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Inorganic : calorimetry
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 Message 16 of 23 in Discussion 
From: MSN Nickname·Steve·  in response to Message 15Sent: 3/4/2006 6:39 PM
>>  Curious where you got the values for  44.01 kJ/mol   <<

I think that should have been 44.1 kJ.  I used standard heats of formation values of H2O (l) and H2O (g) from the appendix of one of my general chemistry textbooks.  DHf° for H2O (l) = -285.85 kJ/mol and DHf° for H2O (g) = -241.8 kJ/mol.  Therefore DH° for the reaction H2O(l) --> H2O (g) will be

DH° reaction  =  [(1 mol)(-241.8 kJ/mol)]  -  [(1 mol)(-285.85 kJ/mol)  =  44.05 kJ
(This is just the "products minus reactants" method for calculating DH° of reactions using the standard enthalpies of formation of the substances.)

That is Dvap at standard temperature, 25°C.  But meanwhile I found some better values.  According to one of my textbooks it is 44.94 kJ/mol at 0°C and 40.67 kJ/mol at 100°C, so it does vary with temperature somewhat.  They are in the CRC Handbook also, under "Steam Tables" in my edition; at "T = 32°F, Evap = 1075.5 Btu/lbm" which converts to 45.07 kJ/mol).  Here's a web site that has some of this information too:  http://www.lsbu.ac.uk/water/data.html.  It gives DHvap = 45.051 kJ/mol at 0°C.  As you can see, the values vary a little from source to source.  I'll just go with 45.05 kJ/mol.
 

>>  If I used DH° sublimation that would just equal 44.01  kJ/mol for fusion + the 6.01  kJ/mol for vaporization?  Hess' Law?  <<

That's right, here's what you have:

H2O (s)  -->  H2O (l)     DH  =  6.01 kJ at 0°C
H2O (l)  -->  H2O (g)     DH  =  45.05 kJ at 0°C
H2O (s)  -->  H2O (g)    DH  =  51.06 kJ at 0°C
 
 
>>  also where would I find the molecular mass of ice - would it be that much different than water?  <<

Since ice is made of water, that's the molecular weight to use, 18.0153 g/mol!
 

>>  So I need 7556.02 kJ of heat to melt the ball and then evaporate it.  <<

That's right; it will be a little more if you use the higher DH of 51.06 kJ/mol.
 
 
Steve


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     re: calorimetry   MSN Nickname·Steve·  3/6/2006 2:29 AM