cem152-ss08-key-exam4A

cem152-ss08-key-exam4A - Name M Q CEM 152 SSO8 PID v8-—...

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Unformatted text preview: Name: M Q CEM 152 SSO8 PID: v8 -— Wat/b Exam 4A — April 16, 2008 Answer each question in the space provided. Clearly mark your final answer. Show all work for full credit. 1. (10 pts) Balance the following redox reaction, which occurs in acidic medium. Cu (s) + No; (aq) a Cu“ (aq) + N02 (g) e) ® CMAO CH2+ 716:“ Q10; —} le‘ —‘7 N01)1 n4:ch Lll~l+mle H 2. (10 pts) In lecture, a solution of NaZSO4 was treated by electrolysis. Two reactions could have occurred at the cathode: reduction of water to form hydrogen [2H20 (1) + 26 -+ H2 (g) + 20H_ (aq)] or reduction of the free sodium ions to form metal sodium [Na+ (aq) + le‘ ——+ Na (3)]. Only the reduction of water was observed. Mark which of the following statements is best represents the relative reduction potentials of the two half reactions, and briefly justify your selection. lEored (Na+)| > lEored IEOred (Nam = [E‘led (H20)| c) 1E°red (Na+)|< IEOred(H20)| w Mr: m H? smllev WWijka Mi Eire“ {ggs wov Mfr/nixed 3. (12 pts) Consider the following statements regarding a spontaneous reaction at constant pressure and temperature. Clearly mark whether each statement is true, false, or sometimes true. T“ a. ASuniv > 0 T - Ecell > 0 T . AGDm < 0 g . AHm, < 0 QaOO" Name: M2 6N CEM 152 SSOS PID: “A .— €€M Exam 4A — April 16, 2008 4. (10 pts) Sketch the electrolysis cell consisting of the two half cells: Na+ + 1e‘ —> Na 130,... = —2.71 V 012 + 26 —-> 201‘ Bored = +1.36 V Indicate the process (oxidation/reduction) that occurs at each electrode, the half- reaction that takes place at each electrode, the sign of each electrode, and the direction of electron flow in the external circuit. may «1 (122+); 5. (12 pts) Consider the following statements regarding the dependency of free energy on temperature. Clearly mark whether each statement is true or false. T a. An exothermic reaction that leads to increased entropy in products is spontaneous at all temperatures. F b. Equilibrium reactions are spontaneous at all temperatures. F c. Endothermic reactions that lead to increased entropy in products tend to be spontaneous at low temperatures. F (1. Reactions that result in a decreased entropy in products are never spontaneous at any temperature. 6. (12 pts) A saturated solution of Cu(OH)2 in water has a pOH = 6.26. Estimate KSp for this compound. Camilla (t2? 20H Name: PID: K56~CWK€M CEM 152 SSOS Exam 4A — April 16, 2008 7. (12 pts) Calculate the standard free energy change for the reaction 2NaC1 (s) —> 2Na (5) + C12 (g) given the half-cell reactions: Na+ +1e‘—>Na E°,.d=—2.71 V (:12 + 2e‘ --> 201‘ 13°er = +1.36 V [5ng : (3:901 (molar/id“) R Eidm (OYLJEOV’AK) ":- szv) — C mm 2 «wow A g a. ~V\ F? D, » (a (firmware—>0 to“) + 7.88 4103.? 2+:75-(C—y» 8. (12 pts) For each of the following pairs, determine the relative entropy between the two systems. Fill in the blank with: GREATER (G), LESS (L), or SAME (S). (bl—TAG» 9- 99‘s» and 10 atm 1 mol H2 (g) at ST P has .... ..entropy than 1 mol H2 (g) at 135 K and 1 atm 1 mol CO2 (g) at STP has .... ..entropy than 1 mol CO (g) at STP 1 mol H20 (1) at 100 °C and 1 atm has .... ..entropy than 1 mol H2O (g) at 100 0C and 1 atm 1 mol 02 (g) at 25 oC and 1 atm has .... ..entropy than 1 mol 02(g) at 25 0C 9. (10 pts) For the redox reaction: |Cu|Cu2+ (1M) || H+(1M) |H2 (g,l bar)| which of the three curves in the plot on the right best represents the change in E0611 when decreasing [Cu2+], given all other standard concentrations are maintained. The stande reduction potentials are: Cu2+ (aq) + 26 —> Cu (s) Bored = +0.34 V 2H+ (aq) + 2e‘ -+ H2(g,1 bar) E‘Led = 0 v A 43.24 v -0.29 V m8 B 43.34 v Explain, in qualitative terms, why the curve you chose has the shape it does. _0 39 V / a50~ “9‘05:x «6.0 Q C .A at 43.44 v -3 —2 -1 QDCC’MZK') szt—JQ/ Q L 10M 10M 13M 1M ""“\ log [Cu +] C 1353‘ E”? ...
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cem152-ss08-key-exam4A - Name M Q CEM 152 SSO8 PID v8-—...

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