Thermodynamics and Gibbs Free Energy Study Pack

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Last updated May 21, 2026

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Thermodynamics and Gibbs Free Energy Study Guide

Unpack the thermodynamic principles that predict whether chemical reactions occur spontaneously, including enthalpy, entropy, and the Gibbs free energy equation ΔG = ΔH − TΔS. This pack covers how temperature shifts spontaneity, how to calculate standard free energy changes from formation data or the equilibrium constant K, and how ΔG° connects directly to whether products or reactants are favored at equilibrium.

Key Takeaways

  • •Gibbs free energy (G) combines enthalpy and entropy into a single state function that predicts whether a process will occur spontaneously at constant temperature and pressure.
  • •A negative ΔG indicates a spontaneous (thermodynamically favorable) process, a positive ΔG indicates a nonspontaneous process, and ΔG = 0 indicates the system is at equilibrium.
  • •The Gibbs equation ΔG = ΔH − TΔS shows that both the enthalpy change and the temperature-scaled entropy change determine spontaneity, meaning temperature can shift whether a reaction is favorable.
  • •Standard Gibbs free energy change (ΔG°) is calculated under defined reference conditions (298 K, 1 atm, 1 M concentrations) and can be derived from standard enthalpies and entropies of formation or from the equilibrium constant K via ΔG° = −RT ln K.
  • •When ΔG° is negative, the equilibrium position favors products; when ΔG° is positive, it favors reactants — linking thermodynamic favorability directly to the equilibrium constant.
  • •The four enthalpy/entropy sign combinations produce outcomes that are always spontaneous, never spontaneous, or temperature-dependent, giving a systematic way to predict reaction behavior across conditions.

Foundations: Enthalpy, Entropy, and the Need for a Unified Criterion

Predicting whether a chemical or physical process will occur on its own requires understanding two separate thermodynamic quantities — enthalpy and entropy — and why neither one alone is sufficient to determine spontaneity.

Enthalpy (ΔH) as a Measure of Heat Flow

  • •Enthalpy change measures the heat exchanged between a system and its surroundings at constant pressure; a negative ΔH means heat is released (exothermic), and a positive ΔH means heat is absorbed (endothermic).
  • •Exothermic reactions were historically assumed to be spontaneous, but many endothermic processes — such as dissolving ammonium nitrate in water — also occur spontaneously, proving enthalpy alone is not the deciding factor.

Entropy (ΔS) as a Measure of Dispersal

  • •Entropy quantifies the dispersal of energy and matter; systems naturally tend toward states with greater numbers of accessible microstates, which corresponds to greater disorder or randomness at the molecular level.
  • •Processes that increase the number of gas-phase molecules, mix substances, or melt solids generally have positive ΔS values; processes that form ordered solids or reduce gas moles typically have negative ΔS values.

The Second Law of Thermodynamics

  • •The second law states that any spontaneous process increases the total entropy of the universe (system plus surroundings), written as ΔS_universe > 0 for spontaneous change.
  • •Because calculating ΔS_surroundings requires knowing the heat flow and temperature of the surroundings separately, chemists needed a single function evaluated only on system properties — which is where Gibbs free energy becomes essential.

Gibbs Free Energy: Definition and the Spontaneity Criterion

Gibbs free energy (G) is a thermodynamic state function defined to consolidate the enthalpy and entropy of a system into one quantity that directly predicts spontaneity at constant temperature and pressure.

The Gibbs Equation

  • •Gibbs free energy change is defined as ΔG = ΔH − TΔS, where T is the absolute temperature in Kelvin, ΔH is the system's enthalpy change, and ΔS is the system's entropy change.
  • •This equation is derived from the second law condition ΔS_universe > 0; multiplying through by −T converts the universe's entropy criterion into a system-only energy quantity.

Interpreting the Sign of ΔG

  • •A negative ΔG (ΔG < 0) means the process is spontaneous — it releases free energy and proceeds in the forward direction without requiring a continuous energy input.
  • •A positive ΔG (ΔG > 0) means the process is nonspontaneous in the forward direction; energy must be continuously supplied to drive it, and the reverse process would be spontaneous instead.
  • •When ΔG = 0, the system is at chemical equilibrium — the forward and reverse rates are equal and there is no net change in composition.

Spontaneity vs. Reaction Rate

  • •Thermodynamic spontaneity describes whether a process is energetically favorable, not how quickly it occurs; a reaction can be highly spontaneous yet proceed imperceptibly slowly due to a large activation energy barrier.
  • •The conversion of diamond to graphite is thermodynamically spontaneous (ΔG < 0) at room conditions but effectively does not happen on human timescales — a reminder that kinetics and thermodynamics are separate considerations.

About this Study Pack

Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.

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