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Specific Heat Capacity vs Cv(Vap) vs Cp(vap)

Posted: Sat Feb 11, 2017 9:15 pm
by Michael_Johanis_2K
"A pressure cooker containing 100.0 g of liquid water at 25 C is placed on the stove until all water becomes superheated steam at 121 C. Assume no steam escapes the pressure cooker.

a) Find q from 25C to 100C
b) Find q during boiling
c) Find any other heat gained or lot by the water as it goes from 25C to 121C

For Part C, I used q = m C DeltaT.
m = 100.0 g, T = 21C, C = 2.01 J/(Cg), which is the specific heat capacity of water vapor. Why is my value of C wrong? I was supposed to use Cv(vap) but I have never used this before on questions involving fusion/vaporization. I have always used the specific heat capacity of (material) vapor -- water vapor in this case, which was wrong. Can someone please explain?

Re: Specific Heat Capacity vs Cv(Vap) vs Cp(vap)  [ENDORSED]

Posted: Sun Feb 12, 2017 9:17 am
by Chem_Mod
A pressure cooker is constant volume as opposed to constant pressure. The standard heat capacity you've been using is the constant pressure heat capacity which is applicable for cases in which the reaction takes place under a constant atmospheric pressure. Because the pressure cooker is constant volume and not constant pressure (the pressure is actually increasing in the cooker), the constant volume specific heat must be used.