Physical state of acids when they are ionized

  • #1
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Main Question or Discussion Point

HCl(l) + H2O H+(aq) + Cl-(aq)

or is it

HCl(g)+ H2O H+(aq) + Cl-(aq)

If HCL is liquid is it dil. or conc. ??
 
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Answers and Replies

  • #2
Borek
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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+.
 
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  • #3
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You are so helpful :smile:
My point is when we use acids in experiments to determine the Ka, what is the physical state of the acid
Can it be a conc or a dil liquid ?
 
  • #4
Borek
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Neither - it is dissolved, (aq).

Liquid and dissolved are two different things.
 
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  • #5
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Neither - it is dissolved, (aq).

Liquid and dissolved are two different things.
(aq) and dil are two different things?
 
  • #6
Borek
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dil most likely means diluted (it is not a standard IUPAC state symbol), so in some cases it is equivalent to (aq). But when you say "diluted liquid" you suggest you started with a liquid and you added water. You can't start with a liquid HCl, just like you can't start with a liquid oxalic acid. You can have liquid acetic acid or sulfuric acid though.

What you posted so far suggests you think about liquid and (aq) as if these were equivalent. They are not. Liquid is a liquid, (aq) means something is dissolved in water. Just because something is liquid doesn't mean it is dissolved in water - for example gasoline is a liquid, but it is not dissolved, it even doesn't mix with water.
 
  • #7
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I think I understand what (aq) means
it means that the substance exists as dissociated ions bonded to the water molecules
 
  • #8
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:smile:
 
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  • #9
Borek
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No, it means it exists as a water solution. Ethanol doesn't dissociate, yet it can be written as C2H5OH(aq) - it just means water solution of ethanol, period.
 
  • #10
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I was confused because this equation is used to get hydrogen ions (hydroxonium ions) and chloride ions in an aqueous solution,

so I was wondering whether the acid that is used is already in an aqueous solution that already contains Hydrogen and chloride ions or is it a pure liquid.

so this equation just represents adding more water to an HCl aqueous solution?
 
  • #11
Borek
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so this equation just represents adding more water to an HCl aqueous solution?
Yes, that's the closest description of the reality.

Note that your statement "this equation" is unclear, as you posted two equations, so it is not entirely clear which one you mean. But the HCl dissociation is

HCl(aq) → H+(aq) + Cl-(aq)
 
  • #12
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Yes, that's the closest description of the reality.



Note that your statement "this equation" is unclear, as you posted two equations, so it is not entirely clear.

Oh sorry :smile:, I meant the first equation where the HCl is in an aqueous solution
 
  • #13
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But the HCl dissociation is
HCl(aq) → H+(aq) + Cl-(aq)

How is HCl(aq) prepared in the first place ??
 
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  • #14
Borek
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By dissolving gaseous HCl in water. If you would want to write the reaction equation it would be

HCl(g) → HCl(aq)

followed by the dissociation.
 
  • #15
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By dissolving gaseous HCl in water. If you would want to write the reaction equation it would be

HCl(g) → HCl(aq)

followed by the dissociation.
:smile: so HCl gas dissolves in water,
Is that by forming bonds between the HCl molecules and the water
Molecules ??
 
  • #16
Borek
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You really want to get that deep? Because details can get ugly.

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.

In the case of HCl things are more complicated, as HCl dissociates immediately, with both H+ and Cl- being strongly hydrated (surrounded by water molecules; water molecules are dipoles and are strongly attracted by small anions and cations).
 
  • #17
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In the case of HCl things are more complicated, as HCl dissociates immediately, with both H+ and Cl- being strongly hydrated (surrounded by water molecules; water molecules are dipoles and are strongly attracted by small anions and cations).

I don't get it, HCl never exists as a whole molecule in water ??
 
  • #18
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You really want to get that deep? Because details can get ugly.

Nothing is better than deep understanding :smile:

I'm grateful, no one helped me before as you do
 
  • #19
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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
 
  • #20
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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 :smile:
 
  • #21
Qube
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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.
 
  • #22
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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.
 
  • #23
Qube
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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).

puvaheje.jpg
 
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  • #24
Borek
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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 :smile:
Quite the opposite - you've hit the nail on the head :thumbs:
 

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