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Heat Capacity and Heat

  1. Nov 23, 2005 #1
    The heat capacity of lead is 0.130 J/g C. How many joules of heat are required to raise the temperature of 28.5 grams of lead from 15 to 37?

    For this problem the formula that must be used is q = m*C*deltaT, where mass is expressed in g?

    q = m*C*deltaT

    q = (0.130 J/g*C)*(28.5 g Pb)*(37 - 15 C) = + 81.5 J

    Is my sign correct?

    The formula q = m*C*deltaT, where m is expressed in grams per g/mol (Pb's molar mass), is not being asked in this question, right?

    Thanks.
     
  2. jcsd
  3. Nov 23, 2005 #2

    Gokul43201

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    Your answer is correct.

    When asked how much heat is required or evolved, you don't have to worry about the sign. The terms "required" and "evolved" already describe the thermicity of the process. If you are asked for the change in enthalpy, however, you must be careful with the sign.
     
  4. Nov 24, 2005 #3
    Sign Conventions

    For example, since "heat evolved" in these problems is used, the answer for both will be expressed with a + sign in kJ, too?


    The value of the heat of reaction for the following reaction is -6535 kJ. What will be the amt. of heat in kJ evolved during the combustion of 12.0 g of C6H6?

    The amt. of heat evolved for a reaction with H2 and Cl2 is -186 kJ. How many kJ of heat would evolve from reaction of 25.0 g Cl2?


    Thanks.
     
    Last edited: Nov 24, 2005
  5. Nov 24, 2005 #4

    Gokul43201

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    The most commonly used convention assigns a negative sign to heat evolved (dH < 0 => exothermic). The idea is that you are trying to monitor the heat content of the system. If the system loses heat, its heat content drops, and H(fin) - H(in) < 0

    However, if someone asks you "how much heat is evolved", it is unnecessary to answer with a negative number. This might make someone think that the heat is in fact absorbed (double negative). So, I dislike the wording of the second question - it is at best redundant.
     
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