Gibbs Free Energy Calculator Formula

Understand the math behind the gibbs free energy calculator. Each variable explained with a worked example.

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Formulas Used

Gibbs Free Energy (ΔG)

delta_g = delta_h - (temp * delta_s / 1000)

Variables

VariableDescriptionDefault
delta_hEnthalpy Change (ΔH)(kJ)-100
delta_sEntropy Change (ΔS)(J/K)-50
tempTemperature (T)(K)298

How It Works

Gibbs Free Energy

Formula

ΔG = ΔH − TΔS

Where:

  • ΔG = Gibbs free energy change (kJ)
  • ΔH = enthalpy change (kJ)
  • T = temperature (K)
  • ΔS = entropy change (J/K, converted to kJ/K by dividing by 1000)

    • If ΔG < 0, the reaction is spontaneous. If ΔG > 0, it is non-spontaneous. If ΔG = 0, the system is at equilibrium.

    Worked Example

    A reaction with ΔH = -100 kJ and ΔS = -50 J/K at 298 K.

    delta_h = -100delta_s = -50temp = 298
    1. 01ΔG = ΔH - TΔS
    2. 02ΔG = -100 - (298 × (-50)/1000)
    3. 03ΔG = -100 - (-14.9)
    4. 04ΔG = -100 + 14.9
    5. 05ΔG = -85.1 kJ (spontaneous)

    Frequently Asked Questions

    What does ΔG tell you?

    ΔG predicts spontaneity at constant temperature and pressure. Negative ΔG means the reaction can proceed spontaneously. It does not predict how fast the reaction occurs.

    Can a reaction with positive ΔH be spontaneous?

    Yes, if the entropy increase (TΔS) is large enough to make ΔG negative. Endothermic reactions can be spontaneous when entropy drives them.

    What is the relationship between ΔG and K?

    ΔG° = -RT ln(K). A large negative ΔG° corresponds to a large equilibrium constant, favoring products.

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