- Define spontaneous change and explain why enthalpy alone does not determine spontaneity.
- Use the sign of Gibbs free energy to classify a forward process at constant temperature and pressure.
- Interpret entropy changes and calculate reaction entropy using products, reactants, and stoichiometric coefficients.
- Distinguish thermodynamic spontaneity and stability from reaction speed.
- Interpret standard Gibbs free energies of formation and use formation values in reaction calculations.
Spontaneous Change and the Limits of Enthalpy
Rust formation is spontaneous at constant pressure. It also has a negative enthalpy change and is exothermic.
A positive enthalpy change does not rule out spontaneity: ice melting at room temperature is spontaneous even though its enthalpy change is positive.
What does a spontaneous reaction do, and why can the sign of enthalpy alone not determine whether a reaction is spontaneous?
A spontaneous reaction proceeds in the forward direction without outside intervention. Enthalpy contributes to spontaneity but is not its determining factor.
The Gibbs Free Energy Criterion
Gibbs free energy predicts whether a reaction proceeds spontaneously in the forward direction.
- At constant temperature and pressure, negative ΔG indicates spontaneity in the forward direction.
- At constant temperature and pressure, positive ΔG indicates that the forward direction is nonspontaneous.
- At constant temperature and pressure, ΔG = 0 corresponds to equilibrium.
At constant temperature and pressure, classify forward processes with negative, positive, and zero ΔG.
Negative ΔG indicates a spontaneous forward process; positive ΔG indicates a nonspontaneous forward process; zero ΔG corresponds to equilibrium.
Temperature Dependence and Reaction Speed
Temperature can change whether a process is spontaneous, and not every reaction is spontaneous at all temperatures.
One sign combination gives an all-temperature result: a reaction with negative ΔH and positive ΔS has negative ΔG and is spontaneous at all temperatures.
Glucose oxidation has a standard entropy change of +233 joules per kelvin per mole and a ΔG° of −2,885 kilojoules per mole at room temperature. It is spontaneous but slow at room temperature.
Glucose oxidation is spontaneous but slow at room temperature. What does this example show about using spontaneity to predict speed?
A spontaneous reaction can still proceed slowly at room temperature, so spontaneity does not establish that a reaction is fast.
Entropy Concepts and Calculation
- Positive ΔS represents an increase in disorder.
- Negative ΔS represents a decrease in disorder.
Gas has greater disorder than liquid, and liquid has greater disorder than solid. Internal degrees of freedom and molecular vibrations can also contribute to entropy changes.
How is a reaction entropy change calculated, and what must be done with stoichiometric coefficients?
Calculate the sum of product entropies minus the sum of reactant entropies, including the stoichiometric coefficients.
Hydrogen Peroxide and Ice Examples
Hydrogen peroxide decomposes into liquid water and oxygen gas. Its ΔS is positive because the reaction produces a gas from liquid reactants.
In the demonstration, yeast acts as a catalyst that makes the decomposition proceed faster, while soap traps the produced oxygen in visible bubbles.
Ice melting has a standard entropy change of +28.59 joules per kelvin per mole. Melting increases entropy because water molecules have more freedom of motion in a liquid than in a solid.
Ice melting has a negative ΔG° at room temperature. How should the forward process be classified at constant temperature and pressure?
The forward melting process is spontaneous at constant temperature and pressure because its ΔG° is negative.
Formation Free Energy, Stability, and Kinetics
The standard Gibbs free energy for forming carbon dioxide from graphite and oxygen gas is −394.36 kilojoules per mole.
- A negative Gibbs free energy of formation means formation from the elements is spontaneous in the forward direction.
- A negative Gibbs free energy of formation means decomposition back into the elements is nonspontaneous.
- A substance with a negative Gibbs free energy of formation is thermodynamically stable relative to its elements.
A substance has a positive Gibbs free energy of formation. What does this indicate about its stability relative to its elements and the reverse direction toward those elements?
It is thermodynamically unstable relative to its elements, and the reverse direction toward the elements is spontaneous.
Common misconceptions
Watch out: Enthalpy alone determines whether a reaction is spontaneous.
Instead: Enthalpy contributes to spontaneity but is not its determining factor.
Watch out: A spontaneous reaction must proceed quickly.
Instead: Glucose oxidation is spontaneous but slow at room temperature, and thermodynamics does not determine reaction rate.
Lesson recap
- Gibbs free energy relates enthalpy, temperature, and entropy through ΔG = ΔH − TΔS.
- At constant temperature and pressure, negative ΔG indicates forward spontaneity, positive ΔG indicates a nonspontaneous forward direction, and zero ΔG corresponds to equilibrium.
- Reaction entropy is the sum of product entropies minus the sum of reactant entropies, with stoichiometric coefficients included.
- Thermodynamics does not determine reaction rate; kinetics describes whether reactions proceed quickly or slowly.
- Reaction Gibbs free energy can be calculated as the sum of product Gibbs formation energies minus the sum of reactant Gibbs formation energies.