Question

Ethanol (C2H5OH) melts at -114 C. The enthalpy of fusion is 5.02 kJ/mol. The specific heats of solid and liquid ethanol are 0.97J / gK and 2.3 J / gK, respectively. How much heath (kJ) is needed to convert 25.0 g of solid ethanol at -125 C to liquid ethanol at -80 C?

Answer 1

mass of ethanol = 25.0 g

moles of ethanol = 25.0 / 46 = 0.543 mol

- 125 oC ---------- - 114 oC   -----------   - 80 oC

                      1            2               3

Q1 = m Cp dT

      = 25 x 0.97 x (-114 + 125)

Q1 = 266.75 J

Q2 = n x delta Hfus

     = 0.543 x 5.02 x 10^3

    = 2724.1 J

Q3 = 25 x 2.3 x (- 80 - (-114))

    = 1955 J

total heat = Q1 + Q2 + Q3

                = 4946 J

heat needed = 4.95 kJ