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Homework Help: Equilibrium and thermodynamics

  1. Apr 7, 2016 #1
    1. The problem statement, all variables and given/known data
    The thermal dissociation equilibrium of CaCO3 (s) is studied under different conditions.
    CaCO3 ⇔ CaO (s) + CO2 (g)
    For this equilibrium, the correct statement(s) is (are)
    (A) ΔH is dependent on T
    (B) Keq is independent of the initial amount of CaCO3
    (C) Keq is dependent on the pressure of CO2 at a given T
    (D) ΔH is independent of the catalyst, if any

    Answers are (A), (B) and (D)

    2. Relevant equations

    3. The attempt at a solution
    I need someone to tell me if my justification for (A) is correct, and clear a doubt I have on (C).

    For (A), since this reaction is taken to be at equilibrium ΔG = 0.
    ΔG = ΔH - TΔS
    ⇒ ΔH = TΔS (at eq)
    Since ΔH ∝ T
    (A) is correct.

    I'm asking this because I think my reasoning might be wrong as I saw a justification involving Kirchhoff's law, http://chemwiki.ucdavis.edu/Core/Ph...namics/State_Functions/Enthalpy/Kirchhoff_Law

    For (C), I'm well aware that equilibrium constants are independent of concentrations of reactants and products, and they are a fixed value for a given reaction at a given temperature.
    However, I'm unable to reason why this is so. In this case Kp = (pco2)eq, i.e. partial pressure of CO2 at equilibrium. So isn't is directly dependent on the pressure of CO2, even though the pressure of CO2 at equilibrium is a fixed value at a given temperature and volume due to ideal gas law?

    Thank you!
  2. jcsd
  3. Apr 7, 2016 #2


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    2017 Award

    Perhaps they meant that Keq is independent of the starting concentration of CO2.
  4. Apr 7, 2016 #3
    The reason A is correct can be explained in terms of Hess's Law.

    Regarding choice C, I think you're "well aware" explanation is right on target.
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