CHM 102 Sinex

Thermodynamics

Part 2

CHM 102 Sinex

Free EnergyG - Gibbs free energy - maximum possible

useful work at constant T and PG = H – TSOur predictive tool for reactions

G > 0 non-spontaneousG = 0 at equilibriumG < 0 spontaneous

For a spontaneous process:maximize entropy minimize energy

CHM 102 Sinex

Standard molar free energy of formation - Gof

Gof = 0 for free elements in standard state

CaCO3 (s) CaO (s) + CO2 (g)

Calculate free energy change for a reaction

Gof : -1129 kJ/mole -604 kJ/mole -394 kJ/mole

Is this a spontaneous reaction?

CHM 102 Sinex

G = H – TSCASE RESULT- the sign of GS > 0H < 0 Spontaneous at ALL temperatures

S > 0H > 0 Spontaneous at high temperatures

S < 0H < 0 Spontaneous at low temperatures

S < 0H > 0

Non-spontaneous at ANY temperature

CHM 102 Sinex

Is this reaction possible?CH4 (g) C (diamond) + 2H2 (g)

@298K CH4diamon

d H2

Hof (kJ/mole) -75 2 0

So (J/K-mole) 186 2 131Go

f (kJ/mole) -51 3 0

Calculate Horxn, So

rxn, and Gorxn

G = H-TS

CHM 102 Sinex

Can it rain diamonds on Neptune and Uranus?

Neptune/Uranusblue color – CH4

10-50GPa2000-3000K

Can the reaction become spontaneous?

G = 0 = H - TS

Teq = H/S (estimate)

CHM 102 Sinex

Hess’ Law

C (graphite) + O2 (g) CO2 (g) H1

C (graphite) + ½ O2 (g) CO (g) H2

CO (g) + ½ O2 (g) CO2 (g) H3

Can't run this reaction, you get CO and CO2

Reactions are additive!

CHM 102 Sinex

C (graphite) + O2 (g) CO2 (g) H1

CO (g) + ½ O2 (g) CO2 (g) H3

C (graphite) + O2 (g) CO2 (g) H1

CO2 (g) CO (g) + ½ O2 (g) -H3

C (graphite) + ½ O2 (g) CO (g) H2 = H1-H3

CHM 102 Sinex

Biological pathways

Glucose + HPO4-2 + H+ [glucose-6-phosphate]- + H2O

Important in the metabolism of glucose

Go’ = 13.8 kJ

Glucose + 6O2 CO2 + H2O

First step in glycolytic pathway:

Is this reaction spontaneous?

CHM 102 Sinex

Coupled reactionsGlucose + HPO4

-2 + H+ [glucose-6-phosphate]- + H2O

Go’ = 13.8 kJATP-4 + H2O ADP-3 + HPO4

-2 + H+

Go’ = -30.5 kJ

Glucose + ATP-4 [glucose-6-phosphate]- + ADP-3

Go’ = 13.8 + -30.5 = -16.7 kJ

CHM 102 Sinex

Free energy and the equilibrium

constantGo = -2.303RTlog KT - kelvin temperature

R - gas constant – 8.314 J/K-mole

Go > 0 K < 1 reactant favoredGo = 0 K = 1Go < 0 K > 1 product favored

CHM 102 Sinex

Diamonds are NOT forever!Cdiamond Cgraphite

Gof : 3 kJ/mole 0

Gorxn = 0 – 3 = -3 kJ

log K = Gorxn x 1000

-2.303 x 8.314 J/K-mole x 298 Klog K = 0.53 K = 100.53 = 3.4

very very very very slow kinetics