Simulation · Chemistry · Class 11
When does a reaction go on its own?
From the lesson Gibbs energy and spontaneity in Thermodynamics. Change the values and watch what happens.
The idea behind it
NCERT § "Gibbs Energy and Spontaneity"
- Gibbs energy is defined as G = H − TS; at constant temperature ΔG = ΔH − TΔS.
- At constant temperature and pressure, ΔG < 0 means spontaneous, ΔG > 0 means non-spontaneous (the reverse is spontaneous), and ΔG = 0 means equilibrium.
- ΔH < 0 and ΔS > 0: ΔG is negative at all temperatures, always spontaneous.
- ΔH > 0 and ΔS < 0: ΔG is positive at all temperatures, never spontaneous.
- ΔH < 0 and ΔS < 0: spontaneous only at low temperature, when |ΔH| > |TΔS|.
- ΔH > 0 and ΔS > 0: spontaneous only at high temperature, when TΔS > ΔH.
- The temperature at which ΔG changes sign is T = ΔH/ΔS, assuming both stay roughly constant; convert ΔS to kJ K⁻¹ mol⁻¹ before dividing into ΔH in kJ mol⁻¹.