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 0.97 J/gK and 2.3 J/gK, respectively. How much heat (kJ) is needed to convert 25.0 g of solid ethanol at -135°C to liquid ethanol at -50°C?

A) 207.3 kJ

B) -12.7 kJ

C) 6.91 kJ

D) 4192 kJ

E) 9.21 kJ

Show all your calculations.

Answer 1

ans)

from above data that

there are three heats involved

1. warm up the solid ethanol to its melting point

2. melt the solid ethanol at its melting point

3. warm up the solid ethanol to -50 C

1. heat = mass * T change * specific heat

the T change is 21 C which is also a change of 21 K

heat = 25.0 g * 21 C * 0.97 J /gC = 509 J

2. find the mol of ethanol . the molar mass of ethanol is 46.0 g / mol

25.0 g C2H5OH * ( 1 mol / 46.0 g) = 0.543 mol

heat = 0.543 mol * ( 5.02 kJ / 1 mol) = 2.73 kJ = 2730 J

3.  heat = mass * T change * specific heat

the temperature change is [ -50 - (-114) ] = 64 C

heat = 25.0 g * 64 C * 2.3 J / gC = 3680 J

the total heat is 509 + 2730 + 3680 = 6919 J = 6.919 KJ

option c right answer