Question

What is the enthalpy change of 15^oC system if the heat capacity of air at 20^oC is 10 J/^oC and heat capacity of water vapor at 20^oC is 20 J/^oC, enthalpy of vaporization is 20 J, and the humidity is 50%?

(a) 50 J

(b) 60 J

(c) 80 J

(d) 110 J

The question was asked during an interview.

I'm obligated to ask this question of Applications of Humidity Chart in portion Humidity Charts & their Use, Analysis of the Degrees of Freedom in a Steady – State Process, Solving Material and Energy Balances Using Process and Unsteady State Material and Energy Balances of Basic Chemical Engineering

Answer 1

Correct answer is (d) 110 J

To elaborate: ∆H = 10(20 – 15) + 0.5*[20(20 – 15) + 20] = 50 + 60 = 110 J.