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Enthalpy Change of Reaction

  1. Jun 10, 2009 #1
    1. The problem statement, all variables and given/known data

    Zn(solid) + Cu2+(aq) -> Cu(solid) + Zn2+(aqueous)
    5.96grams on zinc was added to 25cm^3 of 1M copper(II)sulphate solution in a polystyrene cup.
    Over 6 and a half minutes, the temperature of the solution decreased from 68 degrees Celsius to 19.5 degrees C.

    1. Calculate enthalpy change for the quantities used, making appropriate assumptions
    2. Calculate the enthalpy change for one mole of Zn and CuSO4(aq) and write the thermochemical equation for the reaction

    2. The attempt at a solution

    moles of Zn = 5.96/65.4 = 0.09113... or 149/1635
    therefore, moles of Cu2+ in CuSO4 = 149/1635 (because ratio in above equation is 1 to 1

    mass of CuSO4 in solution = 149/1635 x molar mass of CuSO4 = 14.54 grams

    q = mcDT,
    m = 25 grams, c = 4.2, DT = 68-19.5 = 48.5
    q = 5092.5 joules

    DH = q/n
    = 5092.5/(149/1635)
    = 55880Jmol^-1

    temperature increased so enthalpy is negative
    -> -55880joules per mol

    2. Zn + Cu2+ -> Zn2+ + Cu DH = -55880 joules

    I'm not sure if I'm doing this right...
  2. jcsd
  3. Jun 10, 2009 #2


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    For the moles of Cu2+, you'll need to use the information given about the copper(II)sulphate solution. It's not necessarily the same # of moles as the Zinc ... either Cu or Zn will be the limiting reactant here.

    Other than that, your methods look fine.
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