11/14 Thermodynamics and Chemical Spontaneity Free Online FlashCards

What does spontaneity determine, and what does it not determine?

Spontaneity predicts the thermodynamically favored direction under specified conditions, not the speed. Reaction rate is determined by kinetics.

What does the second law say about a spontaneous process?

For a spontaneous process, ΔS_univ > 0. At equilibrium, ΔS_univ = 0; a process with ΔS_univ < 0 is nonspontaneous in the written direction.

What does entropy measure?

Entropy measures the dispersal of energy and matter. Microscopically, more accessible arrangements generally mean greater entropy, as expressed by S = k ln W.

Which changes generally increase entropy?

Entropy generally increases when a solid melts, a liquid vaporizes, gas-particle count increases, or particles become more dispersed. For one substance: S_solid < S_liquid < S_gas.

How is standard reaction entropy calculated?

ΔS°_rxn = Σ nS°(products) − Σ nS°(reactants), using stoichiometric coefficients from the balanced equation.

What do the signs of ΔH indicate?

At constant pressure, ΔH is the heat absorbed or released: ΔH < 0 is exothermic, while ΔH > 0 is endothermic.

How does Gibbs energy indicate spontaneity?

At constant temperature and pressure, ΔG = ΔH − TΔS. ΔG < 0 favors the forward process, ΔG > 0 favors the reverse, and ΔG = 0 indicates equilibrium.

How is ΔG related to the entropy of the universe?

At constant temperature and pressure, ΔG = −TΔS_univ. Thus, negative Gibbs energy corresponds to a positive entropy change for the universe.

Which ΔH and ΔS signs give temperature-independent behavior?

If ΔH < 0 and ΔS > 0, a process is spontaneous at all temperatures, assuming the values remain approximately constant. If ΔH > 0 and ΔS < 0, it is nonspontaneous at all temperatures.

How can the temperature where ΔG = 0 be estimated?

For ΔH and ΔS with the same sign, the boundary temperature is T = ΔH/ΔS, with consistent units. At this temperature, ΔG = 0.

How is standard reaction Gibbs energy calculated from formation values?

ΔG°_rxn = Σ nΔG°_f(products) − Σ nΔG°_f(reactants). An element in its standard state has ΔG°_f = 0.

How is Gibbs energy calculated under nonstandard conditions?

Under nonstandard conditions, ΔG = ΔG° + RT ln Q. Q is the reaction quotient based on activities; pure solids and liquids are omitted.