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.

Answer 1

Answer:

ΔS = -0.1076 kJ /kg*K

Explanation:

Step 1: Data given

Initial state = 0.8 m³/kg and 25 °C = 298.15 K

Final state = 0. 3³/kg and 287 °C = 560.15 K

Cv = 0.686 kJ/kg*K

Step 2: Calculate the average temperature

The average temperature = (25°C + 287 °C)/2  =156 °C ( = 429 K)

Step 3: Calculate the ΔS

ΔS =(Cv, average) * ln(T2/T1) + R*ln(V2/V1)

ΔS = 0.686 * ln(560.15/298.15) + 0.2598*ln( 0.1/0.8)

ΔS = -0.1076 kJ /kg*K