Table of Contents
ACTIVITY and the ACTIVITY COEFFICIENT
We’ve always learned that
PPT Slide
Word Definition
Mathematical Definition of Activity
Problem with Mathematical Definition of Activity
Mathematical Definition of the Activity Coefficient
New Mathematical Definition for Activity
Debye-Huckel equation log yi = -.51 zi2u1/2 1+ (3.28 bi u1/2) where yi is the activity coefficient u is the ionic strength of the solution bi is the effective diameter of the hydrated ion and zi is the charge on the ion
Since we are now defining activity with the use of the activity coefficient, we must set up a reference system. If you will recall, the definition of the activity coefficient is the deviation of the activity of the current solution to some reference solution.
Definition of a Reference System
Since the activity coefficient is only valid at low concentrations, the function being used is close to or moving towards the reference system. Therefore, the activity coefficient is going towards 1 and is usually put in the equation that way. So
Allowing the activity to equal our function makes the calculations much easier. Now we can just take our function, say molar concentrations (Products over Reactants) and put that into our G equation.
Using the function as our activity is a nice and convenient calculation, but it is not the true value. Real systems can be very complex. The next slide shows a reference system verses a possible real system.
Dilute System Versus Concentrated System
In the real system we would have to take into account the interactions between ions and the structure of the solvent in response to all the ions. But for the purposes of class, we will only worry about the calculations which involve a simpler system that is similar to the reference system.
References
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Author: Leif Munkel
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