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March 29, 2017

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help - confusion :(

The oxidation of copper(I) oxide, Cu2O(s), to copper(II) oxide, CuO(s), is an exothermic process,
2 Cu2O + O2 --> 4CuO

The change in enthalpy upon reaction of 75.30 g of Cu2O(s) is -76.83 kJ. Calculate the work, w, and energy change, ΔUrxn, when 75.30 g of Cu2O(s) is oxidized at a constant pressure of 1.00 bar and a constant temperature of 25°C.

Here's my line of reasoning.. but it's been marked wrong. Thank you in advance.

converting 75.3g of Cu2O to moles of CuO, I got 1.0525mol CuO

W=-delta(n)RT = -(1.0525mol)(8.3144J/molK)(25+273.15)= -2.609 kJ

energy change = -2.609kj+ - 76.83kJ = -79.439kJ

  • Chemistry - ,

    the heat of reaction is 76.83kJ per mole of Cu2O, not CuO.

    Then, work=moles*Cu2O*76.83kJ

    and that is it. Your delta(n)RT is off the thinking mark.

  • Chemistry - ,

    You appear to be calculating the change in volume from mols Cu2O ==> 2CuO. But each is a solid and they don't count do they? Isn't the change in volume the change due to the gas which is oxygen? Shouldn't you calculate how much oxygen was there initially and how much is there at the end (zero of course) and p*delta V then is the work done by the surroundings on the system. The mols O2 you can calculate from mols Cu2O given in the problem.

  • Chemistry - ,

    ok I recalculated and got the w=.006523kJ and the energy change to be -76.82kJ... The energy change is right but the w is wrong for some reason...
    I calculated w as w=-delta(n)RT = -(0-.26312molO2)(8.314452x10^-2 Lbarr/molK)(25+273.15) = 6.5226J=.006523kJ...

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