Question

State whether the following are reversible processes. Explain your reasoning.

(a.) One mole of (ideal) diatomic gas is compressed from 200 cm3 to 100 cm3 at a constant pressure of 100 kPa.

(b.) One mole of (ideal) diatomic gas is adiabatically compressed from 200 cm3 to 100 cm3.

Answer 1

to be a reversible process there should not be any change in entropy.

(A)

So, change in entropy at constant pressure = C_P * ln(T₂ /T₁)

we know P₁V₁/T₁ = P₂V₂ / T₂

P₁ = P₂

So, T₂ / T₁ = V₂ / V₁

So change in entropy = C_P * ln(100/200)

which is not zero because neither ln(0.5) is zero nor the C_P is zero.

So, the process is not reversible.

(B)

So, change in entropy = change in heat / Temperature

because in the adiabatic compression change in heat is zero, so the change in entropy is zero and hence this process is reversible.