Thermodynamics Chapter 20 Thermodynamics Prediction of whether change

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Thermodynamics Chapter 20

Thermodynamics Chapter 20

Thermodynamics Prediction of whether change will occur No indication of timeframe Spontaneous: occurs without

Thermodynamics Prediction of whether change will occur No indication of timeframe Spontaneous: occurs without external intervention Nonspontaneous: requires outside “push”

Entropy and Spontaneity Driving force for a spontaneous change is an increase in entropy

Entropy and Spontaneity Driving force for a spontaneous change is an increase in entropy of the universe Entropy, S: measure of disorder Spontaneous change implies: more order less order fewer ways of arranging particles more

Second Law of Thermodynamics In any spontaneous change, there is always an increase in

Second Law of Thermodynamics In any spontaneous change, there is always an increase in entropy of the universe. Units: J K

Entropy 1877 Ludwig Boltzmann: k = Boltzmann constant, R/NA W = no. of possible

Entropy 1877 Ludwig Boltzmann: k = Boltzmann constant, R/NA W = no. of possible arrangements Third Law of Thermodynamics: The entropy of a perfect crystal at 0 K is zero.

Positional Entropy Why does a gas expand into a vacuum? Expanded state has highest

Positional Entropy Why does a gas expand into a vacuum? Expanded state has highest positional probability of states available.

Other factors in entropy Size: increase in S with increasing size (mass) Molecular complexity:

Other factors in entropy Size: increase in S with increasing size (mass) Molecular complexity: increase in S with increasing complexity Generally effect of physical state >> complexity

Reactions For a spontaneous reaction: S 0 Na. OH(s) + CO 2(g) Na 2

Reactions For a spontaneous reaction: S 0 Na. OH(s) + CO 2(g) Na 2 CO 3(s) + H 2 O(l) 64. 45 213. 7 139 69. 94 J/K Is the reaction spontaneous as written?

Spontaneity and S Spontaneous: Nonspontaneous: At equilibrium: Suniv > 0 Suniv < 0 Suniv

Spontaneity and S Spontaneous: Nonspontaneous: At equilibrium: Suniv > 0 Suniv < 0 Suniv = 0 Ssys can be positive if Ssurr increases enough

Surroundings and Suniv Surroundings add or remove heat Exothermic: qsys qsurr < > 0

Surroundings and Suniv Surroundings add or remove heat Exothermic: qsys qsurr < > 0 0 so Ssurr > 0 Endothermic: qsys > 0 qsurr < 0 so Ssurr < 0

Ssurr and Ssys Ssurr: Ssurr - qsys Ssurr 1/T At constant pressure:

Ssurr and Ssys Ssurr: Ssurr - qsys Ssurr 1/T At constant pressure:

The Math Given: @constant P: Multiply by T: Result:

The Math Given: @constant P: Multiply by T: Result:

Reactions and G G 0: Standard Free Energy Reactants in standard states are converted

Reactions and G G 0: Standard Free Energy Reactants in standard states are converted to products in standard states

Gibb’s Free Energy Overall criterion for spontaneity from the standpoint of the system A

Gibb’s Free Energy Overall criterion for spontaneity from the standpoint of the system A process at constant temp. and pressure is spontaneous in the direction G decreases

G = H - T S H S G Spontaneous? “Good”: H <

G = H - T S H S G Spontaneous? “Good”: H < 0 “Good”: S > 0 “Good”: G < 0 - + - - - ? + + ? + - + At all temperatures At low temperatures At high temperatures Not at any temperature

Summary G < 0 Spontaneous as written G > 0 Not spontaneous as written

Summary G < 0 Spontaneous as written G > 0 Not spontaneous as written Reverse process spontaneous G = 0 At equilibrium

A Closer Look… T S: energy not avail. for doing work G: E avail.

A Closer Look… T S: energy not avail. for doing work G: E avail. as heat – E not avail. for work max. work available (constant T and P) Amount of work actually obtained depends on path

G and Work G Spontaneous Nonspontaneous max. work obtainable min. work required Work

G and Work G Spontaneous Nonspontaneous max. work obtainable min. work required Work and path-dependence wmax (wmin) process performed reversibly theoretical wactual < wmax performed irreversibly real world

Reversible vs. Irreversible Processes Reversible: The universe is exactly the same as it was

Reversible vs. Irreversible Processes Reversible: The universe is exactly the same as it was before the cyclic process. Irreversible: The universe is different after the cyclic process. All real processes are irreversible. Some work is changed to heat.

Free Energy and Pressure Q: reaction quotient from mass action law

Free Energy and Pressure Q: reaction quotient from mass action law

Free Energy and Equilibrium K: At equilibrium: equilibrium constant G = 0 K=Q

Free Energy and Equilibrium K: At equilibrium: equilibrium constant G = 0 K=Q

A B

A B

G and Extent of Reaction A B C D G 0 B <

G and Extent of Reaction A B C D G 0 B < G 0 A G 0 D> G 0 C Spontaneous Nonspontaneous

Temperature Dependence of K Plot ln. K vs. 1/T slope = - H 0/R

Temperature Dependence of K Plot ln. K vs. 1/T slope = - H 0/R intercept = S 0/R *assumes H 0, S 0 relatively T independent

Change Management Change Management Is Change Happening ChangeChange Management Change Management Is Change Happening Change
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