enter image description here

I guess Cp = dH/dT= dq/dT=dS Thus SdT = integral of CpdT. From that ∆H is given and we can calculate ∆G. However, entropy for this reaction should be -7.4J/molK and I am getting like 26J/molK.

Also S is not ∆S which is a big flag in this working.


1 Answer 1


Entropy change is

$$ \Delta S = \int \frac{\delta q_{rev}}{T} $$

Under an assumption that the phase transformation is reversible, this becomes

$$ \Delta_{pt} S = \frac{\Delta_{pt} H}{T_{pt}}$$

Enthalpy and entropy depend on temperature as

$$ \Delta H(T_2) - \Delta H(T_1) = \int C_p dT $$

$$ \Delta S(T_2) - \Delta S(T_1) = \int \frac{C_p}{T} dT $$

The Gibbs energy change is

$$ \Delta G = \Delta H - T \Delta S $$

The Gibbs energy change of the phase transformation at the transformation conditions is calculated directly from the enthalpy change and transition temperature.

To calculate the value at a different temperature, you must first calculate the enthalpy and entropy changes at the different temperature.

  • $\begingroup$ It's probably worth noting that $ \Delta S = \int \frac{\delta q_{rev}}{T}$, rather than $\int \frac{\delta q}{T}$. However because, exceptionally, we can consider phase changes as reversible (I say exceptionally because this is not the case for most real-world macroscopic processes), we have $\delta q= \delta q_{rev}$ and thus $ \Delta S_{phase \,change} = \int \frac{\delta q}{T} $. $\endgroup$
    – theorist
    Apr 25, 2022 at 6:28
  • $\begingroup$ @theorist Amended as such. Thanks. $\endgroup$ Apr 25, 2022 at 13:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.