[Cooper]

Hi,

I am having conceptual difficulty with finding the entropy of formation of urea.

See the standard molar entropies below...

$$C(s)+\frac{1}{2}O_2(g)+N_2(g)+2H_2(g)\rightarrow CO(NH_2)_2(s)\\\Delta S^{\phi}_m=\underset{p}\Sigma-\underset{r}\Sigma\\\Delta S^{\phi}_m=[650.0-5.74-\frac{1}{2}(205.138)-191.61-2(130.684)]JK^{-1}mol^{-1}\\\Delta S^{\phi}_m=+88.71JK^{-1}mol^{-1}$$

I don't see how the entropy can be positive when you are going from three moles of gas to one mole of solid. Am I doing the math wrong?

Thanks

[Corribus]

I think your intuition is right. According to NIST, the standard molar entropy of solid urea is ~104 J/K mol, not 650 J/K mol. Where did you get your value? 

http://webbook.nist.gov/cgi/cbook.cgi?ID=C57136&Mask=2#Thermo-Condensed

[Cooper]

I think your intuition is right. According to NIST, the standard molar entropy of solid urea is ~104 J/K mol, not 650 J/K mol. Where did you get your value? 

http://webbook.nist.gov/cgi/cbook.cgi?ID=C57136&Mask=2#Thermo-Condensed

Thanks! We were just given the values for a homework problem, no idea where they came from.

[Corribus]

It's good to see you're thinking about your answers rather than chugging out numbers. This is the mark of a true budding scientist!