Thermodynamic spontaneity — At constant temperature and pressure, a chemical reaction will proceed spontaneously if the change in Gibbs free energy (ΔG) for the process is:

Introduction:
Predicting spontaneity is central to chemical thermodynamics. Under constant temperature and pressure, Gibbs free energy provides the appropriate criterion for chemical reactions, phase changes, and transport-driven equilibria.

Given Data / Assumptions:

• Closed system exchanging heat and pressure–volume work with a reservoir at fixed T and P.
• No non-PV work considered for the basic criterion.

Concept / Approach:
The Gibbs free energy change ΔG represents the maximum non-PV work obtainable from a process at constant T and P. The second law implies that for spontaneous change at fixed T, P, the total entropy change of universe is positive, which translates to ΔG < 0 for the system.

Step-by-Step Solution:
Set constraints: constant T, P.Apply criterion: spontaneity ↔ ΔG < 0.Equilibrium condition: ΔG = 0; Non-spontaneous forward direction: ΔG > 0.

Verification / Alternative check:
From ΔG = ΔH − TΔS, a negative ΔG can result from exothermicity (negative ΔH), positive entropy change (positive ΔS), or both; at equilibrium ΔG = 0 and chemical potentials are balanced.

Why Other Options Are Wrong:

• Zero indicates equilibrium, not spontaneous net progress.
• Positive means the forward reaction is non-spontaneous (reverse is favored).
• Other statements are irrelevant to the criterion at constant T and P.

Common Pitfalls:
Using ΔA (Helmholtz free energy) which is the criterion at constant T and V; mixing up enthalpy-only criteria.

Final Answer:
Negative