Chemistry 12: Entropy and Gibbs Energy

Spontaneous process

A process that occurs by itself under specialized conditions, without an ongoing input of energy

Nonspontaneous process

A process that needs a continuous input of energy

Entropy (S)

A measure of how dispersed the system’s energy is

Increases entropy

Increasing number of moles of gas, increasing temperature, and increasing molecular complexity have what effect on entropy

Second Law of thermodynamics

Entropy of the universe increases for a spontaneous process (ΔS_univ > 0)

Change in entropy of the surroundings equation

ΔS_surr = -ΔH_sys/T

Third law of thermodynamics

Entropy of a perfect crystal at absolute zero (0K) is zero

Standard entropy

The entropy of one mole of a substance under standard state (298.15 K)

Equation for standard entropy change

ΔS^o_rxn=∑n_pS^o(products) - ∑n_rS^o(reactants)

Gibbs free energy (G)

Thermodynamic property defined in terms of system H and S. Reveals spontaneity

Gibbs free energy base equation

G = H –TS

Standard change of free energy for a chemical reaction (ΔG^o_rxn)

The change in free energy between products and reactants of a chemical reaction under standard conditions (1 atm for gases, 1 M for solutions)

ΔG^o_rxn is multiplied by the same factor

If a chemical reaction is multiplied by a factor, how does this effect ΔG^o_rxn?

ΔG^o_rxn changes sign

If a chemical reaction is reversed, how does this effect ΔG^o_rxn?

Hess’s law

If a chemical equation can be expressed as the sum of a series of steps, ΔG^o_rxn for the overall equation is the sum of the ΔG^o_rxn for each step