Question

A)

Explain what is Fugacity (provide an explanation as detailed as possible). Explain how the fugacity criteria for VLE and Chemical reactions is related to the concept of increasing entropy

B)

What are internal Energy and Enthalpy? how are they related?

C)

A mixture of water-ethanol containing a significant amount of methanol is being distilled (separation by VLE). Someone tells you that the methanol is not a problem because it boils off first as it has a lower boiling point. Is this true? Discuss in detail.

Answer 1

A) Fugacity is the "tendency of a gas to escape". Quantitatively, it is a hypothetical thermodynamic parameter which carries the information about the deviation of a real gas from its ideal behavior. The ratio of fugacity to the partial pressure of a gas is called it's fugacity coefficient. This coefficient is directly related to the residual thermodynamic properties of the vapour phase of the substance, which contains the information about the nonideal behavior of the gas.

The fugacity criteria for VLE equates the fugacity in the vapour and liquid phases for a substance, which through the Gibbs free energy based definition of fugacity relates the eqaulibrium value of ∆G to 0. This is the case when the Gibbs free energy is minimized, which corresponds to the maximization of the entropy of the system (increased to its highest possible value).

B) Internal energy is the sum total of the kinetic energy of the particles of a system in the reference of its center of mass. As a result, internal energy is a quantification of the temperature dependent energy of a system, which in turn is related to the molecular level vibrations.

Enthalpy, is the sum of internal energy and the PV energy contained in a thermodynamic system. (H=U +PV)

C) NO, this is not true. Because the system forms an azeotrope by virtue of which, the methanol water system evaporates by the same composition as it is present in the liquid phase. Thus it becomes pointless to distill the two fractions out beyond a certain composition of enrichment.