GCHEM Destroyer Number 38

[xaznxcountx]

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How much heat is needed to convert 20g of ice at -40 degrees celsius to steam at 300 degrees celsius.

ice = 2.0 j/g.c
water = 4.2 j/g.c
steam = 1.8 j/g.c


fusion = 6.0 kj/mole
vaporization = 41kj/mole

The answer tells you to set up a heating curve and there are 5 delta H's. Can someone who understands this question give me a more conceptual explanation. I understand the math, but I don't get why you wouldn't have only 3 delta H's since you are only going from ice -> water -> steam

GCHEM is my weak spot 🙁

 

[ZenoVT]

heat needed to go from -40->300 = (heat from -40->0) + (heat from ice to water) + (heat from 0-100) + (heat from water to gas) + (heat from 100-300)

q=mc(change of T)
heat from -40 -> 0 = 20*2*40
heat of ice to water = heat of fusion = 6 kj = 6000 j/mol
heat from 0-100 = 20*4.2*100
heat from water to gas = heat of vaporization = 41kj = 41,000j/mol
heat from 100-300 = 20*1.8*200

those are where u get the 5 H's

 

[xaznxcountx]

So although it is going from (H1)ice-(H2)--> (H3)water --(H4)----> steam (H5)



H2= fusion
H4= Vaporization

Can I think of it this way?

 

[ZenoVT]

exactly =D! If you remember that graph on the the fusion and vaporization of water, you could see that there are places where more heat does not change the temperature (no slope) which is where the heat of fusion and vaporization turns ice to liquid and liquid to gas.

 

[xaznxcountx]

O so basically the heat of vaporization and fusion are "intermediate" stages where the phases need to overcome the intermolecular forces right? And heat during these "intermediate" points does nothing to temperature. Nice...

Thanks VT!