unit 9#

  • kinetics vs thermodynamics:

    • kinetics: focuses on pathways between reactants and products (rxn rate)

    • thermodynamics: focuses on whether a process will occur & direction of process

      • considers only initial and final states between reactants & products and not the pathway

  • spontaneous processes: processes that occur without outside intervention

  • entropy (ΔS): molecular randomness or disorder

    • ΔSgas > ΔSaq > ΔSliquid > ΔSsolid

    • ΔSsolution > ΔScomponents of solution

      • since upon mixing more volume is available for the particles so more entropy

  • at high temperatures, most important factor for determining favorability is entropy

  • if a rxn such as this one occurs: A(g) → 2B(g), entropy increases as the rxn progresses since the number of moles of gases increases (very important point)

  • more volume (gases) = more entropy bc there is more space for the (same) molecules to move around & create disorder

  • entropy is measured in kJ/mol rxn

  • ΔSrxn = ΔSproducts - ΔSreactants

  • entropy (ΔS) ∝ temperature (T)

  • endothermic rxn: ΔS negative (ΔH positive)

  • exothermic rxn: ΔS positive (ΔH negative)

  • gibbs free energy (G) determines the thermodynamic favorability of a rxn

  • thermodynamically unfavorable rxns (nonspontaneous rxns) cannot occur without outside intervention

  • just because a rxn is favorable, doesn’t mean that it happens quickly!!!

  • standard change in gibbs free energy : ΔG°

    • ΔG°rxn = ΔG°products - ΔG°reactants

    • ΔG° must be negative to be favorable!!!

  • ΔG° = ΔH° - T(ΔS°)

    • ΔG° is negative at:

      • ΔH° - / ΔS° + → all T

      • ΔH° + / ΔS° - → no T

      • ΔH° + / ΔS° + → high T

      • ΔH° - / ΔS° - → low T

  • thermodynamic favorability:

    • ΔG° → negative

    • ΔH° → negative

    • ΔS° → positive

  • the use of catalysts has no effect on the thermodynamic favorability of a reaction

  • K = e^(ΔG/RT)

  • ΔG° = -RT(lnK)

    • R =

  • linking ΔG° and K:

    • ΔG° > 0 → K < 1 (reactant favored)

    • ΔG° < 0 → K > 1 (product favored)

    • ΔG° = 0 → rxn at equilibrium

  • to make a thermodynamically unfavorable process occur, you can either:

    • use an external energy source

    • couple the unfavorable rxn to a favorable one

      • use hess’ law to determine ΔG for the coupled rxn

      • the sum of the reactions’ ΔG is negative, making the overall process thermodynamically favorable

  • coupled rxns are rxns that share a common intermediate

  • electrochemical cells!!!

    • galvanic cells:

      • anode oxidation negative

      • cathode reduction positive

      • cations to cathode

      • anions to anode

      • electrons from anode to cathode

      • current from cathode to anode

    • electrolytic cells: