How much heat needed?

  • Thread starter DrMcDreamy
  • Start date
  • #1
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Homework Statement



7 kg of ice at 0◦C is converted to steam at 100◦C. How much heat is needed? The heat of fusion for water is 79500 cal/kg, its heat of vaporization is 5.4 × 105 cal/kg, and its specific heat is 1000 cal/kg ·◦C. Answer in units of kcal.

Homework Equations



What is the formula that I am supposed to use?

Q=mc[tex]\Delta[/tex]T ?
 

Answers and Replies

  • #2
114
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You *also* need to use

Q = mL,
where m is the mass and L is the heat of fusion/vaporisation.


R.
 
  • #3
112
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you go from solid to liquid to gas, so you have to calculate the heat for each stage than add up the total of each stage. I think, also, for the vaporization you would use Q =ml where L = latent heat of vaporization.. and Q=ml again for latent heat of fusion/melting

I think thats how you do it...
 
Last edited:
  • #4
114
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You should end up with 3 separate values for Q which you will have to add up.
 
  • #5
68
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So:

Q=mc[tex]\Delta[/tex]T
Q=(7kg)(1000 cal/kg C)(100 C)
Q= 700,000 cal

heat of fusion:
Q=mL
Q=(7kg)(79500 cal/kg)
Q= 556,500 cal

heat of vaporization:
Q=mL
Q=(7kg)(5.4x105 cal/kg)
Q=3,780,000 cal

[tex]\sum[/tex]Q= 700,000 + 556,500 + 3,780,000

[tex]\sum[/tex]Q=5,036,500 cal = 5036.5kcal

Is that how?!
 
  • #6
114
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Indeed :)
 
  • #7
68
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Thank you!
 

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