Chemistry

# Exercises

ChemistryExercises

### 17.1Balancing Oxidation-Reduction Reactions

1.

If a 2.5 A current is run through a circuit for 35 minutes, how many coulombs of charge moved through the circuit?

2.

For the scenario in the previous question, how many electrons moved through the circuit?

3.

For each of the following balanced half-reactions, determine whether an oxidation or reduction is occurring.

(a) $Fe3++3e−⟶FeFe3++3e−⟶Fe$

(b) $Cr⟶Cr3++3e−Cr⟶Cr3++3e−$

(c) $MnO42−⟶MnO4−+e−MnO42−⟶MnO4−+e−$

(d) $Li++e−⟶LiLi++e−⟶Li$

4.

For each of the following unbalanced half-reactions, determine whether an oxidation or reduction is occurring.

(a) $Cl−⟶Cl2Cl−⟶Cl2$

(b) $Mn2+⟶MnO2Mn2+⟶MnO2$

(c) $H2⟶H+H2⟶H+$

(d) $NO3−⟶NONO3−⟶NO$

5.

Given the following pairs of balanced half-reactions, determine the balanced reaction for each pair of half-reactions in an acidic solution.

(a) $Ca⟶Ca2++2e−,Ca⟶Ca2++2e−,$ $F2+2e−⟶2F−F2+2e−⟶2F−$

(b) $Li⟶Li++e−,Li⟶Li++e−,$ $Cl2+2e−⟶2Cl−Cl2+2e−⟶2Cl−$

(c) $Fe⟶Fe3++3e−,Fe⟶Fe3++3e−,$ $Br2+2e−⟶2Br−Br2+2e−⟶2Br−$

(d) $Ag⟶Ag++e−,Ag⟶Ag++e−,$ $MnO4−+4H++3e−⟶MnO2+2H2OMnO4−+4H++3e−⟶MnO2+2H2O$

6.

Balance the following in acidic solution:

(a) $H2O2+Sn2+⟶H2O+Sn4+H2O2+Sn2+⟶H2O+Sn4+$

(b) $PbO2+Hg⟶Hg22++Pb2+PbO2+Hg⟶Hg22++Pb2+$

(c) $Al+Cr2O72−⟶Al3++Cr3+Al+Cr2O72−⟶Al3++Cr3+$

7.

Identify the species that undergoes oxidation, the species that undergoes reduction, the oxidizing agent, and the reducing agent in each of the reactions of the previous problem.

8.

Balance the following in basic solution:

(a) $SO32−(aq)+Cu(OH)2(s)⟶SO42−(aq)+Cu(OH)(s)SO32−(aq)+Cu(OH)2(s)⟶SO42−(aq)+Cu(OH)(s)$

(b) $O2(g)+Mn(OH)2(s)⟶MnO2(s)O2(g)+Mn(OH)2(s)⟶MnO2(s)$

(c) $NO3−(aq)+H2(g)⟶NO(g)NO3−(aq)+H2(g)⟶NO(g)$

(d) $Al(s)+CrO42−(aq)⟶Al(OH)3(s)+Cr(OH)4−(aq)Al(s)+CrO42−(aq)⟶Al(OH)3(s)+Cr(OH)4−(aq)$

9.

Identify the species that was oxidized, the species that was reduced, the oxidizing agent, and the reducing agent in each of the reactions of the previous problem.

10.

Why is it not possible for hydroxide ion (OH) to appear in either of the half-reactions or the overall equation when balancing oxidation-reduction reactions in acidic solution?

11.

Why is it not possible for hydrogen ion (H+) to appear in either of the half-reactions or the overall equation when balancing oxidation-reduction reactions in basic solution?

12.

Why must the charge balance in oxidation-reduction reactions?

### 17.2Galvanic Cells

13.

Write the following balanced reactions using cell notation. Use platinum as an inert electrode, if needed.

(a) $Mg(s)+Ni2+(aq)⟶Mg2+(aq)+Ni(s)Mg(s)+Ni2+(aq)⟶Mg2+(aq)+Ni(s)$

(b) $2Ag+(aq)+Cu(s)⟶Cu2+(aq)+2Ag(s)2Ag+(aq)+Cu(s)⟶Cu2+(aq)+2Ag(s)$

(c) $Mn(s)+Sn(NO3)2(aq)⟶Mn(NO3)2(aq)+Au(s)Mn(s)+Sn(NO3)2(aq)⟶Mn(NO3)2(aq)+Au(s)$

(d) $3CuNO3(aq)+Au(NO3)3(aq)⟶3Cu(NO3)2(aq)+Au(s)3CuNO3(aq)+Au(NO3)3(aq)⟶3Cu(NO3)2(aq)+Au(s)$

14.

Given the following cell notations, determine the species oxidized, species reduced, and the oxidizing agent and reducing agent, without writing the balanced reactions.

(a) $Mg(s)│Mg2+(aq)║Cu2+(aq)│Cu(s)Mg(s)│Mg2+(aq)║Cu2+(aq)│Cu(s)$

(b) $Ni(s)│Ni2+(aq)║Ag+(aq)│Ag(s)Ni(s)│Ni2+(aq)║Ag+(aq)│Ag(s)$

15.

For the cell notations in the previous problem, write the corresponding balanced reactions.

16.

Balance the following reactions and write the reactions using cell notation. Ignore any inert electrodes, as they are never part of the half-reactions.

(a) $Al(s)+Zr4+(aq)⟶Al3+(aq)+Zr(s)Al(s)+Zr4+(aq)⟶Al3+(aq)+Zr(s)$

(b) $Ag+(aq)+NO(g)⟶Ag(s)+NO3−(aq)(acidic solution)Ag+(aq)+NO(g)⟶Ag(s)+NO3−(aq)(acidic solution)$

(c) $SiO32−(aq)+Mg(s)⟶Si(s)+Mg(OH)2(s)(basic solution)SiO32−(aq)+Mg(s)⟶Si(s)+Mg(OH)2(s)(basic solution)$

(d) $ClO3−(aq)+MnO2(s)⟶Cl−(aq)+MnO4−(aq)(basic solution)ClO3−(aq)+MnO2(s)⟶Cl−(aq)+MnO4−(aq)(basic solution)$

17.

Identify the species oxidized, species reduced, and the oxidizing agent and reducing agent for all the reactions in the previous problem.

18.

From the information provided, use cell notation to describe the following systems:

(a) In one half-cell, a solution of Pt(NO3)2 forms Pt metal, while in the other half-cell, Cu metal goes into a Cu(NO3)2 solution with all solute concentrations 1 M.

(b) The cathode consists of a gold electrode in a 0.55 M Au(NO3)3 solution and the anode is a magnesium electrode in 0.75 M Mg(NO3)2 solution.

(c) One half-cell consists of a silver electrode in a 1 M AgNO3 solution, and in the other half-cell, a copper electrode in 1 M Cu(NO3)2 is oxidized.

19.

Why is a salt bridge necessary in galvanic cells like the one in Figure 17.4?

20.

An active (metal) electrode was found to gain mass as the oxidation-reduction reaction was allowed to proceed. Was the electrode part of the anode or cathode? Explain.

21.

An active (metal) electrode was found to lose mass as the oxidation-reduction reaction was allowed to proceed. Was the electrode part of the anode or cathode? Explain.

22.

The mass of three different metal electrodes, each from a different galvanic cell, were determined before and after the current generated by the oxidation-reduction reaction in each cell was allowed to flow for a few minutes. The first metal electrode, given the label A, was found to have increased in mass; the second metal electrode, given the label B, did not change in mass; and the third metal electrode, given the label C, was found to have lost mass. Make an educated guess as to which electrodes were active and which were inert electrodes, and which were anode(s) and which were the cathode(s).

### 17.3Standard Reduction Potentials

23.

For each reaction listed, determine its standard cell potential at 25 °C and whether the reaction is spontaneous at standard conditions.

(a) $Mg(s)+Ni2+(aq)⟶Mg2+(aq)+Ni(s)Mg(s)+Ni2+(aq)⟶Mg2+(aq)+Ni(s)$

(b) $2Ag+(aq)+Cu(s)⟶Cu2+(aq)+2Ag(s)2Ag+(aq)+Cu(s)⟶Cu2+(aq)+2Ag(s)$

(c) $Mn(s)+Sn(NO3)2(aq)⟶Mn(NO3)2(aq)+Sn(s)Mn(s)+Sn(NO3)2(aq)⟶Mn(NO3)2(aq)+Sn(s)$

(d) $3Fe(NO3)2(aq)+Au(NO3)3(aq)⟶3Fe(NO3)3(aq)+Au(s)3Fe(NO3)2(aq)+Au(NO3)3(aq)⟶3Fe(NO3)3(aq)+Au(s)$

24.

For each reaction listed, determine its standard cell potential at 25 °C and whether the reaction is spontaneous at standard conditions.

(a) $Mn(s)+Ni2+(aq)⟶Mn2+(aq)+Ni(s)Mn(s)+Ni2+(aq)⟶Mn2+(aq)+Ni(s)$

(b) $3Cu2+(aq)+2Al(s)⟶2Al3+(aq)+3Cu(s)3Cu2+(aq)+2Al(s)⟶2Al3+(aq)+3Cu(s)$

(c) $Na(s)+LiNO3(aq)⟶NaNO3(aq)+Li(s)Na(s)+LiNO3(aq)⟶NaNO3(aq)+Li(s)$

(d) $Ca(NO3)2(aq)+Ba(s)⟶Ba(NO3)2(aq)+Ca(s)Ca(NO3)2(aq)+Ba(s)⟶Ba(NO3)2(aq)+Ca(s)$

25.

Determine the overall reaction and its standard cell potential at 25 °C for this reaction. Is the reaction spontaneous at standard conditions?

$Cu ( s ) │ Cu 2+ ( a q ) ║ Au 3+ ( a q ) │ Au ( s ) Cu ( s ) │ Cu 2+ ( a q ) ║ Au 3+ ( a q ) │ Au ( s )$

26.

Determine the overall reaction and its standard cell potential at 25 °C for the reaction involving the galvanic cell made from a half-cell consisting of a silver electrode in 1 M silver nitrate solution and a half-cell consisting of a zinc electrode in 1 M zinc nitrate. Is the reaction spontaneous at standard conditions?

27.

Determine the overall reaction and its standard cell potential at 25 °C for the reaction involving the galvanic cell in which cadmium metal is oxidized to 1 M cadmium(II) ion and a half-cell consisting of an aluminum electrode in 1 M aluminum nitrate solution. Is the reaction spontaneous at standard conditions?

28.

Determine the overall reaction and its standard cell potential at 25 °C for these reactions. Is the reaction spontaneous at standard conditions? Assume the standard reduction for Br2(l) is the same as for Br2(aq).
$Pt(s)│H2(g)│H+(aq)║Br2(aq),Br−(aq)│Pt(s)Pt(s)│H2(g)│H+(aq)║Br2(aq),Br−(aq)│Pt(s)$

### 17.4The Nernst Equation

29.

For the standard cell potentials given here, determine the ΔG° for the cell in kJ.

(a) 0.000 V, n = 2

(b) +0.434 V, n = 2

(c) −2.439 V, n = 1

30.

For the ΔG° values given here, determine the standard cell potential for the cell.

(a) 12 kJ/mol, n = 3

(b) −45 kJ/mol, n = 1

31.

Determine the standard cell potential and the cell potential under the stated conditions for the electrochemical reactions described here. State whether each is spontaneous or nonspontaneous under each set of conditions at 298.15 K.

(a) $Hg(l)+S2−(aq, 0.10M)+2Ag+(aq, 0.25M)⟶2Ag(s)+HgS(s)Hg(l)+S2−(aq, 0.10M)+2Ag+(aq, 0.25M)⟶2Ag(s)+HgS(s)$

(b) The galvanic cell made from a half-cell consisting of an aluminum electrode in 0.015 M aluminum nitrate solution and a half-cell consisting of a nickel electrode in 0.25 M nickel(II) nitrate solution.

(c) The cell made of a half-cell in which 1.0 M aqueous bromide is oxidized to 0.11 M bromine ion and a half-cell in which aluminum ion at 0.023 M is reduced to aluminum metal. Assume the standard reduction potential for Br2(l) is the same as that of Br2(aq).

32.

Determine ΔG and ΔG° for each of the reactions in the previous problem.

33.

Use the data in Appendix L to determine the equilibrium constant for the following reactions. Assume 298.15 K if no temperature is given.

(a) $AgCl(s)⇌Ag+(aq)+Cl−(aq)AgCl(s)⇌Ag+(aq)+Cl−(aq)$

(b) $CdS(s)⇌Cd2+(aq)+S2−(aq)at 377 KCdS(s)⇌Cd2+(aq)+S2−(aq)at 377 K$

(c) $Hg2+(aq)+4Br−(aq)⇌[HgBr4]2−(aq)Hg2+(aq)+4Br−(aq)⇌[HgBr4]2−(aq)$

(d) $H2O(l)⇌H+(aq)+OH−(aq)at 25°CH2O(l)⇌H+(aq)+OH−(aq)at 25°C$

### 17.5Batteries and Fuel Cells

34.

What are the desirable qualities of an electric battery?

35.

List some things that are typically considered when selecting a battery for a new application.

36.

Consider a battery made from one half-cell that consists of a copper electrode in 1 M CuSO4 solution and another half-cell that consists of a lead electrode in 1 M Pb(NO3)2 solution.

(a) What are the reactions at the anode, cathode, and the overall reaction?

(b) What is the standard cell potential for the battery?

(c) Most devices designed to use dry-cell batteries can operate between 1.0 and 1.5 V. Could this cell be used to make a battery that could replace a dry-cell battery? Why or why not.

(d) Suppose sulfuric acid is added to the half-cell with the lead electrode and some PbSO4(s) forms. Would the cell potential increase, decrease, or remain the same?

37.

Consider a battery with the overall reaction: $Cu(s)+2Ag+(aq)⟶2Ag(s)+Cu2+(aq).Cu(s)+2Ag+(aq)⟶2Ag(s)+Cu2+(aq).$

(a) What is the reaction at the anode and cathode?

(b) A battery is “dead” when it has no cell potential. What is the value of Q when this battery is dead?

(c) If a particular dead battery was found to have [Cu2+] = 0.11 M, what was the concentration of silver ion?

38.

An inventor proposes using a SHE (standard hydrogen electrode) in a new battery for smartphones that also removes toxic carbon monoxide from the air:
$Anode: CO(g)+H2O(l)⟶CO2(g)+2H+(aq)+2e−Eanode°=−0.53 VCathode:2H+(aq)+2e−⟶H2(g)Ecathode°=0 V¯Overall: CO(g)+H2O(l)⟶CO2(g)+H2(g)Ecell°=+0.53 VAnode: CO(g)+H2O(l)⟶CO2(g)+2H+(aq)+2e−Eanode°=−0.53 VCathode:2H+(aq)+2e−⟶H2(g)Ecathode°=0 V¯Overall: CO(g)+H2O(l)⟶CO2(g)+H2(g)Ecell°=+0.53 V$

Would this make a good battery for smartphones? Why or why not?

39.

Why do batteries go dead, but fuel cells do not?

40.

Explain what happens to battery voltage as a battery is used, in terms of the Nernst equation.

41.

Using the information thus far in this chapter, explain why battery-powered electronics perform poorly in low temperatures.

### 17.6Corrosion

42.

Which member of each pair of metals is more likely to corrode (oxidize)?

(a) Mg or Ca

(b) Au or Hg

(c) Fe or Zn

(d) Ag or Pt

43.

Consider the following metals: Ag, Au, Mg, Ni, and Zn. Which of these metals could be used as a sacrificial anode in the cathodic protection of an underground steel storage tank? Steel is mostly iron, so use −0.447 V as the standard reduction potential for steel.

44.

Aluminum $(EAl3+/Al°=−2.07 V)(EAl3+/Al°=−2.07 V)$ is more easily oxidized than iron $(EFe3+/Fe°=−0.477 V),(EFe3+/Fe°=−0.477 V),$ and yet when both are exposed to the environment, untreated aluminum has very good corrosion resistance while the corrosion resistance of untreated iron is poor. Explain this observation.

45.

If a sample of iron and a sample of zinc come into contact, the zinc corrodes but the iron does not. If a sample of iron comes into contact with a sample of copper, the iron corrodes but the copper does not. Explain this phenomenon.

46.

Suppose you have three different metals, A, B, and C. When metals A and B come into contact, B corrodes and A does not corrode. When metals A and C come into contact, A corrodes and C does not corrode. Based on this information, which metal corrodes and which metal does not corrode when B and C come into contact?

47.

Why would a sacrificial anode made of lithium metal be a bad choice despite its $ELi+/Li°=−3.04 V,ELi+/Li°=−3.04 V,$ which appears to be able to protect all the other metals listed in the standard reduction potential table?

### 17.7Electrolysis

48.

Identify the reaction at the anode, reaction at the cathode, the overall reaction, and the approximate potential required for the electrolysis of the following molten salts. Assume standard states and that the standard reduction potentials in Appendix L are the same as those at each of the melting points. Assume the efficiency is 100%.

(a) CaCl2

(b) LiH

(c) AlCl3

(d) CrBr3

49.

What mass of each product is produced in each of the electrolytic cells of the previous problem if a total charge of 3.33 $××$ 105 C passes through each cell? Assume the voltage is sufficient to perform the reduction.

50.

How long would it take to reduce 1 mole of each of the following ions using the current indicated? Assume the voltage is sufficient to perform the reduction.

(a) Al3+, 1.234 A

(b) Ca2+, 22.2 A

(c) Cr5+, 37.45 A

(d) Au3+, 3.57 A

51.

A current of 2.345 A passes through the cell shown in Figure 17.20 for 45 minutes. What is the volume of the hydrogen collected at room temperature if the pressure is exactly 1 atm? Assume the voltage is sufficient to perform the reduction. (Hint: Is hydrogen the only gas present above the water?)

52.

An irregularly shaped metal part made from a particular alloy was galvanized with zinc using a Zn(NO3)2 solution. When a current of 2.599 A was used, it took exactly 1 hour to deposit a 0.01123-mm layer of zinc on the part. What was the total surface area of the part? The density of zinc is 7.140 g/cm3. Assume the efficiency is 100%.

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