Entropy, Free Energy, and Equilibrium

Transcription

1 Nv. 26 Chapter 19 Chemical Thermdynamics Entrpy, Free Energy, and Equilibrium

2 Nv. 26 Spntaneus Physical and Chemical Prcesses Thermdynamics: cncerned with the questin: can a reactin ccur? A waterfall runs dwnhill A lump f sugar disslves in a cup f cffee At 1 atm, water freezes belw C and ice melts abve C Heat flws frm a htter bject t a clder bject

3 Nv. 26 Spntaneus Physical and Chemical Prcesses spntaneus nnspntaneus

4 Nv. 26 Spntaneus Physical and Chemical Prcesses A spntaneus prcess can depend n temperature nnspntaneus

5 Nv. 26 Entrpy Entrpy (S) : a measure f the randmness r disrder f a system. rder S S = S - S final disrder initial If a change results in an increase in randmness Fr any substance: if S > S, S> final initial Randmness Order slid < liquid << HO () s H 2 2 gas S slid < Sliquid << Sgas O ( l), S> S

6 Nv. 26 Entrpy Slute + Slvent Slutin Randamness increase

7 Nv. 26 Entrpy Bltzman : the entrpy (S) f a system is related t the natural lg f the number f micrstates (W). S = k lnw S = S S final initial S = S S = klnw klnw = kln final initial final initial W W S Final > initial, > W W final initial W W S Final < initial, <

8 Entrpy W=3 Three Pssible Arrangements (states) f Fur Mlecules in a Tw- Bulbed Flask Nv. 26

9 Nv. 26 Entrpy Prcesses that lead t an increase in entrpy ( S > )

10 Nv. 26 Entrpy Prcesses that lead t an increase in entrpy ( S > )

11 Nv. 26 Entrpy State functins: prperties that are determined by the state f the system, regardless f hw that cnditin was achieved. Energy (E), enthalpy(h), pressure(p) vlume(v), temperature(t), entrpy(s) entrpy

12 Nv. 26 First Law f Thermdynamics: E = q+ w Secnd Law f Thermdynamics: The entrpy f the universe increases in a spntaneus prcess and remains unchanged in an equilibrium prcess. Irreversible (Spntaneus) prcess: Reversible (Equilibrium) prcess: S univ = S sys + S surr > S univ = S sys + S surr =

13 Nv. 26 Secnd Law f Thermdynamics Reversible prcess, islated system: S sys = Irreversible prcess, islated system: S sys > Standard mlar entrpy( ): the mlar entrpy values f substances at their standard states S S (O 2 ) = 25. J/K ml S (CO) = J/K ml S (CO 2 ) = J/K ml

14 Nv. 26 Standard mlar entrpy 1. Standard mlar entrpies f elements are nt zer S >S >S 2. Fr a substance: slid liquid gas 3. Fr different substances: entrpies increase with the increasing f mass f mlecules 4. Fr different substances: entrpies increase with the number f atms in the frmula f substance

15 Nv. 26 Entrpy Changes in the System ( S sys ) S The standard entrpy f reactin ( ) is the entrpy change fr a reactin carried ut at 1 atm and 25 C. aa + bb cc + dd S = [ c S ( C) + d S ( D)] [ a S ( A) + b S ( B)] = ns S (prducts) ms (reactants)

16 Nv. 26 Entrpy Changes in the System ( S sys ) What is the standard entrpy change fr the fllwing reactin at 25 C? 2CO(g)+O (g) 2CO (g) 2 2 S = [ c S ( C) + d S ( D)] [ a S ( A) + b S ( B)] = ns S (prducts) ms (reactants) S (CO) = J/K ml S (O 2 ) = 25. J/K ml S (CO 2 ) = J/K ml S = 2 S (CO 2 ) [2 S (CO) + S (O 2 )] S = [ ] = J/K ml

17 Entrpy Changes in the System ( S( sys ) Nv. 26 When gases are prduced (r cnsumed) If a reactin prduces mre gas mlecules than it cnsumes, S >. If the ttal number f gas mlecules diminishes, S <. If there is n net change in the ttal number f gas mlecules, then S may be psitive r negative BUT S will be a small number. What is the sign f the entrpy change fr the fllwing reactin? 2Zn (s) + O g 2( ) 2ZnO (s) The ttal number f gas mlecules ges dwn S <

18 Nv. 26 Entrpy Changes in the Surrundings ( S surr ) Exthermic Prcess S surr > Endthermic Prcess S surr <

19 Third Law f Thermdynamics The entrpy (S) f a perfect crystalline substance is zer at the abslute zer f temperature. S = k lnw W = 1 S = Nv. 26 The entrpy (S) increases

20 Gibbs Free Energy (G) Spntaneus prcess: S univ = S sys + S surr > Equilibrium prcess: S univ = S sys + S surr = Fr a cnstant-temperature temperature prcess: Gibbs free energy (G) G = H sys -T S sys G < G > The reactin is spntaneus in the frward directin. The reactin is nn-spntaneus as written. The reactin is spntaneus in the reverse directin. G = Nv. 26 The reactin is at equilibrium.

21 Gibbs Free Energy (G) G The standard free-energy f reactin ( ) : the free-energy change fr a reactin when it ccurs under standard-state cnditins. aa + bb cc + dd G = [ c G ( C) + d G ( D)] [ a G ( A) + b G ( B)] f f f f = n Gf G (prducts) m G (reactants) f Gibbs free energy (G): A State Functin Nv. 26 G=G -G final initial

22 Nv. 26 Gibbs Free Energy (G) Standard free energy f frmatin ( G f ) : the free-energy change that ccurs when 1 mle f the cmpund is frmed frm its elements in their standard states. G f f any element in its stable frm is zer

23 Nv. 26 Gibbs Free Energy (G) G = H T S

24 Nv. 26 Gibbs Free Energy and Temperature CaCO3() s CaO() s + CO2( g) H = kj S = 16.5 J/K At 25 C, G = H T S -3 = kj k / = 13kJ > kj K

25 Gibbs Free Energy and Temperature CaCO3() s CaO() s + CO2( g) H = kj S = 16.5 J/K CO 2 Equilibrium Pressure T=?, it is a spntaneus reactin? G H T S = kj-t / T H = 117.8K = 835 S kj K = G changes with the change f temperature Nv. 26 C

26 Gibbs Free Energy and Chemical Equilibrium G = G + RT ln Q R : the gas cnstant (8.314 J/K ml) T : the abslute temperature (K) Q: the reactin qutient At Equilibrium G = G + RT ln K = G = RT ln K Nv

27 Nv. 26 Gibbs Free Energy and Chemical Equilibrium G = RT ln K