Question

A diatomic gas sublimates at .005 atm at 175k and at .0005 atm at 158K. The vapor pressures of the liquid are 0.017 atm at 186k and 0.09 atm at 206k. Calculate the enthalpies of fusion, vaporization, and sublimation.

Answer 1

P1 = 0.0005 atm , P2 = 0.005 atm

DHsub = ? T1 = 158 k, T2 = 175 k

ln(p2/p1) = DHsub/R(1/T1 - 1/T2)

ln(0.005/0.0005) = x/8.314((1/158)-(1/175))

x = DHsub = 31.136 kj/mol

for liquid

P1 = 0.017 atm , P2 = 0.09 atm

T1 = 186k T2 = 206k

DHvap = ?

ln(p2/p1) = DHvap/R(1/T1 - 1/T2)

ln(0.09/0.017) = x/8.314((1/186)-(1/206))

   x = DHvap = 26.545 kj/mol

DHsub = DHfus+ DHvap

DHfus = DHsub - DHvap

        = 31.136 - 26.545

        = 4.6 kj/mol