Enthalpy change of the reaction under standard conditions
Entropy change of the reaction under standard conditions
System temperature
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kJ/mol—
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KThe standard Gibbs free energy (ΔG°) is the thermodynamic potential that determines the spontaneity of a reaction at constant temperature and pressure. Its temperature dependence is governed by the Gibbs-Helmholtz equation (integrated assuming constant heat capacities). The thermodynamic equilibrium constant (Keq) is directly linked to this potential:
Free Energy
Equilibrium Const.
The simulator rigorously evaluates both functions. If Keq exceeds the 64-bit precision limits (exponent greater than 700 or less than -700), it is visually restricted, but a warning for extreme thermal asymmetry is triggered.
By substituting ΔG° into the Keq expression and applying the natural logarithm, the linear form of the van't Hoff isochore is obtained. This expression is fundamental because it demonstrates that plotting ln(Keq) vs 1/T yields a straight line, whose slope allows experimental isolation of the enthalpy:
| ΔH° | ΔS° | Thermodynamic Behavior & Example |
|---|---|---|
| < 0 | > 0 | Always spontaneous (ΔG° < 0 for all T). No physical Tinv. Ex: Exothermic combustion generating gases (glucose, methane). |
| > 0 | < 0 | Never spontaneous (ΔG° > 0 for all T). No physical Tinv. Ex: Decomposition of liquid water into gaseous H₂ and O₂ at room T. |
| < 0 | < 0 | Spontaneous at low T (T < Tinv). Enthalpically driven. Ex: Freezing of water (H₂O liq → sol) below 0°C. |
| > 0 | > 0 | Spontaneous at high T (T > Tinv). Entropically driven. Ex: Endothermic thermal decomposition of limestone (CaCO₃). |
The simulator assumes that ΔH° and ΔS° are truly independent of temperature. In thermodynamic reality, both vary with T according to Kirchhoff's Law and the integral of (ΔC_p/T)dT. This linear approximation is very robust for moderate intervals and is widely used in Ellingham Diagrams for metallurgy, but loses severe accuracy at extremely high temperatures or if the substance undergoes critical phase transitions.
The simulator exclusively calculates standard spontaneity (ΔG°). However, the actual in-situ spontaneity (ΔG) also depends on the instantaneous reaction quotient: ΔG = ΔG° + RT ln(Q). A reaction with ΔG° > 0 can, in industrial practice, be driven to be spontaneous if the concentrations of the reactants are forced to be very high (achieving a Q ≪ 1). In the electrochemical context, this same relationship manifests as the Nernst equation: E = E° − (RT/nF)·ln Q, where free energy is measured as cell potential (see the galvanic cell calculator).
| Constant / Ref. | Official Value | Rigorous Source |
|---|---|---|
| R (Ideal Gas) | 8.314462618... J/(mol·K) | CODATA 2018 (NIST) |
| Standard T (Thermo) | 298.15 K (25 °C) | IUPAC Green Book |
| Standard P | 10⁵ Pa (1 bar) | IUPAC 1982 |