US Trends

how does the change in energy of a chemical reaction predict whether or not the reaction will occur

The change in energy of a chemical reaction helps predict whether it can occur spontaneously by using a quantity called Gibbs free energy , ΔG\Delta GΔG. When ΔG<0\Delta G<0ΔG<0, the reaction is thermodynamically able to happen on its own (spontaneous); when ΔG>0\Delta G>0ΔG>0, it is not and needs a continual input of energy.

Quick Scoop: Core Idea

Think of a reaction as “wanting” to move toward lower free energy, the way a ball rolls downhill.

  • ΔG=ΔH−TΔS\Delta G=\Delta H-T\Delta SΔG=ΔH−TΔS, where:
    • ΔH\Delta HΔH = enthalpy change (heat released or absorbed)
    • ΔS\Delta SΔS = entropy change (change in randomness/disorder)
    • TTT = temperature in kelvin
  • If ΔG<0\Delta G<0ΔG<0: reaction is spontaneous in the forward direction.
  • If ΔG=0\Delta G=0ΔG=0: reaction is at equilibrium (it can occur in both directions equally).
  • If ΔG>0\Delta G>0ΔG>0: reaction is non‑spontaneous as written, but the reverse reaction is spontaneous.

Spontaneous here means “can happen without continuous outside energy,” not “will be fast.” Diamond slowly turning into graphite is spontaneous but extremely slow.

Mini‑Section: Exothermic vs Endothermic

Energy changes alone (just “exothermic” or “endothermic”) do not fully decide if a reaction will occur; entropy and temperature matter too.

  • Exothermic reactions (ΔH<0\Delta H<0ΔH<0):
    • Tend to have ΔG<0\Delta G<0ΔG<0, especially if entropy increases.
    • Many are spontaneous because they release energy to the surroundings.
  • Endothermic reactions (ΔH>0\Delta H>0ΔH>0):
    • Can still be spontaneous if they greatly increase entropy (ΔS>0\Delta S>0ΔS>0) and/or temperature is high, making TΔST\Delta STΔS big enough so ΔG\Delta GΔG becomes negative.
* Example idea: dissolving some salts in water absorbs heat (feels cold) but still occurs spontaneously because the ions become more randomly distributed, increasing entropy.

Mini‑Section: What “Predict” Really Means

When chemists say “energy change predicts if a reaction will occur,” they mean “thermodynamics says whether the reaction is allowed to proceed on its own.”

  • ΔG<0\Delta G<0ΔG<0 → reaction is allowed and will move toward products until equilibrium is reached.
  • ΔG>0\Delta G>0ΔG>0 → reaction is not favored; it will prefer reactants unless conditions (like temperature or concentrations) change.
  • At equilibrium, changing conditions (temperature, pressure, concentrations) can change ΔG\Delta GΔG and “push” the reaction in one direction.

However, ΔG\Delta GΔG does not tell how fast a reaction happens. A reaction can be thermodynamically favorable but kinetically slow if it has a high activation energy barrier.

Mini‑Section: Putting It in Simple Terms

You can link the energy change to “will it go?” like this.

  1. Calculate or estimate ΔH\Delta HΔH and ΔS\Delta SΔS.
  2. Use ΔG=ΔH−TΔS\Delta G=\Delta H-T\Delta SΔG=ΔH−TΔS.
  3. Decide:
    • ΔG<0\Delta G<0ΔG<0: reaction can occur on its own under those conditions.
    • ΔG>0\Delta G>0ΔG>0: reaction needs continuous energy input.
    • ΔG=0\Delta G=0ΔG=0: system is balanced at equilibrium.

So the change in energy of a chemical reaction predicts whether it will occur spontaneously by telling us if the products lie at a lower free energy than the reactants when both heat changes and disorder (plus temperature) are taken into account.

Information gathered from public forums or data available on the internet and portrayed here.