Question

Ethyl acetate (CH 3 COOCH 2 CH 3 ) is a common volatile ester, which contributes to the characteristic odor of glues and nail polish remover. It has a boiling point of 77 ◦ C, with ∆vap H = 31.94 kJ·mol −1 . Calculate q, w, ∆U , and ∆H for the following process: Isobaric cooling (1 atm) of 0.5 kg of ethyl acetate from 100 ◦ C to 20 ◦ C. The heat capacity of the liquid is C P,liq = 168.44 J·mol −1 ·K −1 and the heat capacity of the vapor obeys the equation C P,vap = a + bT + c T 2 , (1) where T is the thermodynamic temperature, a = 23.38 J·mol −1 ·K −1 , b = 0.276 J·mol −1 ·K −2 , and c = 4.25 × 10 5 J·mol −1 ·K.

Answer 1

Solution.

1) Convert mass to moles:

2) The process of cooling can be parted into:

a) cooling of vapor from 100 to 77 ^oC.

The mean heat capacity of a gas is

The enthalpy change at constant pressure is

Using the values given, (the equation of heat capacity has been corrected to a resonable form):

This value is calculated for 1 mole of vapor, so the actual value is -2908*5.68 = -16520 J.

b) condensation at 77 ^oC;

c) cooling of liquid from 77 to 20 ^oC. The enthalpy change is

The total enthalpy change is

For a change at constant pressure,

The volume of a liquid is negligible, so the work done is