Entropy

• Entropy, S, is a measure of the number of ways that particles can be arranged and their energies be shared out.

• A higher entropy indicates a more disordered system.

• Substances are more energetically stable at higher entropy, and tend towards a higher entropic state.

• Entropy is affected by 3 factors:

  • Physical state: Solids have the most ordered arrangement so they have the lowest entropy. Gases have the most disordered arrangement so they have the highest entropy.

  • Amount of energy: The more energy quanta a substance has, the more ways they can be arranged and have greater entropy.

  • Number of particles: More particles means there are more ways the particles and their energy can be arranged so they have greater entropy.

• The units for entropy are JK^-1mol^-1

Entropy changes in a reaction

• Entropy change of the system, Δ_sysS, is the entropy change between the reactants and products during a reaction.

• Entropy change of surroundings, Δ_surrS, is the entropy change of the surroundings when a reaction occurs, as energy is transferred between the surroundings and the system.

• The total entropy change, Δ_totS, is the sum of Δ_sysS and Δ_surrS.

• Δ_totS = Δ_sysS + Δ_surrS

• Δ_sysS = S_products - S_reactants

• Δ_surrS = -ΔH/T (ΔH must be converted into Jmol^-1 and T is temperature in kelvin).

Reaction feasibility

• A feasible reaction is one that, once started, will carry on to completion, without any energy being supplied to it.

• For a reaction to be feasible, Δ_totS must be greater than or equal to 0.

• To calculate the minimum temperature of feasibility,

  • Δ_totS = 0

  • Δ_sysS = -Δ_surrS

  • Δ_sysS = ΔH/T

  • T = ΔH/Δ_sysS