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## Main Question or Discussion Point

Hello - I am wondering abut the meaning of chemical activity. Most definitions are something along the lines of "effective concentration," which is fine until you have a real concentration in a lab, and you don't know if you need to calculate the concentration or "effective" concentration of the species. Basically its too ambiguous a definition to be of real use.

Elsewhere you find activity being used in real equations, such as in the definition of p{H}, or in the definition of the equilibrium constant.

Finally after hours of reading on the side, I finally came across the definition in Hansen and McDonald's excellent book, "Theory of Simple Liquids," where they give the definition of activity as exp(μ/kT)*(mkT/2π(hbar^2))^1.5. I don't know about anyone else, but not being a statistical mechanisist, that definition doesn't help too much on the conceptual end. Wikipedia gives a similar definition, but without the multiplying factor (cube of the De Broglie length). The concept of propensity to undergo phase change across interfaces makes sense, but that seems too simple too.

I am currently reading literature on high ionic strength solutions, but everyone talks about the activity coefficient either in highly theoretical terms or as if it just exists and nothing more.

Can anyone give a middle of the road definition that's conceptual, not a series of equations, but not "effective concentration?" Why would a concentration have a real side and an effective side that are different?

Thanks in advance.

Elsewhere you find activity being used in real equations, such as in the definition of p{H}, or in the definition of the equilibrium constant.

Finally after hours of reading on the side, I finally came across the definition in Hansen and McDonald's excellent book, "Theory of Simple Liquids," where they give the definition of activity as exp(μ/kT)*(mkT/2π(hbar^2))^1.5. I don't know about anyone else, but not being a statistical mechanisist, that definition doesn't help too much on the conceptual end. Wikipedia gives a similar definition, but without the multiplying factor (cube of the De Broglie length). The concept of propensity to undergo phase change across interfaces makes sense, but that seems too simple too.

I am currently reading literature on high ionic strength solutions, but everyone talks about the activity coefficient either in highly theoretical terms or as if it just exists and nothing more.

Can anyone give a middle of the road definition that's conceptual, not a series of equations, but not "effective concentration?" Why would a concentration have a real side and an effective side that are different?

Thanks in advance.

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