1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    Ygggdrasil

    User Avatar
    Science Advisor

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted