Change in internal not zero for an Isothermal process? [duplicate]








**This question already has answers here**:
 
 




[Enthalpy and Internal Energy for Isothermal Expansion](/questions/37720/enthalpy-and-internal-energy-for-isothermal-expansion)

 (2 answers)
 


[Can the change in internal energy be nonzero if temperature is constant?](/questions/39645/can-the-change-in-internal-energy-be-nonzero-if-temperature-is-constant)

 (3 answers)
 

Closed 6 years ago.



(*All energy quantities are in $\pu {kJ/mol}$.*) 




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> One molar $H\_2O$ is condensed isothermally and reversibly at $\pu{373K}$. Given $\Delta\_{vap}H^\circ\_{100^\circ}[H\_2O] = \pu{40.656 kJ/mol}$. Find $q, w, \Delta\_r U, \Delta\_r H$.
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Since the process is isothermal $\Delta U = 0$ and $\Delta H\_p = q\_p = -w\_p = -40.656$.


I got $\Delta H\_p = q\_p$ part correct but


For other two
author used $\Delta H - \Delta n(g)RT = \Delta U = 37.55$ and then first law to get $w = 3.101$.


Why $\Delta U$ is not zero ?


