Chemical Thermodynamics – Gibbs Free Energy and Spontaneity Chapter 19 Chemical Thermodynamics – Gibbs Free Energy and Spontaneity

Free Energy Changes Gibbs Free Energy, ΔG, A measure of the maximum useful work that can be done by a chemical reaction. ΔGo = ΔHo - TΔSo Note that ΔG, like ΔS, is dependent on pressure and concentration. Unlike ΔH and ΔS, ΔG is strongly temperature dependent because of T in the equation.

If ΔG < 0 the reaction is spontaneous at standard conditions. If ΔG > 0 the reaction is not spontaneous at standard conditions. If ΔG = 0 the reaction is at equilibrium.

The effect of ΔH and ΔS on spontaneity: ΔG = ΔH - TΔS The effect of ΔH and ΔS on spontaneity: If ΔH > 0, ΔS < 0, then ΔG > 0 at all temperatures not spontaneous If ΔH < 0, ΔS > 0, then ΔG < 0 at all temperatures spontaneous If ΔH > 0, ΔS > 0, then ΔG > 0 at low T ΔG < 0 at high T (spontaneous) If ΔH < 0, ΔS < 0, then ΔG < 0 at low T ΔG > 0 at high T(nonspontaneous)

Calculate ΔGo for the following reaction. Fe2O3(s) + 3 H2(g) → 2 Fe(s) + 3 H2O(g) At what temperature does the reaction become spontaneous?

You can also calculate ΔGo from ΔGfo values at the back of the book. Ex - Calculate the standard Gibbs Free Energy for the reaction: FeCl2 → Fe2+ + 2 Cl-

Calculation of ΔG from ΔGo ΔG = ΔGo + RTlnQ Free energy activity Ex FeCl2(s) → Fe2+ + 2 Cl- If [Fe2+] = 0.0010 M and [Cl-] = 0.0010 M calculate ΔG. (Note that this turns out to be spontaneous at low concentration.)

Relationship between ΔG and K ΔGo = -RTlnK If K > 1, ΔG < 0, rxn is spontaneous at standard conditions. If K < 1, ΔG > 0, rxn is nonspontaneous at std cond. If K = 1, ΔG = 0, rxn is at equilibrium at std. cond.

Ex - Calculate ΔGo for the ionization of water at standard conditions.

Notes quiz https://docs.google.com/spreadsheet/embeddedform?formkey=dDl6M0dWcjZrSHBrVURXLWk1LXhSNnc6MQ