1. We showed previously how to calculate enthalpy changes for chemical reactions

Question

1. We showed previously how to calculate enthalpy changes for chemical reactions at various temperatures by using hypothetical paths, which required calculating enthalpy changes upon heating and cooling. Another common occurrence is for the gaseous reactant and product species to not all be present at standard partial pressures (of 1 atm). Then standard reaction enthalpies cannot be used. What is then needed to relate reaction enthalpies at nonstandard pressures to those at standard pressures is the ability to evaluate quantities like Such an integral is challenging to evaluate because we need to know something about the function (aH /ap)r to be able to perform the integration. It would be desirable to relate this quantity to more common state variables, which may either be directly measured or calculated based on useful models, such as those of the ideal gas and the van der Waals gas. Show that generally av

Explanation / Answer

H= U+PV

dH= dU+PdV+VdP

but from 1st law, dU= dQ-PdV, = TdS-PdV

hence dH= TdS-PdV+PdV+VdP= TdS+ VdP

(dH/dP)T= T*(dS/dP)T+ V, but from maxwell relation, (dS/dP)T= -(dV/dT)P

hence dH= V-T*(dV/dT)P

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