Will a non-spontaneous process occur if the temperature is increased?

Transcription

1 Name Chem 163 Sectin: Team Number: ALE 20. Calculating S & Predicting Spntaneity f Reactin (Reference: Silberberg 5 th editin) Will a nn-spntaneus prcess ccur if the temperature is increased? The Mdel: Calculating the Change in Entrpy fr a Reactin The entrpy f a (nn-aqueus in) substance at any temperature abve 0 K is psitive and is related t the substance s Third Law Entrpy (i.e., S = 0 at 0 K fr a perfect crystal ) thrugh the substance s heat capacity. Since chemists are typically interested in reactins ccurring at a temperature f 25 C, the standard state mlar entrpies (i.e., S, where the superscript naught infrms the reader that a gas exerts a pressure f 1 atm and slids and liquids are under a pressure f 1 atm) f substances are usually tabulated fr a temperature f 25 C. In Chem 161 we learned that we can use tabulated standard heats f frmatin ( f a reactin via Hess s Law (eqn 1) H f H = n p H (p) n r H (r) (1) sum ver prducts sum ver reactants ) t calculate the heat where n i is the i th species stichimetric cefficient in the balanced chemical equatin. What makes Hess s Law pssible fr heats f reactin is that enthalpy is a state functin. Key Questin rxn p f 1. Since entrpy is a state functin, there must be a versin f Hess s Law fr changes f entrpy in a system when a chemical reactin ccurs. Cnsider the generic chemical reactin shwn as eqn 2. a A + b B d D + e E (2) Write the frmula that wuld be used t determine the change in entrpy fr the eqn 2 based n the tabulated values f standard mlar entrpies f the reactants f and prducts. (Nte: Tabulated standard mlar entrpies are S and nt S.) r f Page 1 f 6

2 Exercises 2. Fr the fllwing reactins: Predict whether the entrpy f the system will decrease r increase and briefly explain why; and Then use the table f thermdynamic data t determine the standard change in entrpy at 25 C fr each f the fllwing reactins and cmpare the sign f S with what yu predicted. a. 2 KClO 3 (s) 2 KCl(s) + 3 O 2 (g) Predictin: Entrpy increases Entrpy decreases (circle yur chice) Explanatin: Calculatin f S: (Circle yur answer!) b. NO(g) + SO 3 (g) NO 2 (g) + SO 2 (g) Predictin: Entrpy increases Entrpy decreases (circle yur chice) Explanatin: Calculatin f S: (Circle yur answer!) c. 2 NaOH(aq) + FeCl 2 (aq) Fe(OH) 2 (s) + 2 NaCl(aq) (Hint: First write the net inic equatin.) Net inic Equatin: Predictin: Entrpy increases Entrpy decreases (circle yur chice) Explanatin: Calculatin f S: (Circle yur answer!) S fr Fe(OH) 2 (s) = J ml -1 K -1 (surce: NIST) S fr Fe 2+ (aq) = J ml -1 K -1 (frm multiple surces; the value fr Fe 2+ (aq) in Silberberg is incrrect!) Page 2 f 6

3 The Mdel: Predicting the Spntaneity f a Reactin The system is simply the chemical reactants and prducts under investigatin. The surrundings is everything else, and the system and the surrundings tgether cmprise the universe (i.e., an islated system unt itself). What separates the system frm the surrundings is perhaps sme barrier (e.g., a beaker in which the system is underging a chemical reactin), which is classified based n whether matter, heat energy, and/r wrk energy can pass frm ne side t the ther freely. We can represent the system and its surrunding by a simple diagram. In the diagram, the temperature f the surrundings is represented by T. Since the surrundings are s large (It s the rest f a universe cmpared t the small system under investigatin!), it is cnvenient t think f the surrundings as a infinite thermal reservir. Small amunts f heat depsited int r withdrawn frm the surrundings wn t significantly affect the temperature f the surrundings. Key Questins 3. A nnspntaneus prcess requires a cntinuus input f energy frm an utside surce in rder t ccur (e.g. phtsynthesis requires light energy t ccur.) On the ther hand, a spntaneus prcess ccurs withut any utside assistance in the frm f energy (ccurs naturally). When I think f a prcess ccurring spntaneusly, all I need t d is cnsider the Sun. Cmpared t the rest f the Universe, it is a cncentrated mass f mstly hydrgen (abut kg wrth). In the Sun s cre the temperature is ~ K, and at its surface the temperature is ~ K. The temperature f the vacuum f space is 2.7 K. Nuclear fusin takes place within the Sun, the primary prcess being Hydrgen-1 fusing t becme Helium-4. By-prducts f this nuclear reactin are heat and phtns. a. Are the prcesses ccurring within the Sun endthermic r exthermic? (Circle yur chice.) b. While certainly the Sun isn t very large cmpared t the rest f the Universe, in the figure belw the sun s cncentrated energy (the cmbined mass and electrmagnetic) is represented by and the surrunding circle the current extent f the Universe. In the circle t the right, sketch what the later picture will lk like after the Sun has expended all f its fuel fr fusin. c. Is the nw r the later picture f the Sun s energy mre disrdered? Briefly explain yur answer. 4. Nature prefers prcesses in which: (Circle the crrect answer.) i. heat is released frm the system int the surrundings; and entrpy f the system increases. ii. heat is released frm the system int the surrundings; and entrpy f the system decreases. iii. heat is absrbed by the system frm the surrundings; and entrpy f the system increases. iv. heat is absrbed by the system frm the surrundings; and entrpy f the system decreases. Page 3 f 6

4 5. If a prpsed reactin is exthermic and the entrpy f the chemical system increases, the reactin: i. will always be nn-spntaneus, but can ccur if wrk is dne n the system. ii. can be spntaneus, but it will depend n the temperature at which the reactin is taking place. iii. will always be spntaneus, but it desn t necessarily ccur instantaneusly. 6. If a prpsed reactin is endthermic and the entrpy f the chemical system decreases, the reactin: i. will always be nn-spntaneus, but can ccur if wrk is dne n the system. ii. can be spntaneus, but it will depend n the temperature at which the reactin is taking place. iii. will always be spntaneus and ccurs instantaneusly. 7. There are numerus examples f endthermic prcesses in which the entrpy f the system increases. An everyday example is the melting f ice: H 2 O(s) H 2 O(l) The diagram t the right represents ice in a rm, the temperature f which is abve 0 C. a. Explain why the entrpy f the system is increasing. b. Heat flws frm cld t ht / ht t cld. (Circle the crrect answer.) c. Draw an arrw, labeled with q, in the abve diagram that describes the directin in which heat flws between the ice and its surrundings. d. What kind f temperature must the surrundings have relative t the ice s that heat will flw in the directin that is the answer t part (c): lw r high? (Circle yur answer.) 8. There are numerus examples f exthermic prcesses in which the entrpy f the system decreases. Think abut everyday prcesses that yu are familiar with that fall in this categry. Describe it belw and draw a diagram that shws the system in its universe. Indicate hw heat must flw between the system and its surrunding by drawing an arrw labeled with q. Determine whether the prcess ccurs at lw r high temperatures. Page 4 f 6

5 Exercises Fr each f the fllwing thermchemical equatins, predict whether the reactin has a S < 0 r S > 0. (D nt calculate this quantity using the table f thermdynamic data in yur textbk.) Then if the reactin will be: (I) spntaneus at all temperatures; (II) spntaneus at lw temperatures (but nn-spntaneus at high temperatures); (III) spntaneus at high temperatures (but nn-spntaneus at lw temperatures); r (IV) nn-spntaneus at any temperature. Briefly explain yur reasning in each HgO(s) 2 Hg(l) + O 2 (g) H = 90.8 kj Reasning: Spntaneus at Fe(s) + 3 O 2 (g) 2 Fe 2 O 3 (s) H = kj Reasning: Spntaneus at 11. N 2 (g) + 3 Cl 2 (g) 2 NCl 3 (g) H = 460 kj Reasning: Spntaneus at NH 4 NO 3 (s) 2 N 2 (g) + 4 H 2 O(g) + O 2 (g) H = kj Reasning: Spntaneus at Page 5 f 6

6 13. Distinguish between the terms spntaneus and instantaneus. Give an example f a prcess that is spntaneus but very slw, and ne that is fast but nt spntaneus. 14. Distinguish between the terms spntaneus and nnspntaneus. Can a nnspntaneus prcess ccur? Explain. 15. Why is the S vap f a substance always larger than S fus? 16. Withut ding a calculatin, predict if S sys fr each prcess is greater than r less than zer. Be able t defend yur respnses. a.) Gasline vaprs mixing with air in a car engine: b.) Ht air expanding: c.) Breath cndensing in cld air: 17. Withut ding a calculatin predict cmpund f each pair has the greater mlar entrpy. Be able t defend yur respnses. (Circle yur respnse.) a.) NO 2 (g) r N 2 O 4 (g) b.) CH 3 OCH 3(l) r CH 3 CH 2 OH (l) c.) HCl (g) r HBr (g) (Hint: cnsider their IMF s.) 18. Withut cnsulting Appendix B, arrange each f the fllwing grups in rder f decreasing standard mlar entrpy, S 0. Be able t defend yur respnses. a.) The fllwing three metals: Mg, Ca, Ba: b.) Hexane (C 6 H 14 ), benzene (C 6 H 6 ), cyclhexane (C 6 H 12 ) c.) PF 2 Cl 3 (g), PF 5 (g), PF 3 (g) Page 6 f 6