Thermodynamics: Entropy and Enthalpy

Entropy and Enthalpy

• Entropy (S) is a measure of disorder or randomness in a system. Higher entropy indicates greater disorder.
• Enthalpy (H) is a measure of the total heat content of a system at constant pressure.
• Reactions tend to favor lower enthalpy (more stable) and higher entropy (greater disorder).

Minimum Enthalpy Favoring

• Reactions tend to proceed towards a state of lower enthalpy.
• Exothermic reactions (releasing heat) generally have a lower enthalpy product than reactant, favoring the reaction products.
• The change in enthalpy (ΔH) for a reaction is the difference in enthalpy between products and reactants.
• If ΔH is negative, the reaction is exothermic and enthalpy decreases.

Entropy Favoring Side with More Moles

• Reactions tend to favor a state of higher entropy (greater disorder).
• Reactions with an increase in the number of moles of gas tend to have a higher entropy, favouring the product side of the equation.
• The change in entropy (ΔS) for a reaction is the difference in entropy between products and reactants.
• If ΔS is positive, the reaction increases disorder and entropy increases.
• The greater the increase in moles of gaseous products, the greater the increase in entropy and the more favorable the reaction becomes.
• For example, decompositions of solids to produce gases will generally have positive ΔS terms due to the increased disorder of the gaseous products compared to the solid reactants.
• Reactions involving the dissolution of ionic compounds into solution often have a positive ΔS. Ions are more disordered in solution.

Gibbs Free Energy (ΔG)

• Gibbs free energy (ΔG) combines enthalpy and entropy to determine the spontaneity of a reaction.
• ΔG = ΔH - TΔS
  • ΔG is negative for spontaneous reactions. If ΔG is positive, the reaction is non-spontaneous.
  • Temperature (T) is important; the effect of entropy becomes more significant at higher temperatures.
• A reaction does not proceed just because enthalpy or entropy is favorable; ΔG must be negative overall. The change in Gibbs Free energy must be negative for a reaction to take place spontaneously.

Combined Effect of Enthalpy and Entropy

• The combined effects of enthalpy and entropy determine the spontaneity of a reaction.
• A reaction with a negative enthalpy and a positive entropy change is always spontaneous at any temperature.
• A reaction with a positive enthalpy and a negative entropy change is always non-spontaneous at any temperature.
• The effect of temperature becomes crucial for reactions where either the enthalpy or entropy change is positive or negative. For example, if the enthalpy change is positive and the entropy change is positive, the reaction may be spontaneous at high temperatures, but non-spontaneous at low temperature.

Conclusion

• Reactions are driven by the interplay of enthalpy, entropy and Gibbs Free Energy.
• Understanding the changes in enthalpy, entropy and free energy allows predictions to be made of the direction spontaneity of reactions.
• Predicting whether a reaction will occur is important for many applications, from industrial synthesis to metabolic processes in biology.