Thermodynamically Favored Process (TFP) Thermodynamically Favored Process. Thermodynamics III

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1 Lecture 17 Thermdynamics III Entrpy Free Energy Thermdynamically Favred Prcess (TFP) Thermdynamically Favred Prcess Prceeds withut any assistance rm utside the system. Water evaprates at 25 C Irn rusts in the presence O 2 and H 2 O NaCl disslves in water Nn-Thermdynamically Favred Prcess Nn-Thermdynamically Favred Prcess Assistance rm utside the system is necessary in rder t induce the desired change. Water des nt bil at 75 C and 1atm. Water des nt reeze at 15 C. Fe 2 O 3(s) + 3 C (s) 2 Fe (s) + 3 CO (g) at 25 C Thermdynamically Favred Prcess (TFP) A prcess that is thermdynamically avred in ne directin is nn-thermdynamically avred in the ther directin. H 2 O (l) H 2 O (s) H 2 O (s) H 2 O (l) at 10 C (TFP) at 10 C (nn-tfp) Thermdynamically Favred Prcess (TFP) Exthermic reactins are ten thermdynamically avred Nt Nature tends t avrs prcesses that thtcause a reductin in energy. In an exthermic reactin, the bnds in the prducts cntain less energy than the bnds in the reactants. The excess energy is released as heat. Thermdynamically Favred Prcess (TFP) Endthermic reactins can be thermdynamically avred Evapratin is thermdynamically avred. H 2 O (l) H 2 O (g) H = kj C 3 H 8(l) C 3 H 8(g) H = kj Disslving sluble cmpunds is thermdynamically avred. NaCl (s) Na + (aq) + Cl - (aq) H = +3.9 kj Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 1

2 Entrpy (S) A measure the disrder a system. A greater degree disrder is avrable. A psitive value r S is avrable. S = S (prducts) S (reactants) Increases in entrpy are avred Yur huse gets dirty Making a stir-ry The impssibility a maintaining a perect lawn Spilling a glass milk Breaking a windw Laws Thermdynamics 1 st Law Thermdynamics The energy cntained within the universe is cnstant. 2 nd Law Thermdynamics The entrpy the universe is cnstantly increasing. S > 0 r the llwing prcesses: Melting Vaprizatin Reactins where the prducts are in the same phase as the reactants but cntain mre particles than the reactants Making mst slutins Adding heat Increasing the vlume a gas S > 0 when melting S > 0 when vaprizing H 2 O(s) Very Organized Small S H 2 O(l) Less Organized Larger S H 2 O(l) Relatively Lw S H 2 O(g) High S Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 2

3 S > 0 when prducts have mre particles 4 C 3 H 5 N 3 O 9(l) 6 N 2(g) + 12 CO 2(g) + 10 H 2 O (g) + O 2(g) 4 liquid mlecules are cnverted int 29 gaseus mlecules. Increasing the number mles increases the number ptential arrangements. Changing rm liquid t gas increases the number ptential arrangements. S > 0 S > 0 (usually) when making slutins with slids and/r liquids A inic cmpund is very rganized. A slutin cntaining the same ins is much mre disrdered. S < 0 when making slutins with liquids and gases When disslving a gas in a liquid the entrpy will decrease. S > 0 when adding heat S vaprizatin The rapid and chatic mvements the gas particles are greatly reduced by the mlecules in the slutin. S < 0 Entrpy (S) S usin Melting pint Biling pint Temperature S > 0 when adding heat, as the distributin KE increases 250 K S > 0 when vlume increases as particles Number ga 500 K 1000 K H 2 O(g) H 2 O(g) Kinetic Energy High S Higher S Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 3

4 Calculating S Ex) Calculating S S rxn Sum = Σ ns (prducts) Σ ns (reactants) stichimetric ceicients Ex) Calculate S r the llwing reactin at 298K. N 2(g) + 2 O 2(g) N 2 O 4(g) S = Σ n S (prducts) - Σ ns (reactants) Hw can yu tell i a prcess is thermdynamically avred? Exthermic reactins are avrable. H < 0 Tw methds r determining i a prcess is thermdynamically avred 1) S universe Prducing a greater degree disrder is avrable. S > 0 2) G (Gibbs Free Energy) The Universe S universe The Surrundings The system S universe = S system + S surrundings I S universe > 0, the reactin is thermdynamically avred. Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 4

5 S surrundings and H S universe Heat lw links the system t the surrundings. Exthermic Reactins S surr increases ( S surr > 0) Heat The system Endthermic Reactins S surr decreases ( S surr < 0) Heat The system S universe = S system + S surrundings I a prcess: increases the entrpy the system ( S sys > 0), and is exthermic ( S surr > 0), it must als be thermdynamically avred ( S universe > 0). Tw methds r determining i a prcess is thermdynamically avred Gibbs Free Energy (G) 1) S universe ree energy change (kj) temperature system (K) G = H - T S entrpy change (kj/k) 2) G (Gibbs Free Energy) enthalpy change (kj) I G < 0, the reactin is thermdynamically avred. Gibbs Free Energy (G) Tw Methds r Calculating G The maximum amunt energy that can be used t d wrk The energy used t create disrder G = H - T S The energy transerred as heat I G < 0, the reactin is thermdynamically avred. 1) G = H - T S Calculate H using calrimetry, Hess s Law, r enthalpy rmatin values. Calculate S using entrpy values. Calculate G using G = H - T S 2) Calculate G using G values. Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 5

6 Ex1) Gibbs Free Energy ( G) Ex1) Find G r a reactin i: H = 218 kj and S = 765 J/K at 32 C. Is the reactin thermdynamically avred? Ex2) Gibbs Free Energy ( G) Ex2) At what temperature des the previus reactin becme thermdynamically avred? Assume that H and S d nt change as temperature changes? Δ G =ΔH TΔ S = 0 (set G = 0 and slve r T) ΔH 218 kj T = = = 285 K ΔS kj/k Temperature and Thermdynamic Favrability G = H - T S H S Thermdynamic Favrability + TFP at all temperatures + nn-tfp at all temperatures TFP at lw temperatures nn-tfp at high temperatures + + TFP at high temperatures nn-tfp at lw temperatures Ex) Thermdynamic Favrability Ex) H = kj and S = J/K r the llwing reactin. 2 N O 2 N + O 2 ( g ) 2( g) 2( g) Is this prcess thermdynamically avred? Justiy yur answer. Tw methds r calculating G 1) G = H - T S Calculate H using calrimetry, Hess s Law, r enthalpy rmatin values. Calculate S using entrpy values. Calculate G using G = H - T S 2) Calculate G using G values. Free Energy Frmatin G is the ree energy change that ccurs when ne mle a cmpund is made rm its elements in their standard states. G r an element in its standard state is zer. G rxn Sum = Σ n G (prducts) - Σ n G (reactants) Stichimetric Ceicients Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 6

7 Ex) Free Energy Frmatin Prblem Ex) Find Grxn r the thermite reactin under standard cnditins. Fe 2 O 3(s) + 2 Al (s) 2 Fe (s) + Al 2 O 3(s) rxn G = Σ n G (prducts) Σ n G (reactants) Thermdynamically avred prcess are nt necessarily ast! The rate a reactin is determined by chemical kinetics, nt thermdynamics. Many TFPs d nt ccur at a measurable rate. e.g. Oxidizing is a TFP. It can take several hundred years r an irn beam t rust away. Using external energy when G > 0 External energy can drive reactins when G > 0 Electricity can be used t recharge a battery. Phtns can supply the energy required t remve electrns rm atms. e.g. The cnversin CO 2 and H 2 O t glucse thrugh phtsynthesis. 6 CO2 + 6 H2O C6H12O6 + 6 O 2 Δ G = kj/ml This ccurs thrugh multiple steps that are initiated by the absrptin several phtns. Cupling reactins when G > 0 Initial step in the metablic breakdwn glucse C H O + HPO + H C H O P + H O Δ G = kj/ml Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 7