2024.9.04
Chemistry at pirika.com > Chemstry > Chemical engineering > Reprint: gas-liquid equilibrium estimation by ASOG. >
In practice, this ethanol/water, 50% mol solution boils at around 80 °C. The boiling point is 7 °C lower. Raoult’s law does not hold.
How can we treat real solutions as easily as ideal solutions? Various predecessors have thought about this, and the concept of active volume (Activity：ai) was invented by Gilbert Lewis. Activity is the ‘effective molar concentration’ in a real solution and is defined by the following equation.
ai≡γi*xi (1-4)
This γi is called the activity coefficient.
If the molar concentration xi in Raoult’s law is replaced by the effective molar concentration ai, P=P1*a1+P2*a2, the form of the equation is the same as in Raoult’s formula.
This means that the activity coefficient is introduced as a correction factor to treat the real solution as an ideal solution in the form of the equation.
In many cases, the form in which the activity coefficient is explicitly left out is used, as in the following equation.
P=P1*γ1*x1+P2*γ2*x2 (1-5)
Next section: 1.4 Calculation of activity coefficients
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