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Activity and chemical potential

  1. Apr 16, 2012 #1
    can someone help me in understanding the difference between chemical potential and activity?
    Why it is important to know/determine activities of certain species in material growth? Given that activity is dimensionless quantity, what is it measuring?

    Also, what is the activity of pure solids, liquids, and gases? Is it important to specify temperature, pressure, etc. when asking for species' activites?

    Thank you in advance.
  2. jcsd
  3. Apr 16, 2012 #2
    Roughly speaking, the chemical potential of species X is a function of its activity. In the dilute limit ,i.e, when the particles of species X do not "see" each other, the activity is simply the partial pressure of the gas or the concentration of solid species in aqueous solution. On the other hand if the particles of species X can "see" each other (in other words can interact with each other), then we express this effect emperically by specifiying activity coefficient. As an example, the activity of dissolved solid in aquous solution would be concentration x the activity coefficient.
  4. Apr 17, 2012 #3
    Thanks for the reply. I am still, however, confused about the activity concept.
    So, does it measure the strength of interaction of the species?
    Why is it important to know it?

  5. Apr 17, 2012 #4
    The activity is a measure of the abundance of the species. What I described in my previous post is the how the notion of reactivity is distinct from the familiar concepts of gas partial pressure and solute concentration. I summarize again that in the dilute limit there is no distinction, while in the high abundance limit (interacting species) there is a distinction.

    It is importance depends on the context. For example, a corrosion scientist would care about the activity of chloride ions in a solution to be able to predict the corrosion rate of a metal. Another example, a surface scientist would care about the activity of oxygen gas to study its adsorption on metal surfaces.

    I recommend reading an introductory text in electrochemistry (or may be corrosion science or physical chemistry) to understand the definition. If you want to go one step deeper , you can read a statitichal mechanics text to see in general how interactions between particles lead to deviation from ideal behavior.

    Edit: I checked Wikipedia page on activity(chemistry) and it seems also a good introduction to the topic.
    Last edited: Apr 17, 2012
  6. Apr 17, 2012 #5


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    Science Advisor

    Maybe you would find the following book valuable:

    Klotz, Rosenberg, Chemical Thermodynamics
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