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Chemistry
Gibbs Free Energy and Thermodynamics
Gibbs Free Energy and Thermodynamics
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Gibbs free energy connects heat, entropy, and temperature to predict whether a chemical process is thermodynamically favorable. It matters because chemists use ΔG to judge reactions, phase changes, electrochemical cells, and biochemical pathways. A negative ΔG means a process can occur spontaneously under the stated conditions, while a positive ΔG means it is not spontaneous as written.
Key Facts
- ΔG = ΔH - TΔS, where T must be in kelvin.
- If ΔG < 0, the process is spontaneous in the forward direction.
- If ΔG > 0, the process is nonspontaneous in the forward direction.
- If ΔG = 0, the system is at equilibrium.
- Standard free energy and equilibrium are related by ΔG° = -RT ln K.
- For electrochemical cells, ΔG° = -nFE°cell.
Vocabulary
- Gibbs free energy
- Gibbs free energy is the energy available to do useful work in a system at constant temperature and pressure.
- Enthalpy
- Enthalpy is the heat content of a system at constant pressure, represented by H.
- Entropy
- Entropy is a measure of how spread out energy and matter are in a system, represented by S.
- Spontaneous process
- A spontaneous process is one that is thermodynamically favored and can occur without continuous outside input.
- Equilibrium
- Equilibrium is the state where the forward and reverse processes have no net change and ΔG equals zero.
Common Mistakes to Avoid
- Using Celsius instead of kelvin for T is wrong because thermodynamic equations require absolute temperature.
- Forgetting to convert entropy units is wrong because ΔH is often in kJ/mol while ΔS is often in J/mol·K, so the units must match before calculating ΔG.
- Assuming negative ΔG means a reaction is fast is wrong because ΔG predicts thermodynamic favorability, not reaction rate.
- Treating ΔG and ΔG° as the same is wrong because ΔG depends on actual concentrations or pressures, while ΔG° applies to standard conditions.
Practice Questions
- 1 A reaction has ΔH = -120 kJ/mol and ΔS = -200 J/mol·K at 298 K. Calculate ΔG and decide whether the reaction is spontaneous.
- 2 For a reaction at 350 K, ΔH = 45 kJ/mol and ΔS = 150 J/mol·K. Calculate ΔG and identify whether products or reactants are thermodynamically favored.
- 3 A reaction is endothermic and has a positive entropy change. Explain why increasing temperature can make the reaction spontaneous.