Question

From the following data, calculate the total heat (J) needed to convert 0.394 mol ethanol gas at 401°C and 1 atm to liquid ethanol at 25.0°C and 1 atm.

Boiling point at 1 atm 78.5°C

cgas 1.43 J/g·°C

cliquid 2.45 J/g·°C

ΔH°vap 40.5 kJ/mol

Answer 1

Solution-

From the question we have 0.394 mol ethanol so lets convert it into g

= 0.394*46

=18.124 g

Steps
A. Now first find the heat required to cool down the gas from 401 C to 78.5 C

heat change = mass x C x change in temperature = 18.124 g * 1.43 J/g C * -322.5 C

= -8358.33 J

B. To condense the gas to liquid the heat change required

Now we have to find at 78.5 C = mols * heat of vaporisation/mol

this = 0.394 mols * - 40.5 kJ/mol

= -15.95kJ (-1595J)

C. And finally the heat required to cool down the liquid from 78.5 C to 25 C

mass x C x deltaT

= 18.124 g*2.45J/g C * - 53.5 C = -2375.60J

Then add up : Total heat required = (-8358.33)+ (-1595) + (-2375.60)

= -12328.93J