pretty easy."

"easy" in this context is clearly a reference to the question and not a reference to the reaction, and I reject your weak attempt to prove you're smarter than me. 🙂

Perhaps you should revisit your comprehension of English grammar. And your understanding of mass conservation while you're at it.

Of course I'm not smarter than you. I wouldn't ever suggest such a laughable thing.