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Homework Help: Enthalpy calculation

  1. Jan 24, 2014 #1
    1. The problem statement, all variables and given/known data

    A sample of sulfur, mass 2.56g, is burned in excess oxygen inside a calorimeter of heat capacity 923 J/K and insulated by 815g of water. If the sole product of combustion is SO2(g), what is the expected increase in temperature?

    2. Relevant equations

    q = mcΔT
    q = CΔT

    3. The attempt at a solution

    I am given that the ΔHof = -296.8 kJ/mol for SO2(g) and the enthalpy of formation of oxygen gas and sulfur is 0.

    S(s) + O2(g) → SO2(g)

    So, I did:

    (1g / 32.06 g/mol)(-296.8 kJ/mol) = - [(4.184 g/molK)(815g)ΔT + (923 J/K)(ΔT)]

    After isolating for ΔT and converting units to J, i found my answer as 2.136 K but this is incorrect. Any comments on where I went wrong? Thanks!
  2. jcsd
  3. Jan 24, 2014 #2
    A negative heat of reaction means that, to maintain the temperature constant at the initial temperature, you need to remove heat. So the reaction gives off 296.8 kJ/mol. So the temperature in your calorimeter has to rise. There shouldn't be a minus sign on the rhs.
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