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Heat and phase (vaporization)

  1. Jul 21, 2015 #1
    1. The problem statement, all variables and given/known data
    How many kilocalories of heat are needed to change 143N of ethyl alcohol at 65°C to vapour?

    2. Relevant equations
    Lv = Q/m

    3. The attempt at a solution
    I'm really stuck. I have no idea how to work the units of N with heat problems. I've looked online and in my book and have no idea how to get to the solution but I know the final answer is 3090kcal. A nudge in the right direction is greatly appreciated!
     
  2. jcsd
  3. Jul 21, 2015 #2

    SteamKing

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    Well, what other information do you have to work with?

    What's the BP of ethanol?

    What's the specific heat of ethanol in the liquid phase?

    What's the heat of vaporization for ethanol?

    Do you know what a newton is?
     
  4. Jul 21, 2015 #3
    Ok so I have:
    the BP= 78.5 C
    the specific heat = 0.58 kcal/kg
    and the heat of vaporization = 204 kcal/kg
    and 1 N = 1kg*m/s^2
    I still don't know where to go with this. I'm in my first ever physics course and it is completely independent. Only notes I have to work with is the book and there is nothing that I can compare this to.
     
  5. Jul 21, 2015 #4
    Correction: specific heat = 0.58 kcal/kgC
     
  6. Jul 21, 2015 #5
    If it weighs 143N, what is its mass? At what temperature is that 204 kcal/kg? Have you heard of Hess's Law?

    Chet
     
  7. Jul 21, 2015 #6

    SteamKing

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    Chet, it's not clear why Hess's Law would be helpful here. There's no chemical reaction occurring, AFAIK, just vaporizing some ethanol.
     
  8. Jul 21, 2015 #7
    Hi SteamKing,

    Thanks. Even though hess's law appears to officially apply only to heats of reaction, the basic principle is still the same for phase change: The change in enthalpy between two thermodynamic equilibrium states of a system is independent of path. I guess I thought that this was a more general statement of hess's law. The basic principle can still be applied to determining the heat of vaporization at one temperature if you know the heat of vaporization at another temperature and the heat capacities of the vapor and liquid. Of course, you may have to make a small correction for the effect of pressure on enthalpy of the liquid phase, even if the gas phase is treated as an ideal gas. But this shouldn't be any big deal.

    Chet
     
  9. Jul 21, 2015 #8

    SteamKing

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    Chet, I appreciate the explanation, but I think that Hess's Law is a little advanced for the OP. He appears to be self-studying physics for the first time (See Post #3).
     
  10. Jul 21, 2015 #9
    Yeah, I can see that. But there doesn't seem to be an easier way of doing it than that (at least not to me).

    Chet
     
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