I2(s)⟶I2(g) Heat of reaction depend upon temperature i.e., it varies with temperature, as given by Kirchoff's equation, ∆HT2=∆HT1+
∫
T1
T2∆CpdT where ∆Cp=Cp of product −Cp of reactant ∴∆Cp=0.031−0.055=−0.024cal∕g Now, ∆HT2−∆HT1=∆Cp(T2−T1) ∆H(250)−∆H(200)=−0.024(523−473) ∆H(250)=24−50×0.024=22.8cal∕g