acidity (chemistry) help!

[yestomeds]

Hi all, I have 2 questions about acids/bases

1. "the number of ions in solution does to depend on the base strength." So, the question has KOH (strong base) and KoAc (weak base) in an aqueous solution, and we're measuring the current (how the diff solutions can conduct electricity).

The answer says that both bases are salts (first of all, why/how is this the case), and therefore will completely disassociate into water, therefore, the ion concentration is the same in both solutions. <-- I know this probably seems simple enough to you but can anyone break this down further and make it even more simplified for me... please... #confused

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2. Why does a stronger acid have a higher boiling point than does a weaker acid?
I would have thought that a stronger acid... more easily dissociates, is more unstable, thus you'd need less energy to go and boil it? No?

(Separately speaking, this is from a TBR acids/bases passage, passage 5, p. 271. The solutions are on p. 284, and tbh, though their solutions are great, I do.not.understand.the.explanation(all.7.sentences).for.this.question.at.all.).

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[ImmunoLove]

Ok typing on iPhone app so this prolly will be insufficient (PM me if this is the case), but long story short:

1. A salt is a combo of cations and anions (meaning the sharing of electrons is so disparate that it's beyond polar covalent, it's ionic). Without actually knowing the Q, I'm assuming this is some kind of colligative property question, and you're supposed to figure out the number of ions in solution. It's unclear whether each molecule of the different bases produces the same number of ions (and thus you start with same molar concentrations), or if each molecule dissociates into a different number of ions (but the molar concentrations are different such that you end up with the same "ion concentration").

2. Any time you're thinking of acids and bases, always analyze them with respect to their conjugate species. A strong acid has a low pka and its conjugate base (a weak base) is more stable that the conjugate base of a weak acid. So, a strong acid is strong because it dissociates all the way, and its entropy is (2+ ions vs 1 acid molecule) is greater than the enthalpy of staying in the single molecule (oh, GIbbs free energy). Its conjugate base is weak because it's so stable, it doesn't need to snag a cation the way a strong base (conjugate of a weak acid) would.

Whereas the strength of the acid/base has to do with the intramolecular bond (in the same molecule), the boiling point has to do with the strength of the intermolecular bond (eg dissociated acid/base and not dissociated acid/base with its solution of water, respectively).

You're right that strong acids are more unstable, but that instability has to do with the bonds within the acid Itself (intramolecular bonds), which is why it's a strong acid (it is unstable and dissociates all the way). once it dissociates, it forms more ions (than an equimolar solution of weak acid), and these ions form stronger intermolecular bonds with H2O than the polar covalent counterparts of weak acids (plus, the stronger the acid, the more the dissociation, and the greater number of bonds with H2O, and subsequently more energy is required to boil the solution (recall colligative properties, melting point depression, boiling point elevation, and liquid-phase expansion).

Ok this was a lousy response but hopefully it's enough for you to glean a thing or two...

 

[sazerac]

Alternate answer for #2:

The boiling point depends on the number of particles of solvent in the solute. The more particles, the higher the boiling point.

Imagine you tossed 50 molecules of a strong acid HA into solution. What would happen? Strong acids completely dissociate, so there would be 50 H+ particles and 50 A- particles, a total of 100 particles. That boiling point is going way up!

Now imagine tossing 50 molecules of a weak acid into the solvent. Huh, a weak acid only partially dissociates, let's say half of them dissociate. You have 25 H+ particles, 25 A- particles, and 25 stubborn HA molecules that just won't dissociate because they are a weak acid. That is 75 particles, so the boiling point will go up a little. Certainly not as much as a solution with 100 particles in it.

 

[ImmunoLove]

Alternate answer for #2:

The boiling point depends on the number of particles of solvent in the solute. The more particles, the higher the boiling point.

Imagine you tossed 50 molecules of a strong acid HA into solution. What would happen? Strong acids completely dissociate, so there would be 50 H+ particles and 50 A- particles, a total of 100 particles. That boiling point is going way up!

Now imagine tossing 50 molecules of a weak acid into the solvent. Huh, a weak acid only partially dissociates, let's say half of them dissociate. You have 25 H+ particles, 25 A- particles, and 25 stubborn HA molecules that just won't dissociate because they are a weak acid. That is 75 particles, so the boiling point will go up a little. Certainly not as much as a solution with 100 particles in it.

This is correct (and also why I stated "than an equimolar soln"--being an ochem 2 lecturer makes one precise).

Also, love the screen name. Great bev !