CHEM1312H - Gibbs Free Energy & Equilibrium

standard change in gibbs energy

-can be found using tabulated values or ΔG° = ΔH° - TΔS°

-most chemical reactions are not under standard conditions, and we will use ΔG insteas as the reaction proceeds from reactants to products

what is the significance of ΔG (non-standard)

-ΔG measures the reaction progress toward equilibrium

-ΔG can be measured at any point in the reaction and has infinite values, and is thus dircectly related to Q (reaction quotient)

ΔG_rxn (non-standard)

-ΔG_rxn = ΔG°_rxn + RTlnQ

-ΔG_rxn° - rfee energy at standard conditions in kJ/mol

-ΔG_rxn - free energy at any moment in kJ/mol

-R - universal gas constant 8.314 J/molK

-T - temp in kelvin

-Q reaction quotient (Q_p is used for reactions involving gases and Q_c are for substances dissolved in solution)

ΔG

-using ΔG, we can measure how far the reaction is from equilibrium

-if ΔG < 0, the reaction is spontaneous in the forward direction (Q < K and the reaction proceeds toward products to reach equilibrium)

-if ΔG > 0, the reaction is non-spontaneous in the forward direction (Q > K and the reaction proceed toward reactants to reach equilibrium

what if ΔG = 0

-the minimum point on the curve means that the reaction has reached equilibrium and at equilibrium, Q = K

-ΔG°_rxn = -RTlnK

-K - K_p for gas phase reactions and K_c for reactions in solution

-if you are given molar concentrations for gaseous phase reactions, you MUST convert from K_c to K_p

-note that there is only one value for ΔG° since there is only one value for the equilibrium constant at a given temperature

sign on ΔG°_rxn

-if ΔG°_rxn > 0, K < 1 (lnK is a negative value) and the process is reactant-favored at equilibrium (if K << 1, ΔG°_rxn is positive and large)

-if ΔG°_rxn < 0, K > 1 (lnK is a positive value) and the process is product-favored at equilibrium (if K >> 1, ΔG°_rxn is negative and large)

-if ΔG°_rxn = 0, K = 1 (ln1 is zero)