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### Topic: Activities  (Read 4484 times)

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#### Darwin

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##### Activities
« on: January 07, 2010, 12:23:22 PM »
Hi

I have a question that has bothered me for long now, and I can't really find an answer. For an e.g. HCl(aq) solution, is the following relation true?

aHCl2 = aH+*aCl-

or should it rather be aHCl = aH+*aCl-?

where a is the activity.

#### Schrödinger

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##### Re: Activities
« Reply #1 on: January 07, 2010, 01:22:31 PM »
Hi

I have a question that has bothered me for long now, and I can't really find an answer. For an e.g. HCl(aq) solution, is the following relation true?

aHCl2 = aH+*aCl-

or should it rather be aHCl = aH+*aCl-?

where a is the activity.

What seems logical to you and why?
"Destiny is not a matter of chance; but a matter of choice. It is not a thing to be waited for; it is a thing to be achieved."
- William Jennings Bryan

#### Darwin

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##### Re: Activities
« Reply #2 on: January 07, 2010, 01:46:05 PM »
What seems logical to you and why?

Well, normally for a reaction like HCl   H+ + Cl-, we write [HCl(aq)] = [H+] = [Cl-], so [HCl]2 = [H+][Cl-], and the exact same thing with molalities. So my logical answer is that aHCl2 = aH+aCl-. However, in a standard solution book that I have, it is indicated that aHCl = aH+aCl-, though without any concomitant explanation. I'm wondering if that's wrong..

#### Borek

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##### Re: Activities
« Reply #3 on: January 07, 2010, 05:13:01 PM »
I am not aware what aHCl is, unless it is activity of dissolved but undissociated HCl, similarly to - say - aCH3COOH - activity of undissociated acetic acid. Usually we assume activity of uncharged species is just their concentration (activity coefficient is 1).
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#### Schrödinger

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##### Re: Activities
« Reply #4 on: January 09, 2010, 11:36:07 AM »
Well, normally for a reaction like HCl  H+ + Cl-, we write [HCl(aq)] = [H+] = [Cl-], so [HCl]2 = [H+][Cl-], and the exact same thing with molalities. So my logical answer is that aHCl2 = aH+aCl-.

Why do think it is [HCl]2 ?
"Destiny is not a matter of chance; but a matter of choice. It is not a thing to be waited for; it is a thing to be achieved."
- William Jennings Bryan