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ChemistryNEET 2025First law, enthalpy and Hess's law

Thermodynamics

The standard heat of formation, in kcal/mol, of Ba2+\mathrm{Ba^{2+}} is: [Given: standard heat of formation of the SO42−\mathrm{SO_4^{2-}} ion (aq) = –216 kcal/mol, standard heat of crystallisation of BaSO4(s)\mathrm{BaSO_4(s)} = –4.5 kcal/mol, standard heat of formation of BaSO4(s)\mathrm{BaSO_4(s)} = –349 kcal/mol]

  1. A.–128.5✓
  2. B.–133.0
  3. C.+133.0
  4. D.+220.5

Solution

ΔfH∘(BaSO4)=ΔfH∘(Ba2+)+ΔfH∘(SO42−)+ΔHcryst\Delta_fH^\circ(\mathrm{BaSO_4}) = \Delta_fH^\circ(\mathrm{Ba^{2+}}) + \Delta_fH^\circ(\mathrm{SO_4^{2-}}) + \Delta H_{cryst}: −349=x−216−4.5⇒x=−128.5-349 = x - 216 - 4.5 \Rightarrow x = -128.5 kcal/mol.

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