Question

Oxygen gas is compressed in a piston-cylinder device from an initial state of 0.8 m3 /kg and 25°C to a final state of 0.1 m3 /kg and 287°C. Determine the entropy change of the oxygen during this process. Assume constant specific heats at the averaged temperature.

Answer 1

Answer: -0.105 kJ/kgK

Step-by-step explanation:

Given

T1 = 25 °C = 25 + 273 = 298 K

T2 = 287 °C = 287 + 273 = 560 K

V1 = 0.8 m³/kg

V2 = 0.1 m³/kg

The average temperature, T(a) would be (T1 + T2) / 2

T(a) = (298 + 560) / 2

T(a) = 858/ 2

T(a) = 459 K

The entropy change of oxygen during the process is

s1 - s2 = c(v) In T2/T1 + R In V2/V1

Where c(v) = 0.69 kJ/kgK

s1 - s2 = 0.69 In 560/298 + 0.2598 In 0.1/0.8

s1 - s2 = 0.69 * 0.6308 + 0.2598 * -2.0794

s1 - s2 = 0.4352 - 0.5402

s1 - s2 = -0.105 kJ/kgK