# Talk:Thermodynamic activity

WikiProject Chemistry (Rated Start-class, High-importance)
This article is within the scope of WikiProject Chemistry, a collaborative effort to improve the coverage of chemistry on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Start  This article has been rated as Start-Class on the project's quality scale.
High  This article has been rated as High-importance on the project's importance scale.

## Redirect

I don't understand why this redirected to the disambiguation page. I've fixed it and tried to write a bit of an article from other sources on the net, but I'm only here because I was looking for information, and I'm not an expert on the topic. Please, chemists, help me!!!

Redirect doens't work (see my new section)173.189.72.93 (talk) 23:17, 18 November 2014 (UTC)

## Unit; examples

What's the unit for measuring activity (I'm assuming it's unitless)? Could we find an example of a numerical value of a common solution?

Do the activities of all the components of a mixture add up to one?

The article currently fails to give a usable definition of the term.

RandomP 16:38, 19 September 2006 (UTC)

Interestingly, the definitions in the German and English articles describe a different quantity. As I know it, activity is unitless, but defined against an arbitrary standard 1 mol dm-3. Therefore, a concentration of 1.0 mol dm-3 and a coefficient of 0.9 gives an activity of 0.9. --Vuo 21:35, 19 September 2006 (UTC)
Thanks! Do you have any examples that involve specific solutions that might be well-known?
"unitless, but defined against an arbitrary standard", at least to me, appears to make no sense whatsoever. A concentration of 1 mol/l and a coefficient of .9 should surely give an activity of .9?
not really suggesting doing anything, yet, but in the interest of the sanity of any high school students who might not be allowed the same sloppiness with units in their science classes, I'd like to see sloppy use of units clearly marked on WP, I guess ...
RandomP 21:46, 19 September 2006 (UTC)
Actually, the French article seems to have a lot of good content. --Vuo 22:51, 19 September 2006 (UTC)
The activity in Chemistry is one of the few units which doesn't have a unit. So this is completely normal(It's because when you divide mol.L-1 with mol.L-1 the result has no unit).

## Usable definition, exact as it was defined

• Original Lewis' definition is:
${\displaystyle a_{i}={\frac {f_{i}}{f_{i}^{o}}}}$

so ratio of fugacities. DEfinition is valid for SOLIDS, LIQUIDES and GASES (not only for gases)!! You can choose denominator (reference fugacity) arbitrarily !! (only the usage is 101325 as perfect gas for gasses, and fugacity of pure component for solids)

• Lewis definition of fugacity is:
${\displaystyle d\mu _{i}=RTd\ ln(f_{i})}$
• LInk both equetion we get:
${\displaystyle \mu _{i}-\mu _{i}^{o}=RT\ln {\frac {f_{i}}{f_{i}^{0}}}=RT\ln {a_{i}}}$ —Preceding unsigned comment added by 212.5.210.202 (talk) 10:34, 25 September 2007 (UTC)

## Merger/Translation

Obviously the french and german articals cannot be merged without translation. If users who speak french or german or preferrably both could judge which is best it could be requested that that article be translated and merged with the english article. Vuo Has already suggested that the French article has a lot of good content what are others peoples views and it could then be requested for translation from whichever comes out on top. --Ksbrown 21:03, 22 September 2006 (UTC)

I have added it to the french translation board: Wikipedia:Translation_into_English/French#:Activit.C3.A9_chimique Ksbrown 15:39, 23 October 2006 (UTC)
The french article was merged with the english article and no original information from the english article was lost. The translation was done by SamiKaero. -Ksbrown 17:04, 24 October 2006 (UTC)

## Merger proposal

Hello,

I came across Activity coefficient, and felt that this article does a significantly better job of covering the topic and propose that what is good from that article be merged here, but not by me! Thanks User A1 (talk) 13:38, 22 December 2007 (UTC)

Thanks for this proposal. I was unaware of the existence of Activity coefficient when I re-vamped this article. Obviously that article is more comprehensive. What, if anything, do you think should be kept from this one?
I'm not an expert at merging, so I'm worried about the Wiki citations to the two articles being correctly processed.Petergans (talk) 10:22, 31 December 2007 (UTC)
Perhaps both articles are needed because it it a vast area. My understanding was that the article on activity coefficient will focus on how to practically calculate the activity coefficients in the specific cases, and not dwell on what activity is. This is similar to the situation in articles "equilibrium"-"equilibrium constant", "heat transfer"-"heat transfer coefficient", and numerous others. Just my C\$0.02. Stan J. Klimas (talk) 14:40, 16 February 2008 (UTC)

## P*solvent and Ksolute

Hi. Would it be possible for somebody to write explicitly what P*solvent and Ksolute in section "definition" are? Thanks!

## ${\displaystyle \mu _{i}^{*}(T,p)}$ vs ${\displaystyle \mu _{i}^{\Theta }(T)}$

You can find in the article Ideal_solution, that

If the chemical potential of pure liquid ${\displaystyle i}$ is denoted ${\displaystyle \mu _{i}^{*}}$, then the chemical potential of ${\displaystyle i}$ in an ideal solution is

${\displaystyle \mu _{i}=\mu _{i}^{*}+\Delta \mu _{i,\mathrm {mix} }=\mu _{i}^{*}+RT\ln x_{i}}$

I guess, that ${\displaystyle \mu _{i}^{*}}$ depends on T and p as well: ${\displaystyle \mu _{i}^{*}(T,p)}$. Whereas here at Activity (chemistry) one finds

In ideal mixtures we can write the dependency of the chemical potential of a species i on the composition (written as mole fraction x of i) as:

${\displaystyle \mu _{i}(T,p)=\mu _{i}^{\Theta }(T)+R\cdot T\cdot \ln(x_{i})}$

and µi0 is the value of µi under standard conditions.

This means, that ${\displaystyle \mu _{i}^{\Theta }(T)}$ is only function of the temperature and not the pressure, since the pressure is to be taken at ${\displaystyle p=p^{\Theta }}$. But then where has the pressure dependence of ${\displaystyle \mu _{i}(T,p)}$ in this second formula disappeared? --szaboka87.184.93.231 (talk) 17:16, 27 November 2008 (UTC)

## Gas constant

I suspect that in the page the gas constant R is erroneously used instead of the Boltzmann constant kB, but I may be wrong, as it depends on the units of measure. Can someone comment? E prova sta beta (talk) 13:23, 16 September 2009 (UTC)

The values R and kB are the same, just multiplied by avagadro's number. R and kB are both the same constant, but the units can be chosen arbitrarily, provided that they can be converted back to either j/(mol.K) or j/(at.K). Engineers and chemists tend to prefer R, and physicists tend to prefer kb. See Boltzmann_constant User A1 (talk) 02:19, 17 September 2009 (UTC)
The title of the article is "Activity (chemistry)" so using the gas constant (R) does seem appropriate. (+)H3N-Protein\Chemist-CO2(-) 14:23, 25 September 2012 (UTC)

## Notation of mole/amount fractions

How do people feel about reconfiguring some of the notation used in the article. The amount fraction (or mole fraction), which is denoted as "x", appears quite often due to its connection with activity. However, it is important to note that when a solution is in equilibrium with its vapor, the mole fractions of each component are not the same in the solution and gas phases. Would people be opposed to using the notation "y" for the gas-phase mole fraction and "x" for solution-phase mole fraction? This is a fairly common "modern" notation that many texts have adopted. Sirsparksalot (talk) 16:16, 8 December 2012 (UTC)

If we include a mention of solution-vapour equilibrium, then I agree that we need two symbols for the two values of mole fraction, as in the Raoult's law article (where I see that 3 weeks ago you defined the symbols for the equation with yi and xi and also explained why the equation is useful - thank you). However at the moment such equilibria don't seem to be in the article, so what notation now in the article would you reconfigure? Dirac66 (talk) 19:42, 8 December 2012 (UTC)
My main reasoning for such a change was due to the fact that activities are discussed in context of both gases and solutions and that the solution sections don't explicitly state that xi is the mole fraction in the solution. Since most solutions of practical interest are made with volatile components, they will be in some sort of equilibrium with the vapor. In fact, isn't this how most solution activities are determined, by making use of this equilibrium to equate the chemical potentials of the the gas and solution? I have had several conversations with people who get confused when discussing Raoult's law because they want to use xi (which is typically used to represent the solution mole fraction) to determine the partial pressure in the gas phase using Dalton's law (which also uses xi when discussed on its own). (I should admit that I was one of these people myself when first learning about Raoult's law. :) ) I have found it convenient to adopt the yi/xi notation that several texts use to avoid the confusion. In any case, it is only a minor edit, but wanted to run it by people before investing time and effort on something that people may not like. PS - Thanks for fixing my typo, don't know how I missed that. Sirsparksalot (talk) 00:13, 9 December 2012 (UTC)
OK, I understand the reason, but I am still trying to understand exactly where you want to make the changes. I think the best way (and perhaps the way you mean) would be to systematically use yi for all mention of gas-phase mole fractions. Note that the Dalton's law article now does use yi, and the mole fraction article mentions (last sentence of intro) that the IUPAC Gold Book recommends y for gas mixtures.
So what to change? The subsection Standard states#Gases has 3 x's which can all be changed to y. And the subsections Definition#Activity coefficient and Standard states#Mixtures in general have some x's which can refer to either gas or solution, so we need to point out that they are x in solution but y in gas. But probably not every time x is used, as it would become repetitious. Dirac66 (talk) 03:08, 9 December 2012 (UTC)
Yes, that is essentially what I meant, to replace all gas-phase mole fractions with yi. Although, now that you mention it, it is more difficult than I had initially thought due to the "mixtures in general" parts of the article. I'll see what I can put together then maybe put up a test version to see what other people think about the changes before I commit to anything. Sirsparksalot (talk) 19:55, 10 December 2012 (UTC)

## Standard state for dissolved salt

Could someone insert some details about choosing the pure solid as the reference state.--188.26.22.131 (talk) 10:10, 3 July 2013 (UTC)