Question

How can we calculate the calorific value of a fuel at constant pressure?

(a) QC.V. = QC.P. – (Δn)×R×T

(b) QC.V. = QC.P. + (Δn)×R×T

(c) QC.P. = QC.V. – (Δn)×R×T

(d) QC.V. = QC.P. – (Δn)×R×T×S

I have been asked this question during a job interview.

My question is taken from Calorific Value topic in portion Fuels and Combustion of Engineering Chemistry II

Answer 1

Right choice is (c) QC.P. = QC.V. – (Δn)×R×T

To explain I would say: If there is a decrease in the number of gaseous molecules formed after reaction, then Δn will have a negative value and consequently will be higher than QC.V.

QC.P. = QC.V. – (Δn)×R×T

where QC.P. is calorific value at constant pressure, QC.V. is calorific value at constant volume, R is gas constant, T is temperature and S is entropy.