# Equilibrium and thermodynamics

1. Apr 7, 2016

### erisedk

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. Apr 7, 2016

### Ygggdrasil

Perhaps they meant that Keq is independent of the starting concentration of CO2.

3. Apr 7, 2016

### Staff: Mentor

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.