Question

What is the enthalpy change of 30^oC system if the heat capacity of air at 38^oC is 5 J/^oC and heat capacity of water vapor at 38^oC is 10 J/^oC, enthalpy of vaporization is 10 J, and the humidity is 60%?

(a) 14 J

(b) 39 J

(c) 65 J

(d) 94 J

The question was asked during an interview for a job.

Asked question is from Applications of Humidity Chart topic in chapter 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

Right answer is (d) 94 J

Explanation: ∆H = 5(38 – 30) + 0.6*[10(38 – 30) + 10] = 40 + 54 = 94 J.