3-32 No. 1

Calculate the entropy change when 1 lb. of ammonia is cooled from 500°F to -150°F at a constant pressure of 1 atm. Express the results in Btu/lb.°R. Calculate the absolute entropy of solid NH3 at its melting point at 1 atm. pressure in Btu/lb.°R.

NH3 data:

a = 6.5846

b = 6.1251 x 10-3

c = 2. 3663 x 10-6

d =-1.5981 x 10-9

b.p. =-33.4 °C

m.p. = -77.7 °C

Cp(cal/mol-K) = a + bT + cT^2 + dT^3 (where T is in degrees K)

S = 46.03 cal/mol °K (absolute entropy of gas at 25°C)

lv = 5581 cal/g-mole

lf = 1352 cal/g-mole

Cp(solid) = 0.502 cal/g-°K

Cp(liquid) = 1.06 cal/g-°K