thermodynamics: entropy and gibbs free energy

spontaneity

processes that are spont in one direction are not spont in the other direction

you need the gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) to tell if the rxn is spont or not

entropy change, S

the measure of the number of ways in which matter can be arranged and energy can be distributed in a system
(measure of the no. of possibilities of arrangement)

FACTORS

change in no. of gaseous particles

mixing of particles (no rxn)

change in phase

change in temp

more particles, ΔS increase

more ways to arrange the particles, ΔS increase

decreasing ΔS: gas > liq > sol

temp increase, more KE, ΔS increase

Gibbs free energy, G

ΔG = ΔH - TΔS (units: kJ/mol or J/mol)

prediction of spontaneity

ΔG > 0

ΔG < 0

ΔG = 0

not spont in the forward direction, spont in the reverse direction

spont in the forward direction, not spont in the reverse direction

both forward and backward are spont

system has reached dynamic equilibrium, no net change in amt of products and reactants

four cases (T in Kelvins is always +ve)

  1. ΔH -ve and ΔS +ve, ΔG < 0 always spont
  1. ΔH +ve and ΔS -ve, ΔG > 0 never spont
  1. ΔH +ve and ΔS +ve, ΔG < 0 at high T, only spont at high T
  1. ΔH -ve and ΔS -ve, ΔG < 0 at low T, spont only at low T

crossover temperature

where ΔG = 0 when ΔH and ΔS are both +ve

assumptions when calculating

ΔS and ΔH remain constant at all temps
(only true as long as no phase change)