Calculate the amount of heat that must be released to convert 20.0 g of mercury vapor at 387^@"C" to liquid mercury at 307^@"C" (in kJ)?

1 Answer
Jun 6, 2015

Heat added or removed from a substance when there is a change of temperature without a change in phase (H) = m.s. Deltat

where m is mass of substance
s is specific heat
Deltat is absolute change in temperature

However, we need to appreciate the underlying steps in the thermal transformation taking place. The change takes place in 3 steps:

  1. Extraction of heat to lower temperature of vapour from 387^o C to 357^o C (phase transition temperature).
  2. Phase transition from vapour to liquid without change in temperature ( Heat added/removed = Heat of phase transition/unit mass * mass of substance )
  3. Extraction of heat to lower temperature of liquid from 357^o C to 307^o C.

Step I: H_1 = 20*0.104*(387-357) Vapour
=> H_1 = 20*0.104*30 = 62.4 J

Step II: H_2 = 292*20 = 5840 J Phase transition

Step III: H_3 = 20*0.138*(357-307) Liquid
=> H_3 = 20*0.138*50 = 138 J

Total heat removed = H_1+H_2+H_3 = 62.4 + 5840 + 138 J = 6.040 kJ