How are enthalpy and entropy used together to predict reaction feasibility?

Here’s a explanation:

• Enthalpy (ΔH) is about heat energy: does the reaction release or absorb heat?
• Entropy (ΔS) is about disorder: does the reaction make things more random or more ordered?
• Gibbs free energy (ΔG) combines both to tell if a reaction can happen:

ΔG = ΔH − TΔS

• ΔG < 0 → reaction can happen spontaneously
• ΔG > 0 → reaction cannot happen spontaneously

Rules of thumb:

1. If a reaction releases heat (ΔH −) and increases disorder (ΔS +) → it will happen naturally.
2. If a reaction absorbs heat (ΔH +) and reduces disorder (ΔS −) → it will not happen naturally.
3. Temperature can affect reactions where ΔH and ΔS have opposite signs:
   • High temperature helps reactions that increase disorder.
   • Low temperature helps reactions that release heat.

In short: heat and disorder together decide if a reaction can happen on its own.