Question

If your air conditioner is more than several years old, it may use the chlorofluorocarbon CCl₂F₂ as the heat transfer fluid. (CFCs such as this are being replaced as rapidly as possible by substances less harmful to the environment.) The normal boiling point of CCl₂F₂ is -29.8°C, and the enthalpy of vaporization is 20.11 kJ/mol. The gas and the liquid have specific heats of 117.2 J/mol·K and 72.3 J/mol·K, respectively. How much heat is evolved when 50.0 g of CCl₂F₂ is cooled from +40°C to -40°C?

Answer 1

Moles of CCl2F2 = mass / Molar mass = 50 g / 120.9 g/mol = 0.413565 mol

Heat released when cooling CCl2F2 form 40 C to - 29.8 C = specific heat of gas x moles x change in temp

        = 117.2 J/molK x 0.413565 mol x (40- ( -29.8)   = 3383.2 J = 3.383 KJ

Heat released for phase transformation of gas to liquid at - 29.8 C = enthalphy fo vaporisation x moles

             = 20.11 KJ/mol x 0.413565 = 8.317 KJ

Heat released in cooling liquid CCL2F2 from - 29.8 to -40 C = specific heat x moles x temp change

          = 72.3 J/molK x 0.413565 mol x (-29.8 - ( -40)   = 305 J = 0.305 KJ

Total heat released = sum of three process = 3.383 + 8.317 + 0.305 = 12 KJ