Example 5. Noniedeal Mixtures

The first four examples all used simulations on the n-pentane + n-heptane system. This mixture is about as close to ideal (in the thermodynamic sense) as can be found. Now let us look at the acetone + chloroform system and investigate how non-idealities play a role in mixture behavior.

With the Controls tab selcted in the simulation window, click on the Acetone (r) + Chloroform (b)  radio button. For this system

    Urb >> Ubb = Urr     and     Sb = Sr

This means that the cross interactions between the red and blue molecules (Urb) are larger than either the blue-blue (Ubb) or red-red (Urr) interactions even though the sizes of the two molecules are about the same. This is because hydrogen bonding can occur between the H atom on the chloroform molecule and the carbonyl oxygen in the acetone molecule. Chloroform has a donor hydrogen but not hydrogen-bond acceptor; acetone has an acceptor but no hydrogen that is polar enough to act as a donor. Interactions between unlike components that are quite disimilar from the pure-component interactions give rise to preferential clustering within the fluid. For example, red and blue molecules prefer to be together. This gives rise to activity coefficients considerably different than unity. Can you predict whether the activity coefficients are greater or less than unity based on the relative strengths of the interactions?

Look at the Excess G tab. Note the sign of the excess Gibbs energy. Does this indicate positive or negative deviations from Raoults law? Is this consistent with the activity coefficients shown on the Gamma tab?

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More Examples: Example 1 | Example 2 | Example 3 | Example 4