Question

An unknown quantity of ammonia (assume ideal gas behavior) is contained within a cylinder. Initially the gas is at 25°C and 1 bar of pressure. The gas is heated at constant P = 1 bar until the volume is observed to be 4Vbar. Calculate:

A. q per mole

B. w per mole

C. ΔHbar

E. ΔSbar

Answer 1

Assume Cp is constant over the temperature range

Specific heat capacity of NH3 Cp = 36.78 J/mol·K

At constant pressure

Part a

q = Cp x (T2-T1)

Initial temperature T1 = 25 + 273 = 298 K

V1/T1 = V2/T2

T2 = V2T1/V1 = 4 x 298 = 1192 K

q = 36.78 J/mol·K x (1192 - 298)K

= 32881.32 J/mol x 1kJ/1000 J

= 32.88 kJ/mol

Part b

Work done W = - P (V2 - V1) = - R (T2 - T1)

= - 8.314 J/mol·K x (1192 - 298)K

= - 7432.716 J/mol x 1kJ/1000 J

= - 7.43 kJ/mol

Part c

At constant pressure

ΔH = q = 32.88 kJ/mol

Part d

ΔS = Cp ln (T2/T1)

= 36.78 J/mol·K x ln (1192/298)

= 50.98 J/mol·K