Why Correct: Josiah Willard Gibbs (1839–1903) was an American scientist who developed the concept of Gibbs free energy and formulated the equation ΔG = ΔH - TΔS, which determines whether a chemical reaction is spontaneous at constant temperature and pressure.
Distractor Analysis: Linus Pauling was an American chemist known for work on chemical bonding and molecular structure. Robert Boyle was an Irish chemist who formulated Boyle's law relating gas pressure and volume. Svante Arrhenius was a Swedish chemist who proposed the theory of electrolytic dissociation and the Arrhenius equation for reaction rates.
Takeaway: Gibbs' contributions to thermodynamics include Gibbs free energy, the phase rule, and vector analysis, making him foundational to chemical thermodynamics.

Why Correct: The immediate consequence enabling spontaneity is ΔG < 0, which directly indicates thermodynamic favorability under constant temperature and pressure conditions, allowing the reaction to proceed without external energy input according to ΔG = ΔH - TΔS.
Distractor Analysis: Negative ΔH (exothermic) often accompanies spontaneity but isn't the defining consequence—some spontaneous reactions are endothermic. Positive ΔS (entropy increase) can drive spontaneity but isn't the immediate consequence in all cases. Temperature decrease lowering activation energy is kinetically relevant but not the thermodynamic consequence of spontaneity.
Takeaway: Spontaneity is determined by ΔG < 0, with negative ΔG being the direct thermodynamic consequence that makes reactions proceed without external energy input.

Why Correct: The Gibbs free energy change (ΔG) is the definitive criterion for spontaneity at constant temperature and pressure. A negative ΔG ensures the reaction proceeds spontaneously, regardless of the individual signs of ΔH or ΔS. This follows from the equation ΔG = ΔH - TΔS, where ΔG < 0 indicates spontaneity.
Distractor Analysis: A negative ΔH (exothermic) favors spontaneity but does not guarantee it, as a highly negative ΔS could make ΔG positive at certain temperatures. A positive ΔS (increase in disorder) also favors spontaneity but alone is insufficient if ΔH is sufficiently positive. ΔU (internal energy change) is not the standard criterion for spontaneity under constant pressure conditions; ΔG is used instead.
Takeaway: While ΔH and ΔS influence spontaneity, only ΔG provides a conclusive determination. Confusing ΔH or ΔS as the sole determinant is a common error, as both contribute to ΔG through the relationship ΔG = ΔH - TΔS.

Why Correct: The fundamental equation ΔG = ΔH - TΔS directly relates Gibbs free energy change to enthalpy change, entropy change, and absolute temperature in Kelvin, as per thermodynamic principles.
Distractor Analysis: Option B rearranges the equation incorrectly by adding TΔS instead of subtracting it. Option C solves for ΔS but presents it as the primary relationship rather than the standard form. Option D incorrectly adds TΔS instead of subtracting it, which would reverse the temperature-entropy effect on spontaneity.
Takeaway: The equation ΔG = ΔH - TΔS is central to predicting reaction spontaneity, where negative ΔG indicates spontaneity, with both enthalpy and entropy contributions weighted by temperature.