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Calculating heat capacity

  1. Jan 13, 2015 #1
    1. The problem statement, all variables and given/known data
    A bomb calorimeter is used to measure the overall heat output. It is calibrated by burning 1.00g of methanol (Change in enthalpy of combustion- 715 kJ mol–1) in O2 which produces a temperature rise of 8.40 K. Use this information to determine the heat capacity of the calorimeter. Why must the calorimeter be saturated with water vapour?

    Going over past papers and seen this question. Just a little confused as to how to do it. Previous capacity questions ive done, have given the power and time, which i multipled for q and and divided by the change in temperature for the heat capacity.

    I understand the to get q= change in combustion x no.of moles
    so -22.34

    Then plug it into Cv= q/Change in temp

    to get the heat capacity of the calorimeter at -2.66 KJ/K

    But it doesn't make sense to me? isn't the definition of heat capacity, the amount of energy needed to raise the temp by 1k?
    So a negative answer cant be right?
     
  2. jcsd
  3. Jan 13, 2015 #2
    The 715 is the amount of heat that would have had to be removed to keep the temperature constant (that's where the minus sign comes from). It wasn't removed, but, instead, went into raising the temperature of the calorimeter. So the calorimeter received the 715. The heat capacity is positive.

    Chet
     
  4. Jan 13, 2015 #3
    So do I use the same equation but instead of -715 the positive value 715?
    So.. multiply 715 by the moles of methanol then divide by the 8.4k temp rise? to get 2.2kJ/K?
     
  5. Jan 13, 2015 #4
    Yes.
    I get 2.66, in agreement with your original answer.

    Chet
     
    Last edited: Jan 13, 2015
  6. Jan 13, 2015 #5
    Hey, also helpful to note that the equations are as follows:

    Heat Capacity (Amount of heat needed to raise the temperature of an object by one degree) : C = Q/∆T Where Q is Heat Energy and T is temperature.

    Specific Heat Capacity (Amount of heat needed to raise the temperature of 1kg of an object by 1 degree) : c = Q/m∆T where Q is Heat Energy, m is mass and T is temperature.

    Also keep in mind temperature is in degrees Celsius and ALWAYS absolute value. Don't write it as negative.
     
  7. Jan 13, 2015 #6
    Hi Tashad. Welcome to Physics Forums.

    Did you really think that Armitage12 did not already know these things?

    Chet
     
    Last edited: Jan 13, 2015
  8. Jan 13, 2015 #7
    Hi Chet. Thanks for the kind welcome. I just browsed to the "Introductory Physics" section and assumed its where all the simple questions would be asked. My apologies.
     
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