Given two processes: (i) 1/2 (s) + 3(g) - 2(l); δ H = -635 kJ and (ii) (l) + (g) - (s); δ H = -137 kJ. The value of delta_f H of (s) is

Question

Given two processes: (i) 1/2 $P_4$(s) + 3$Cl_2$(g) -> 2$PCl_3$(l); δ H = -635 kJ and (ii) $PCl_3$(l) + $Cl_2$(g) -> $PCl_5$(s); δ H = -137 kJ. The value of delta_f H of $PCl_5$(s) is

Prashant Kotian · Accepted Answer

Correct answer: B. We want the formation of (s): 1/4 (s) + 5/2 (g) -> (s). Divide equation (i) by 2: 1/4 (s) + 3/2 (g) -> (l); δ H = -317.5 kJ. Add equation (ii): (l) + (g) -> (s); δ H = -137 kJ. Summing these: 1/4 (s) + 5/2 (g) -> (s); δ H = -317.5 + (-137) = -454.5 kJ/mol.

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