🌿非-Equilibrium Example 2: Photosynthesis

Photosynthesis is the process by which plants convert carbon dioxide and water into glucose and oxygen, using sunlight as an energy source. It is non-equilibrium for different reasons than combustion — but entropy and enthalpy still explain why.

Why Photosynthesis is Non-Equilibrium

Enthalpy: Photosynthesis is highly endothermic (ΔH = +2803 kJ/mol). Building glucose from CO₂ and H₂O requires an enormous amount of energy. This energy comes from sunlight. Without continuous light input, the reaction stops immediately. Thermodynamically, the reverse reaction (breaking glucose apart) is favoured — which is exactly what happens in cellular respiration.

Entropy: Photosynthesis decreases entropy (ΔS < 0). Six molecules of CO₂ gas and six molecules of liquid water (small, simple, disordered) are assembled into one large, highly organised glucose molecule (C₆H₁₂O₆). Going from many small molecules to one large, complex molecule is a decrease in disorder.

Combined effect: Both ΔH and ΔS oppose the forward reaction. Endothermic AND entropy decreases. This makes photosynthesis non-spontaneous (ΔG is positive). It requires a continuous external energy input (sunlight) to proceed. The moment the sun goes down or a cloud blocks the light, the reaction stops. This is the definition of a non-equilibrium, non-spontaneous process.

Importantly, photosynthesis is the reverse of combustion (in terms of overall chemistry). Combustion breaks glucose apart, releasing energy. Photosynthesis builds glucose, requiring energy. They are opposite processes driven in opposite directions by enthalpy and entropy.

🌿 Photosynthesis — Enthalpy & Entropy Analysis

Reaction: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)

ΔH = +2803 kJ/mol (endothermic — absorbs energy from sunlight, opposes forward)

ΔS < 0 (many small molecules → one large organised molecule, opposes forward)

Result: Both effects oppose the forward reaction. Non-spontaneous. Requires continuous energy input (sunlight). Stops when energy source removed.