How do you determine if a reaction is spontaneous?

To determine if a reaction is spontaneous, you look at the Gibbs free energy change (ΔG), which combines the effects of enthalpy (ΔH), entropy (ΔS), and temperature (T).

Key points:

1. Spontaneous reaction:
   • Occurs naturally without external input of energy.
   • If ΔG is negative (ΔG < 0), the reaction is spontaneous.
2. Non-spontaneous reaction:
   • Requires energy to proceed.
   • If ΔG is positive (ΔG > 0), the reaction is non-spontaneous.
3. Factors affecting spontaneity:
   • Enthalpy (ΔH): Exothermic reactions (ΔH < 0) favor spontaneity.
   • Entropy (ΔS): Reactions that increase disorder (ΔS > 0) favor spontaneity.
   • Temperature (T): High temperature can make reactions with ΔS > 0 more likely to be spontaneous, even if ΔH > 0.

Simple rule of thumb:

• Spontaneous reactions often release heat and/or increase disorder.
• Non-spontaneous reactions need energy input to occur.

Example in everyday life:

• Ice melting at room temperature is spontaneous because it increases entropy, even though it absorbs heat.
• Water freezing at room temperature is non-spontaneous because it decreases entropy.