Gibbs Free Energy and Spontaneity and the meaning of the universe…

Gibbs Free Energy and Spontaneity and the meaning of the universe…

Télécharger la présentation

Gibbs Free Energy and Spontaneity and the meaning of the universe…

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

1. Relationship Between DS surr & H DS surr affected by heat transfer into or out of closed system Entropy of the surroundings will be affected only by the heat transferred into or out of any closed system. Water DS Surr  (system) q Heat added to surroundings: • K.E. surr increases; Molecules are moving faster. • Disorder increases since there is more randomness • Entropy increases. Note:qp = - DH sysµDS surr

2. Heat flow; Temperature Dependent • Remember that heat transfer is temperature dependent. Heat will transfer more efficiently with changes at low temperature than at high temperature. • i.e., 100€ to an International College student is worth more than to Raul. • @ High temperature, molecules are already moving fast, an extra 10°C will not increase their velocities as much as molecules at very low temperature. • -D Hsys = D Ssurr units: J . • T mol K • Therefore since: DS univ = DS sys + DS surr • DS univ = DS sys - DHsys • T

3. DS univ and Spontaneity • Criteria for Spontaneity in terms of the system: • DS univ = DS sys + DS surr • DS univ = DS sys - DHsys • T • DS univDH sys (-) • (+) SpontaneousDS sys(+) Spontaneous

4. Spontaneity in terms of TDS univ • Criteria for Spontaneity in terms of the system: • DS sys + DS surr = DS univ (1) • DS sys - DHsys =DS univ(2) • T • Note rearranging eqn. 2 • -TDS sys + DH sys = -T DS univ J. Willard Gibbs realized that -TD Suniv can be defined as a new function provided that DT = 0

5. J. Willard Gibbs The Free Energy change (DG) is a measure of spontaneity of a process and of the useful energy available from such a process. J. Willard Gibbs (1839-1903) was not particularly well known in his day, nor is his name widely recognized today, yet he is considered by some to be among the greatest scientists ever born in America. He was awarded th first doctorate in engineering granted in the United States, by Yale University. Gibbs became a professor of mathematical physics at Yale when he was 32 years old and began to publish a series of papers related to thermodynamics and equilibrium. Perhaps because his work was so theoretical, it was largely unappreciated at the time, though its great value was recognized by James Clerk Maxwell. Gibb’s work, if not his name, remains current and vital to this day.

6. DG and Spontaneity • Defining a new State function DG: • -T DS univ= DG @ DT, DP= 0 • Consider • DG = - T DS univ • DG < 0 Spontaneous DS univ > 0 • DG = 0 @ equilibrium DS univ = 0 • DG > 0 nonspontaneous DS univ < 0 • (rev is spontaneous)

7. DG: Pictorial View • Gibbs’ Free Energy can be defined in terms of the enthalpy of the system (DH sys)and the entropy of the system (DSsys) • -T DS univ =DH sys - TDS sys = DG • DG = DH - T DS DG = 0 DG > 0 DG < 0 Reverse reaction occur nonspontaneous in forward direction Forward reaction occur Spontaneous in forward direction @ equilb See later that : DG g Keq or Q DG > 0 DG < 0

8. DG: Equations of Free Energy • Gibbs’ Free Energy can be used to determine the Standard free energy (°) of formation • DG = DH - T DS • DG°f = DH°f - T DS°f °Standard State • f -formation from elements • If data is not for formation process, • then equation is slightly adjusted according to: • DG° = DH° - T DS° • Or from tabulated thermodynamic data: • DG° rxn = S n D G°f(prod) - S n D G°f(react)

9. DG: Evaluation of Free Energy • Consider the calculation for the following reaction: • CH3OH (g) + O2(g) g CO2(g) + H2O (g) • Determine DG°rxn 2 CH3OH (g) + 3 O2(g) g 2 CO2(g) + 4 H2O (g) DH°rxn - 201.2 0 -393.5 -241.82 c DS°rxn + 237.6 205.0 -213.6 -188.83 c DG°rxn - 161.9 0 -394.4 -228.57 c Evaluate by: DG°rxn = DH° rxn - T DS° rxn DX° rxn = S n D X°f(prod) - S n D X°f(react) Or DG°rxn = S n D G°f(prod) - S n D G°f(react)

10. Effect of temperature on Free Energy • Temperature influence on Free Energy and Spontaneity • DG = DH - T DS both DH, DS (+) • (1000) (1) • lg. # sm. # What is the sign of DG ? Temperature will dictate outcome of DG. Tlow: Temperature small DH - TDS gDG (+) dominates negligible nonspontaneous Thigh: Temperature large DH - TDS gDG (-) negligible dominates spontaneous

11. Temperature Relationship and DG • Consider Temperature affect on thermodynamic parameters • DH -TDS T DG Spontaneity • a - + all - spon: T not impt • b + - all + nonspon: T not impt • c - - low - spon: DH impt • d - - high + nonspon : DS impt • e + + low + nonspon : DH impt • f + + high - spon: DS impt • From this table, a spontaneous process can be made nonspontaneous • i.e., c & d by increasing Temperature.

12. Spontaneity: Example • Example : • (c) N2F4(g)g 2NF2(g)DH° - T DS° DG° • 85 kJ 198 J/K • @ T Low DH° dominates DG° (+) \Nonspontaneous • @ T High -TDS° dominates DG° (-) \Spontaneous • Example : • (c) What temp will spontaneity switch for the reaction: • N2(g) + 3H2(g)D 2NH3(g)DH° - T DS° DG° • - 92 kJ -198.5 J/K • @ T Low DH° dominatesDG° (-) \Spontaneous • @ T High -TDS° dominatesDG° (+) \Nonspontaneous • To go from spontaneous to nonspontaneous, DG° = 0 • … T = - 92kJ = 463.5 K below spontaneous • -0.198 kJ / K above, nonspontaneous

13. Phase Change Process • What determines the spontaneity of a phase change? • DH: s l g • DS: s l g • Two factors competing: • Which dominates will determine phase change. • Note: In a phase change: s Dl is at equilib. • or DG° = 0 • 0 = DH° - TDS° DH° = TDS° • With signs for DH & DS are the same T = DH° • DS° Endo DH(+) Endo DH(+) Exo DH(-) Exo DH(-) DS(+) DS(+) DS(-) DS(-)

14. Free Energy and work • Science and Technology use physical and or chemical processes because these can do work. • Economics: To make money €, the work to be preformed must be a possibility and efficient. • DG provides information on spontaneity:DG (+) or (-) provides information on the spontaneity of the process @ DP, DT = 0 • Wasting time:DG is useful because it prevents the wasted effort on process with no inherent tendency to occur. • DG isn’t whole story, Kinetics also important: Note that thermodynamically favorable process may still not occur to any appreciable extent because of the Kinetics. • - It makes sense to find a catalyst to speed up the reaction. • - Prevents wasting time and resource of seeking a catalyst on a reaction that won’t even work.

15. DG Equations • DG° DH° - TDS° - RT ln Keq S n DG°prod - S n DG° reaction DG - RT ln Q