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  • The standard heat of formation for gaseous    is -11 kcal mo

The standard heat of formation for gaseous  \small NH_3  is -11 kcal mol-1 at 298 K. Given that at 298 K,the constant pressure heat capacities of gaseous \small N_2, \small H_2 and  \small NH_3  are 7 , 6 and 8 cal/mol respectively. Find change in enthalpy (in Kcal/mol) at 558K:

Option: 1

12.17


Option: 2

-9.83


Option: 3

-12.17


Option: 4

9.83


Answers (1)

best_answer

Heat of formation of NH3 is represented according to the equation:

\mathrm{\frac{1}{2}N_2(g)+\frac{3}{2}H_2(g))\rightarrow NH_3(g)}

\mathrm{\Delta H^{\circ}_{298K}=-11.0\ kcal\ mol^{-}}

Now, we know from Kirchoff's Equation

\frac{\Delta H_2-\Delta H_1}{T_2-T_1}=\Delta C_P

 \mathrm{where\ \Delta C_P = \sum_{f(P)}i_PC_p-\sum_{f(R)}i_RC_p}

Putting the given values in the equation, we have

\mathrm{\frac{\Delta H_2-(-11.1)}{558-298}=(8.0-\frac{1}{2}\times 7.0-\frac{3}{2}\times 6.0)\times 10^{-3}}

\mathrm{\Delta H_2=-12.17\ kcal\ mol^{-1}}

 

Posted by

seema garhwal

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