What prevents a spontaneous process like free expansion from being reversible?
**Gamma determination** γ = C_p/C_v, C_p - C_v = R, for monatomic f=3 C_v=3/2 R C_p=5/2 R γ=1.67, diatomic f=5 C_v=5/2 R C_p=7/2 R γ=1.4, adiabatic relation P V^γ = const allows γ determination from P-V measurements, slope of log P vs log V = -γ. Free expansion (e.g., gas into a vacuum) is irreversible because it involves non-equilibrium states with no work done to restore the original state. The system cannot return to its initial condition without external intervention. Using first law ΔU = Q - W, W = ∫ P dV, isobaric W = P ΔV, isothermal W = n R T ln(V₂/V₁),
Ref: NCERT > Physics Book > Thermodynamics > Adiabatic Processes and Gamma Determination