Identify the property: Which thermodynamic property of a working substance changes in proportion to reversible heat transfer and is used to measure irreversibility?

Introduction / Context:
Among thermodynamic properties, one uniquely links heat transfer to temperature in a way that quantifies directional constraints and irreversibility. That property is entropy, central to the Second law and energy quality analysis.

Given Data / Assumptions:

• Reversible heat transfer δQ_rev at absolute temperature T.
• Property S with differential dS = δQ_rev / T for reversible processes.
• General inequality ΔS_total ≥ 0 for all real processes.

Concept / Approach:

Entropy increases when reversible heat is added (dS = δQ_rev / T). In any real (irreversible) process, entropy production is positive. Enthalpy and internal energy are energy measures; they do not directly quantify irreversibility. External energy is not a standard thermodynamic property.

Step-by-Step Solution:

Verification / Alternative check:

Isentropic (ΔS = 0) idealizations represent reversible adiabatic steps, confirming entropy's role as the irreversibility yardstick.

Why Other Options Are Wrong:

Enthalpy/internal energy measure energy content but do not by themselves indicate reversibility.External energy and specific heat are either non-standard or material parameters, not the irreversibility measure.

Common Pitfalls:

Confusing zero heat transfer (adiabatic) with zero entropy change; only reversible adiabatic is isentropic.

Final Answer:

entropy