Entropy

entropy (S)

• the distribution of energy and/or matter in a system

  • the more ways energy can be distributed, the higher the entropy

    • how chaotic a system is

• what can increase entropy:

  • change in state (gas has the highest entropy)

  • increased movement of particles e,g through heating

  • increased number of particles e.g. 2H2O2 → 2H2O + O2(g)

  • greatest increase usually results from an increase in the number of gaseous particles

• higher entropy is energetically favourable as energy of the system is more spread out in a disordered state

• the entropy of a perfect crystal at 0K is 0 because:

  • there is a perfectly ordered arrangement of the constituent particle of a pure crystalline substance and there is no disorder at all

calculating standard entropy changes

• units of entropy are in J K^-1 mol^–1

• entropy changes depending on state of matter (gas has the highest entropy)

• coefficients used to balance equation must be used when calculating overall entropy change

what is free energy

• what’s left over to do useful work after a reaction is carried out

• enthalpy change is difference between energy put in to break bonds and energy out when making bonds

• entropy change is the cost of carrying out the reaction, so free energy is what you’re left with

gibbs free energy equation

• units of ΔG^ꝋ are in kJ mol^–1

• ΔH_r^ꝋ are in kJ mol^–1

• units of T are in K

• units of ΔS_system^ꝋ are in J K^-1 mol^–1. so usually must be converted to kJ K^–1 mol^–1 by dividing by 1000

calculating ΔG^ꝋ from other ΔG^ꝋ values

• ΔG^ꝋ = ΣΔG_products^ꝋ – ΣΔG_reactants^ꝋ

spontaneous reactions

• spontaneous = feasible

• for a reaction to be spontaneous ΔG^ꝋ must be negative or zero

summary of factors affecting gibbs free energy