The spontaneous direction of the reaction will always be in the direction in which the red shading overlaps the greater number of energy levels, resulting in the maximum dispersal of thermal energy.

The red shading indicates the range of energy levels that are accessible to the system at each temperature.(The greater the entropy, the more closely-spaced are the quantized microstates.) Each pair of energy-level diagrams depicts the relative spacing of the microscopic energy levels in the reactants and products as reflected by the value of Δ S°.

The other two plots on each diagram are only for the chemistry-committed. Similarly, the lines representing Δ H are even more curved.

Owing to the slight temperature dependence of Δ S, the TΔ S plots are not quite straight lines as shown here.

The reaction A → B will occur spontaneously only when Δ G is negative (blue arrows pointing down.).

Their most important differentiating features are the position of the Δ H line (above or below the is TΔ S line), and the slope of the latter, which of course depends on the sign of Δ S.

The above cases and associated plots are the important ones do not try to memorize them, but make sure you understand and can explain or reproduce them for a given set of Δ H and Δ S. To calculate S for a chemical reaction from standard molar entropies, we use the familiar products minus reactants rule, in which the absolute entropy of each reactant and product is multiplied by its stoichiometric coefficient in the balanced chemical equation. But because the reverse reaction is kinetically inhibited, NO 2 can exist indefinitely at ordinary temperatures even though it is thermodynamically unstable. This reaction is not spontaneous at any temperature, meaning that its reverse is always spontaneous.

Absolute molar entropy equation