Non-Equilibrium Systems

Combustion

When hydrocarbons undergo complete combustion in an excess of oxygen, carbon dioxide and water are formed. These reactions are strongly exothermic:

\text{CH}_4 (g) + 2\text O_2 (g) \rightarrow \text{CO}_2 (g) + 2\text H_2 \text O (l)

The release of heat energy in this exothermic reaction drives the reaction in the forward direction. This reaction is spontaneous as the Gibbs free energy change is negative. (ΔG < 0).

The high activation energy for the reverse reaction reduces the chance that products will recombine to form reactants. Also, as combustion reactions occur in open systems, the products can escape into the surroundings and therefore cannot recombine to produce reactant molecules.

Photosynthesis

Photosynthesis occurs in the chloroplasts of green plant cells. Solar energy is absorbed by chlorophyll molecules inside the chloroplasts.

Chlorophyll selectively absorbs red and violet light in the visible spectrum. This energy is used to convert carbon dioxide and water into glucose and oxygen, as seen below:

6\text{CO}_2 (g) + 6\text H_2 \text O (l) \rightarrow \text C_6 \text H_{12} \text O_6  (s) + 6 \text O_2 (g)

The Gibbs free energy change for photosynthesis is positive (ΔG > 0) and therefore the reaction is non-spontaneous. This reaction requires solar energy to occur.

The reaction products are lost from the system and this prevents the reverse reaction from ocurring.