Syllabus Dot Point

Analyse examples of non-equilibrium systems in terms of the effect of entropy and enthalpy, for example:

• combustion reactions
• photosynthesis

1. Combustion Reactions

Combustion is a classic example of an irreversible, non-equilibrium system. It is a "spontaneous" reaction that, once started, drives itself to completion.

Example: Combustion of Octane (Petrol)

The Reaction Equation:

$$ 2\text{C}8\text{H}{18}(l) + 25\text{O}_2(g) \to 16\text{CO}_2(g) + 18\text{H}_2\text{O}(g) $$

Thermodynamic Analysis

• Enthalpy ($\Delta H < 0$):
  • The reaction is highly exothermic.
  • It releases a massive amount of heat energy to the surroundings.
  • Effect: The release of energy favors the forward reaction (lowering the internal energy of the system).
• Entropy ($\Delta S > 0$):
  • System Entropy: In this specific equation, we go from 25 moles of gas (reactants) to 34 moles of gas (products). This increase in gas particles significantly increases the disorder of the system.
  • Surroundings Entropy: The heat released ($\Delta H < 0$) disperses into the surroundings, increasing the entropy of the universe significantly.
  • Effect: The increase in disorder favors the forward reaction.

Conclusion

Since both enthalpy (energy release) and entropy (increased disorder) favor the products, the reaction is driven strongly to the right. It effectively goes to completion and does not establish an equilibrium.

2. Photosynthesis

Photosynthesis is a non-spontaneous process in thermodynamic terms. It is a system that builds order from disorder, requiring a constant input of energy.

The Reaction Equation:

$$ 6\text{CO}_2(g) + 6\text{H}_2\text{O}(l) \xrightarrow{\text{light}} \text{C}6\text{H}{12}\text{O}_6(s) + 6\text{O}_2(g) $$

Thermodynamic Analysis