phase change energetics

Question

The following information is given for mercury, Hg, at 1 atm:

boiling point = 357 °C	Hvap(357 °C) = 59.3 kJ/mol
melting point = -38.9 °C	Hfus(-38.9 °C) = 2.33 kJ/mol
specific heat liquid = 0.139 J/g°C

At a pressure of 1 atm, what is H in kJ for the process of condensing a 26.3 g sample of gaseous mercury at its normal boiling point of 357 °C.

 kJ

Answer 1

Condensing is essentially the opposite of vaporization. So, you can use the value of ∆Hvap but change the sign. To vaporize 1 mol liquid Hg requires 59.3 kJ. So when 1 mol of gaseous Hg condenses back into liquid Hg, 59.3 kJ of heat energy is given off (∆H = -59.3 kJ)

Convert 26.3 g Hg to moles and multiply by -59.3 kJ/mole to get your final answer.