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March 27, 2017

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Nitrogen gas has values of Cv and Cp of 20.8 and 29.1 J K-1 mol-1, respectively. When a certain amount of nitrogen is heated at constant pressure, what fraction of the heat Q is used to increase the internal energy of the gas, and how is the remaining energy used?

A) 3/2 of Q is used to increase internal energy, the rest used for work.
B) All of Q is used to increase the internal energy of the gas and there is no remaining energy.
C) ~ 0.7 (70%) of Q is used to increase the internal energy and the rest goes to pressure-volume work.
D) Most of Q is used up as work in the expansion of the gas at constant pressure.
E) 58% of Q does work, and the rest goes to increasing the internal energy of the gas.

  • Chemistry - Science (Dr. Bob222) - ,

    Thermodynamics is not my field. I would go with C but get a second opinion.
    I don't think A is right because for a diatomic gas Cp is 7/2R.
    B. B can't be right because at constant P there will be pdV work.
    C. The difference between Cv and Cp is 8.3 and that is about 28.5% of 29.1 so that much energy goes to work leaving about 71.5% for heat and that's close to 70%.
    D. Doesn't sound reasonable
    E. I tried various manipulations to get 58% and couldn't do it so I don't know how they arrived at this as a possible answer.
    I would be interested in knowing the correct answer (and WHY of course) if it isn't C.

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