ΔG = ΔH - TΔS
Where ΔG is Gibbs free energy, ΔH is enthalpy change, T is absolute temperature (K), and ΔS is entropy change.
Reaction spontaneity refers to whether a chemical reaction will occur on its own without external intervention. A spontaneous reaction is thermodynamically favorable and will proceed in the forward direction under the given conditions. This does not mean the reaction happens quickly - spontaneity only indicates the thermodynamic tendency, not the rate of reaction.
The spontaneity of a reaction is determined by the Gibbs free energy change (ΔG). When ΔG is negative, the reaction releases free energy and is spontaneous. When ΔG is positive, the reaction requires energy input and is non-spontaneous. At ΔG = 0, the system is at equilibrium with no net change occurring.
Spontaneity depends on the balance between two competing factors: enthalpy (ΔH) and entropy (ΔS). Exothermic reactions (negative ΔH) release heat and favor spontaneity, while reactions that increase disorder (positive ΔS) also favor spontaneity. The temperature determines how much weight entropy carries in the equation.
ΔH < 0 and ΔS > 0
Always spontaneous at all temperatures. Both enthalpy and entropy favor the reaction.
ΔH > 0 and ΔS < 0
Never spontaneous at any temperature. Both factors oppose the reaction.
ΔH < 0 and ΔS < 0
Spontaneous at low temperatures. Enthalpy favors, entropy opposes.
ΔH > 0 and ΔS > 0
Spontaneous at high temperatures. Entropy favors, enthalpy opposes.
Temperature plays a crucial role in determining spontaneity when enthalpy and entropy have opposing effects. The term TΔS in the Gibbs equation shows that entropy's contribution increases with temperature. This is why some reactions that are non-spontaneous at low temperatures become spontaneous at high temperatures (and vice versa).
The temperature at which a reaction transitions from non-spontaneous to spontaneous (or equilibrium) can be found when ΔG = 0, giving T = ΔH/ΔS. This crossover temperature is important for understanding reaction behavior across different conditions.
Spontaneity calculations assume ideal conditions and equilibrium thermodynamics. Real reactions may be influenced by kinetic barriers, catalysts, non-standard conditions, or non-ideal behavior. A spontaneous reaction may still be extremely slow without appropriate activation energy or catalysis.
This calculator provides theoretical predictions based on thermodynamic data. Actual reaction behavior may differ due to concentration effects, pressure changes, or other factors not captured in standard calculations. Always verify results with experimental data for critical applications.