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Physical state of acids when they are ionized

by ElmorshedyDr
Tags: acids, ionized, physical, state
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ElmorshedyDr
#19
May13-14, 06:16 PM
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Quote Quote by Borek View Post

In general, dissolution doesn't mean creating typical chemical bonds between a solvent and a solute. However, they have to interact in some way (using Van der Waals' forces). Gases like nitrogen or oxygen, dissolve in water this way.

I thought that solubility is only restricted to substances that dissociate in the solvent

Otherwise (if it forms only van der waal's forces) it's calle miscibility
Yanick
#20
May13-14, 07:59 PM
P: 380
You have some terms mixed up. Solubility doesn't mean anything dissociates. It just means that you can make a solution. For example n-butanol is soluble in water to an extent. There is no dissociation, the polarity induced by the hydroxyl group in n-butanol allows for solvation of the molecule by the water. Miscibility, from my understanding, is infinite solubility. Ethanol, methanol and propanol are miscible with water.

To illustrate this point we will get back to n-butanol, with a finite solubility in water (73 g/L according to Wikipedia). If you add a small amount of n-butanol to water, it will dissolve. Keep adding these amounts and you will reach a point where the solution is saturated and you will have a separation of the phases. You will observe two layers, one with the aqueous solution of water and n-butanol and one with n-butanol. This occurs because n-butanol has a finite solubility and once you hit the limit, no more solution can form.

Miscible liquids will never hit that limit. You can keep adding ethanol to water until you have 99% ethanol and 1% water. All that happened is that at some point, rather philosophical and arbitrary IMO, you switched from water being the solvent and ethanol the solute to water being the solute and ethanol the solvent. In other words you went from an aqueous ethanol solution to an ethanolic water solution, if you get my drift.

Now I will wait for Borek to come tell me that everything I said is wrong
Qube
#21
May13-14, 08:19 PM
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Quote Quote by Borek View Post
HCl is a gas at STP, to make it liquid you need either low temp (boiling point around -85 °C) or high pressure. But even then it is hardly an acid, as HCl molecule is covalent. It requires water to dissociate and produce H+.

Are all acids/bases only acids or bases in solution?

I'm thinking that even though HCl is covalent as a gas it can still act as an acid because it is polar covalent and the H+ is pretty electrophilic.
Yanick
#22
May13-14, 09:14 PM
P: 380
Acid base chemistry can be done in the gas phase as well and I believe much of the early work on determination of pKa values was performed this way. Quantum mechanical calculations are also typically easier to perform on gas phase systems. From what little I know of these measurements and calculations, I believe the energies are measured and pKa's are inferred from the ΔG in the gas phase.

You should keep in mind that the high dielectric constant of water is one of the main reasons that molecules such as HCl gas can dissociate in aqueous solutions. Another interesting tidbit to those familiar with the formalism of acid base calculations is that you can use other solvents, such as say ammonia, to perform acid base chemistry. In the ammonia case you would have equations like

HA + NH3 → A- + NH4+

and you would have to define some other equilibrium constant to determine acidities and basicities of various species because Ka and pKa are reserved for the dissociation reactions in water.
Qube
#23
May13-14, 11:51 PM
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P: 442
Right, that's what I thought, because I was once confused about why certain substances do not exist - i.e. are very unstable (think smelling salts like ammonium carbonate). These undergo a large-extent acid/base reaction (both Bronsted and Lewis I think).

Borek
#24
May14-14, 02:30 AM
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Quote Quote by Yanick View Post
To illustrate this point we will get back to n-butanol, with a finite solubility in water (73 g/L according to Wikipedia). If you add a small amount of n-butanol to water, it will dissolve. Keep adding these amounts and you will reach a point where the solution is saturated and you will have a separation of the phases. You will observe two layers, one with the aqueous solution of water and n-butanol and one with n-butanol. This occurs because n-butanol has a finite solubility and once you hit the limit, no more solution can form.
One minor thing - one phase is a saturated water solution of n-butanol, the other phase is a saturated n-butanol solution of water. (Which gives an interesting way of thinking about miscibility - for miscible substances there is no gap between saturated solutions of A in B and B in A; that's one of explanations that can be given for the existence of a critical solution temperature).

Now I will wait for Borek to come tell me that everything I said is wrong
Quite the opposite - you've hit the nail on the head


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