Question

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)?

Answer 1

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`