Practice ExercisesOxidation-Reduction Reactions and Electrochemistry 483

Multiple-ChoiceFor the first four problems below, oneor more of the following responses willapply; each response may be usedmore than once or not at all in thesequestions.

1. Nernst equationII. spontaneous

III. reductionIV oxidationV electrolysis

1. The loss of electrons is atn)_______ process

(A) I(B) II(C) III(D) IV(E) V

2. In order to produce aluminum,large amounts of electricity are usedto perform the _of aluminum oxide.

(A) I(B) II(C)III(D) IV(E) V

3. This is used to determine cellvoltages when standard stateconditions are not present.

(A) I(B) II(C) III(D) IV(E) V

4. cannot occurwithout alsooccurring within the same system.

(A) I, III(B) II, V(C) III, IV(D) IV, V(E) V, II

5. Balance the following half-reactionin acid solution:

NO~ ---)NH~

When balanced with the smallestwhole-number coefficients, thesum of all the coefficients is

(A) 13(B) 26(C) 15(D) 23(E) 21

484 AP Chemistry

6. For the reaction between the permanganate ion and iron(II), the, reaction is

Which of the following is the correct form of the Nernst equation for thisreaction?

(A) E - EO __R_T In.:;,-[M_n_2+-=][=-=-Fe_3+-,,-H-=-::-H_20--=-]

- cell 5?J [MnO:][Fe2+][H+]

o RT [Mn2+][Fe3+Y(B) E = E - - In -"--~'-"---::-"--

cell 5?J [MnO:][Fe2+nH+]

o RT [Mn2+][Fe3+Y(C) E = E - - In --"--~"-"--~--;::-

cell 5?J [MnO:] [Fe2+n H+]8

o RT [Mn2+][Fe3+Y(D) E = E - - In --"--~"-"--~--;::-

cell 8?J [MnO:][Fe2+nH+r

o RT [Mn2+ ][Fe3+ y(E) 5E = 5E - - In _-"--~!.!o--;:--"---;::-

cell 8?J [MnO:][ Fe2+n H+]8

7. The following reactions are knownto occur spontaneously:

Cu + 2Ag+ ~ Cu2+ + 2Ag

Zn + 2Ag+ ~ Zn2+ + 2Ag

Zn + Cu2+ ~ Zn2+ + Cu

The activity series for the threeelements from the strongest to theweakest reducing agent is

(A) Cu > Ag > Zn(B) Zn> Cu > Ag(C) Ag > Cu > Zn(D) Ag > Zn > Cu(E) Zn > Ag > Cu

8. The standard reduction potentialfor Pb02 ~ Pb2+ is +1.46 V, andthe standard reduction potential forFe3+ ~ Fe2+is +0.77 V What is thestandard cell voltage for thereaction

(A) -0.08 V(B) +0.69 V(C) +2.33 V(D) -0.69 V(E) -2.33 V

Oxidation-Reduction Reactions and Electrochemistry 485

9. Which of the following 13. Which of the following elementscompounds includes an element has the largest number of possiblewith an oxidation number of +5? oxidation states?

(A) ClO~ (A) Fe(B) MnO~ (B) CI(C) NO; (C) Ca(0) SO~- (0) Mn(E) NO~ (E) Na

10. Which of the following metals 14. For the reactiondoes NOT react with water toproduce hydrogen? zr + Cl2 ~ zcr + 12

(A) Zn the standard cell voltage is(B) Li +0.82 V What is the equilibrium(C)Ca constant for this reaction at(0) Na 45 DC? (R= 8.314 V C morl K-I,(E) Rb and ?Jf = 96,485 C mol")

11. In the electrolysis of an aqueous (A) 9.8 X 1025

solution of CU(N03)2' which of (B) 1.0 x 10-26

the following is expected to occur? (C) 1.6 X 105

(A) formation of 02 at the anode(0) 6.3 X 10-6

(B) formation of H2 at the(E) 2.27

cathode 15. Which of the following cell(C) deposition of copper metal voltage values represents a

on the anode nonspontaneous reaction that(0) formation of hydroxide ions produces the greatest amount of

at the cathode product? (Assume the same(E) formation of H+ at the number of electrons is transferred

cathode in each reaction.)

12. Sodium metal cannot be (A) +2.31 Velectrolyzed from an aqueous (B) +0.23 VNa2S04 solution because (C) -0.12 V

(A) the voltage needed is too(0) -1.68 V

high for any available(E) -1.14 V

instrument to achieve 16. A metal is electrolyzed from(B) wateris reduced to 02 before aqueous solution by using an

Na+ electrical current of 1.23 A for(C) Na+ has a high over-potential 21/2h, and 3.37 g of metal is

that keeps it from being deposited. In a separatereduced experiment the number of

(0) H+ has a more favorable electrons used for the reduction ofreduction potential than Na+ the metal is 2. What is the metal?

(E) Na+ does electrolyze, but it (A) AIimmediately reacts with (B) Niwater again (C) Sn

(0) Mg(E) Au

486 AP Chemistry

17. A galvanic cell is set up undernonstandard state conditions, andEceU is measured as -0.16 VWhich of the following is trueabout this system?

(A) The galvanic cell is set upincorrectly because of thenegative cell voltage value.

(B) If the contents of the twocells are mixed, the reactionwill proceed in the forwarddirection.

(C) The reaction is definitelynonspontaneous.

(D) If the contents of the twocells are mixed, the reactionwill proceed in the reversedirection.

(E) The reaction is definitelyspontaneous.

18. A galvanic cell is constructed; andwhen the temperature of the cellis increased from 20°C to 30 °C,the cell voltage changes by a factorof 1.034. Which of the followingconclusions may be drawn fromthis observation?

(A) I1Go must be positive sincethe reaction is notspontaneous.

(B) The increase is due solelyto the temperature term inI1Go = -RT In K

(C) The information indicatesthat 115° for this reactionmust be negative.

(D) The reaction is spontaneousbecause I1GO is negative.

(E) The voltage change must bedue to the change in theequilibrium constant withtemperature.

19. Which of the following is NOTcommonly produced byelectrolysis?

(A) NaOCl (bleach)(B) Al(C) Fe(D) NaOH(E) H2

20. Which of the followingcompounds includes an elementthat has the same oxidationnumber as the chlorine in sodiumchlorate, NaClO/

(A) K3Fe(CN)6, (B) KMn04(C) Al(N03)3(D) (NH4)2S04(E) KCl04

21. With a current of 1.25 A, howmany minutes will be required todeposit 2.00 g of copper on aplatinum electrode from acopper(II) nitrate solution?(Faraday'sconstant =96,485 C mol")

(A) 4859(B) 81.0(C) 40.5(D) 1.35(E) 2430

22. What is the minimum number ofelectrons needed to balance thefollowing half-reaction with wholenumber coefficients?

(A) 1(B) 2(C) 5(D) 10(E) 12

23. Which of the following pairs ofconstants are NOT mathematicallyrelated to each other?

(A) equilibrium constant andGibbs free energy

(B) rate constant and activationenergy

(C) standard cell voltage andequilibrium constant

(D) standard cell voltage and rateconstant

(E) Gibbs free energy andstandard cell voltage

Oxidation-Reduction Reactions and Electrochemistry 487

24. Which of the following statementsis FALSE?

(A) Reduction involves a gain ofelectrons.

(B) Batteries are galvanic cells.(C) A spontaneous reaction always

has a positive E ~ell.

(D) Electrolysis reactions alwaysproduce a gas at at least oneelectrode.

(E) Galvanic cells can be used todetermine equilibriumconstants.

ANSWER KEYl.D 6. C 11. A 16. B 21. B2. E 7. B 12. D 17. D 22. D3. A 8. B 13. B 18. B 23. D4. C 9. E 14. A 19. C 24. D5. D 10. A 15. C 20. C

See Appendix 1 for explanation of answers.

Free-Response

(a) Balance the following skeleton reaction in acid solution

r + 10; ~ I;

(b) Describe in words what to do to convert the balanced equation in part (a)into a reaction in basic solution.

(c) Write the Nernst equation for the reaction in part (a).

(d) A galvanic (voltaic) cell is set up with a copper wire immersed in a Cu2+solution and a strip of magnesium immersed in a Mg2+ solution. If themagnesium ion concentration is 0.00200 M and the cell voltage is measuredas 2.15 volts, what is the concentration of the copper ion solution?

(e) A solution containing Cu2+ is electrolyzed for 45.0 minutes. The mass ofa platinum electrode increases from 38.3421 to 38.4876 grams when thecopper is reduced onto it. What was the average electrical current throughthis electrolytic cell?

528 AP Chemistry

Important ConceptsArrhenius theoryBrensted- Lowry theoryLewis theoryNeutralizationTitrationBuffersStrong vs weak acids and basesPredicting relative strengths of acidsHydrolysis

Important Equations

pH = -log [H+] and pOH = -log [OH-]

H K I ([conjugate base]]

p = p a + og . .[conJugate acid]

[H+][A-] [HB+][OH-]K= K==-----=--=------=a [HA] b [B]

Kw = [H+ ][ 0 H- ]

2. Which ion is the strongest base?

.Practice Exercises

For the first four problems below, one or more of the following responses will apply;each response may be used more th~n once or not at all in these questions.

Multiple-Choice

I. Cu(NH3)~+II. KOH

III. HCO~IV: CO~-v: SO~-

1. Which of the above is the productof a Lewis acid reacting with aLewis base?

(A) I and II(B) II(C) I and III(D) IV(E) IV and V

(A) I(B) II(C) III(D) IV(E) V

3. Which of these imparts a color to asolution?

(A) I(B) II(C) III(D) IV(E) V

4. When an equal number of molesof each is mixed, which of thesecan be used to prepare a buffersolution?

(A) Iand II(B) IIand III(C) III and IV(D) IV and V(E) V and I

5. Which of the following has thehighest pH?

(A) 0.100 MHCI(B) 0.200 MHCzH30Z

(C) 0.100 MNazC03(D) 0.200 MNaCl(E) 0.500 MNaCzH30Z

6. Which of the following CANNOToccur together in solution?

(A) H3P04 and HzPO~(B) HCO; and CO~-(C) Na+ and SO~-(D) C20~- and HZCZ04

(E) HPO~- and PO!-

7. When 0.250 mol ofNaOH is addedto 1.00 L of 0.100 MHl04, thesolution will contain

(A) HPO~-(B) H2PO~(C) PO~-(D) A and B(E) A and C

8. A buffer with a pH of 10.0 isneeded. Which of the followingshould be used?

(A) ethanoic acid with a f\, of1.8 X 10-5

(B) ammonia with a ~ of1.8 X 10-5

(C) nitrous acid with a I\. of7.1 X 10-4

(D) H2PO~ and PO!- with a I\. of4.5 X 10-13

(E) dimethylamine with a ~ of1.05 X 10-3

Acids and Bases 529

9. pH is equal to pI\.

(A) when [conjugate acid] =[conjugate base]

(B) at the end point of a titration(C) in the buffer region(D) in the Henderson-Hasse1balch

equation(E) at equilibrium

10. The pH of a 1.23 X 10-3M solutionof Ba(OH)2 aqueous solution is

(A) 2.61(B) 2.43(C) 11.39(D) 13.52(E) 11.57

11. An indicator has a I\. of 6.4 X 10-6,

the conjugate acid is red, and theconjugate base is yellow. At whatpH will the solution be red?

(A) 5.2(B) 5.5(C) 4.0(D) 4.7(E) 6.4

12. A buffer has a pH of 4.87. If thebuffer is made from a weak acid(I\. = 3.30 X 10-5), and its conjugate

b h[conjugate base]

ase, t e ratio is[weak acid]

(A) 4.87(B) 4.48(C) 1.00(D) 2.45(E) 0.41

13. Which of the following statementsis correct?

(A) HCl02 is a stronger acid thanHCI03·

(B) HI is a weaker acid than HCl.(C) CH3COOH is a stronger acid

than CH2BrCOOH.(D) HN03 is a stronger acid than

HNOz'(E) H3P04 is a stronger acid than

HCI04•

530 AP Chemistry

14. What is the pH of a 0.100 Msolution ofK2HPOi (For H3P04,

pK; = 2.15; pI<; = 7.20; p~ =12.35.)

(A) 1.00(B) 13.00(C) 9.78(D) 6.67(E) 4.10

15. Which of the following is thecorrect method for preparing abuffer solution?

(A) Mix the correct amounts of aweak acid and its conjugate base.

(B) Neutralize a weak base partiallywith strong acid.

(C) Neutralize a weak acid partiallywith a strong base.

(D) Add the appropriate amount ofstrong acid to an acid salt.

(E) All of the above methods maybe used to prepare buffers.

16. The only acid that is both a strongand a weak acid on dissociation is

(A) hydrochloric acid(B) perchloric acid(C) nitric acid(D) sulfuric acid(E) phosphoric acid

17. Which of the following is the acidanhydride of a monoprotic acid?

(A) CaO(B) 503(C) FeO(D) CO2

(E) N205

18. Which of the following CANNOTbe either a Lewis acid or a Lewis base?

(A) CH4(B) Cu2

+

(C) CO(D) Fe3

+

(E) NH3

19. In the complex ion Cu(NH3)~+ theNH3 is called

(A) a cation(B) a ligand(C) a Lewis acid(D) an anion(E) a conjugate acid

20. The pH of a 0.125 M solution of aweak base is 10.45. What is the p~of this base?

(A) 3.5 x 10-11

(B) 6.4 x 10-7

(C) 2.8 X 10-4

(D) 2.3 X 10-3

(E) 1.2 X 10-2

21. A solution containing HF is titratedwith KOH. At the end point of thetitration the solution contains

(A) equal amounts ofHF andKOH

(B) H20, H+, OH-, K+, F, andHF

(C) K+ and F(D) KF and H20(E) K+, F-, and H20

22. A buffer at pH 5.32 is preparedfrom a weak acid with a pi\, = 5.15.If 100 mL of this buffer is diluted to200 mL with distilled water, the pHof the dilute solution is

(A) 5.62(B) 5:02(C) 5.32(D) The identity of the acid

is needed to answer thequestion.

(E) The concentrations of theacid and the salt are needed toanswer the question.

I CHALLENGE I23. If 50.0 mL of a 0.0134 MHCl

solution is mixed with 24.0 mL of a0.0250 MNaOH solution, what isthe pH of the final mixture?

(A) 1.87(B) 12.40(C) 5.29(D) 3.02(E) 10.98

Acids and Bases 531

I CHALLENGE ~

24. If 50.0 g offormic acid (HCH02,K"= 1.8 X 10-4)and 30.0 g ofsodium formate (NaCH02) aredissolved to make 500 mL ofsolution, the pH of this solution is

(A) 4.76(B) 3.76(C) 3.35(D) 4.12(E) 3.02

ANSWER KEY

1. C 6. D 11. C 16. D 21. B2. D 7. E 12. D 17. E 22. C3. A 8. B 13. D 18. A 23. D4. C 9. A 14. C 19. B 24. C5. C 10. C 15. E 20. B

ee Appendix 1 for explanations of answers.

Free-ResponseAnswer the following questions using the concepts and procedures discussed in thischapter and preceding chapters.

(a) Rank the strengths of phosphoric acid, sulfuric acid, and perchloric acid.Explain your reasoning.

(b) Calculate the pH, pOH, [H+], and [OH-] of a 0.0035 molar solution ofBa(OH)2'

(c) Sketch and fully label a titration curve for 50.0 mL of 0.0100 M ammoniatitrated with 0.0200 M HBr.

(d) Describe a comprehensive method for preparing a buffer. Be sure tomention all the data you need and calculations that need to be performed.

(e) Write the chemical reaction for propanoic acid (CH3CH2COOH) reactingwith ammonia. Identify the conjugate acid-base pairs, and if the position ofthe equilibrium lies toward the product side, indicate their strengths.