Entropy and feasibility of reactions

Entropy and feasibility of reactions

Understanding Entropy

  • Entropy (S) is a measure of the disorder or randomness of a system.
  • In any system, there is a natural tendency for the entropy to increase.
  • The unit for entropy is joules per mole per kelvin (J mol⁻¹ K⁻¹).

Entropy and Physical States

  • Gases have the highest entropy because their particles can move freely and randomly.
  • Solids have the lowest entropy as their particles are fixed in place.
  • Liquids have an intermediate entropy value as their particles have some freedom of movement.
  • The entropy of a system usually increases during a chemical reaction. This is due to the conversion of reactants into products, often forming gaseous products from solid or liquid reactants.

Calculating Entropy Change

  • The total entropy change for a reaction (∆Stotal) can be calculated by adding the entropy change of the environment (∆Ssurroundings) and the entropy change of the system (∆Ssystem).
  • ∆Ssystem can be calculated using the equation: ∆Ssystem = ΣSproducts - ΣSreactants
  • ∆Ssurroundings can be calculated from the enthalpy change of the reaction (∆H) and the temperature in kelvin (T), using the equation: ∆Ssurroundings = -∆H / T

Entropy and the Feasibility of Reactions

  • A reaction is feasible if the total entropy change for a reaction (∆Stotal) is positive, which implies the overall disorder of the universe increases due to the reaction.
  • If ∆G (Gibbs free energy) of a reaction is negative, the reaction is spontaneous and feasible under constant temperature and pressure. ∆G can be calculated from the equation: ∆G = ∆H - T∆Ssystem
  • However, a positive ∆Stotal does not guarantee a reaction will occur. Other factors such as kinetic barriers (e.g., activation energy) may prevent a feasible reaction from occurring.

Examples of Entropy Changes in Reactions

  • For endothermic reactions, the heat absorbed from the surroundings increases the entropy of the system, contributing to a positive ∆Stotal.
  • Dissolving a solid in a solvent usually results in an increase in entropy due to increased disorder.
  • Chemical reactions that produce more moles of gas result in an increase in entropy.