heat of sublimation

[yebo]

I am confused because the answer to EK 1001 chemistry question 633 states that an approximate value for the heat of sublimation is the heat of vaporization plus the heat of fusion.

So I guess that maybe the key work here is *approximate* but shouldn't the value for sublimation be the heat of vaporization plus the heat of fusion PLUS the heat need to increase the temp of the liquid to its vaporization temp? Otherwise it would seem that it would require less heat to sublime something than to go from solid to liquid to gas, which doesn't seem right...

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[AKMCAT]

Hmm. I think its = mc(specific heat of solid)delta T+ Heat of fusion+ mc(liquid)delta T+ heat of vaporization. Im not sure why thats the answer I could be wrong.

 

[yebo]

I agree - thats what it seems like to should be to me. So I was confused by the answer they gave as the correct one.

I guess my only doubt was that since its sublimation and you aren't going through the liquid phase you don't have to put extra energy into heating up the liquid, right?

 

[DrknoSDN]

Approximation is more than 99% accurate for water. Probably applies to many substances.

Water:
Heat of Fusion = 79.8 Calories/gram
Specific Heat = 1 Cal/gram <--- insignificant
Heat of Vaporization = 539 Cal/gram

Sources:
http://en.wikipedia.org/wiki/Enthalpy_of_fusion#Reference_values_of_common_substances
http://en.wikipedia.org/wiki/Heat_capacity#Measurement_of_heat_capacity
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html