Calculate the standard enthalpy change for synthesis of ammonia gas from following data i. 2 H _ 2( ~g ) + N _ 2( ~g ) N ₂ H _ 4( ~g ) ; _ r H ₁^0=95.4 ~kJ ii. N ₂ H _ 4( ~g ) + H _ 2( ~g ) 2 NH _ 3( ~g ) ; _ r H ₂^0=-187 6 ~kJ

Step-by-step solution

The standard enthalpy change for the target reaction N _ 2( ~g ) + 3 H _ 2( ~g ) 2 NH _ 3( ~g ) is determined using Hess's Law by summing the given reactions and their enthalpy changes. 2 H _ 2( ~g ) + N _ 2( ~g ) N ₂ H _ 4( ~g ) with _ r H ₁^0 = 95.4 kJ N ₂ H _ 4( ~g ) + H _ 2( ~g ) 2 NH _ 3( ~g ) with _ r H ₂^0 = -187.6 kJ Adding the reactions, the intermediate N ₂ H _ 4( ~g ) cancels, yielding the target reaction. The enthalpy change is _ r H ^0_ target = 95.4 + (-187.6) = -92.2 kJ . Among the options, A matches