Entropy, Gibbs Energy and Spontaneity
A process is spontaneous at constant temperature and pressure when the Gibbs energy change dG = dH - T dS is negative.
What is Entropy, Gibbs Energy and Spontaneity?
A process is spontaneous at constant temperature and pressure when the Gibbs energy change dG = dH - T dS is negative.
Key formula / rule: dG < 0 indicates spontaneity, dG = 0 equilibrium.
Key points
- Evaluate the spontaneity of a process using Gibbs energy.
- Derive the temperature at which a reaction changes spontaneity.
Common exam trap
Judging spontaneity from enthalpy alone.
Definitions
- Term
Entropy, Gibbs Energy and Spontaneity
- Meaning
A process is spontaneous at constant temperature and pressure when the Gibbs energy change dG = dH - T dS is negative.
- Term
Entropy, Gibbs Energy and Spontaneity — explanation
- Meaning
Entropy measures dispersal of energy, and the Gibbs criterion combines the enthalpy and entropy contributions into a single test. Temperature decides which term dominates.
Learning objectives
Evaluate the spontaneity of a process using Gibbs energy.
Derive the temperature at which a reaction changes spontaneity.
Formulae
- Key point
dG < 0 indicates spontaneity, dG = 0 equilibrium.
- Key point
Entropy increases for processes that increase disorder or gaseous moles.
- Key point
dG(standard) = -RT ln K links thermodynamics to equilibrium.
Prerequisites
CHE-U1-TH-T2-S1-C2
Common mistakes
Judging spontaneity from enthalpy alone.
Confusing spontaneity with reaction speed.
Keywords
Entropy
Gibbs
Energy
Spontaneity
Practice preview
For a spontaneous process, which of the following statements is generally true regarding the total entropy change (dS_total)?…
easy
Calculate the Gibbs energy change (dG) for a reaction at 300 K if the enthalpy change (dH) is -40 kJ/mol and the entropy change (dS) is -50 J/K mol.…
medium
For the vaporization of water at 1 atm, dH = 40.7 kJ/mol and dS = 109 J/K mol. Above what temperature will the vaporization of water be spontaneous at 1 atm?…
hard