Calculate the heat energy released when 20.4 g of liquid mercury at 25.00 °C is converted to solid mercury at its melting point.

Question

Calculate the heat energy released when 20.4 g of liquid mercury at 25.00 °C is converted to solid mercury at its melting point.

Constants for mercury at 1 atm

heat capacity of Hg(l) 28.0 J/(mol⋅K)

melting point 234.32 K

enthalpy of fusion 2.29 kJ/mol

Answer 1

melting point/freezing point for Hg = 234.32K

starting temperature = 25ºC + 273.15 = 298.15K

step 1: heat released going from 298.15K to 234.32K = q = mC∆T

q = (20.4 g x 1 mol Hg/200.6 g)(28.0 J/molK)(63.83K) = 181.75 J

step 2: heat released freezing the Hg @ 234.32K (no change in temp; phase change)

q = m∆Hf = (20.4 g x 1 mol/200.6g)(2290 J/mol) = 232.88 J

Total heat released (negative value)...

181.75 J + 232.88 J = 415 J of heat released (note: this value would have a negative sign)