Q6. Consider the reaction at 300 K
H 2 g   +   C l 2 g   2 H C l g                     H ° = - 185   k J
If 3 moles of H2 completely react with  mole of Cl2 to form HCl. What is U ° the for this reaction ?
(A) 0              (B) -185 kJ                 (C) 555 kJ                (D) None of these 

Dear Student,

We know that,H°=U° +nRTFor the reaction, H2(g) + Cl2(g)  2HCl(g)1 mol of H2 and Cl2 gives 2 mol of HClSo, 3 mol of H2 and Cl2 gives 3×2=6 mol of HClng = moles of gaseous products-moles of gaseous reactants         = 6-(3+3)         =6-6         =0So, U°=H°=-185 kJHence, correct answer is (B)

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