Determine typical cell potentials for oxidation-reduction reactionsUse standard reduction potentials to determine the much better oxidizing or reducing agent from among several possible choices

The cell potential in Chapter 17.2 Galvanic Cells (+0.46 V) outcomes from the difference in the electrical potentials for each electrode. While it is difficult to identify the electrical potential of a single electrode, we can assign an electrode the worth of zero and also then use it as a recommendation. The electrode favored as the zero is presented in Figure 1 and also is dubbed the conventional hydrogen electrode (SHE). The SHE consists of 1 atm of hydrogen gas bubbled via a 1 M HCl solution, generally at room temperature. Platinum, which is chemically inert, is supplied as the electrode. The reduction half-reactivity preferred as the reference is


2 extH^+(aq ext,;1;M);+;2 exte^-; ightleftharpoons; extH_2(g ext,;1; extatm);;;;;;E^circ = 0; extV

E° is the conventional reduction potential. The supermanuscript “°” on the E denotes traditional problems (1 bar or 1 atm for gases, 1 M for solutes). The voltage is characterized as zero for all temperatures.

You are watching: Compute the cell potential at 25 c.

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Figure 1. Hydrogen gas at 1 atm is bubbled with 1 M HCl solution. Platinum, which is inert to the action of the 1 M HCl, is supplied as the electrode. Electrons on the surconfront of the electrode combine through H+ in solution to develop hydrogen gas.

A galvanic cell consisting of a SHE and Cu2+/Cu half-cell deserve to be provided to determine the conventional reduction potential for Cu2+ (Figure 2). In cell notation, the reactivity is


extPt(s)mid extH_2(g ext,;1; extatm)mid extH^+(aq ext,;1;M)parallel extCu^2+(aq ext,;1;M)mid extCu(s)
longrightarrow l} extAnode;(oxidation): & extH_2(g) & 2 extH^+(aq);+;2 exte^- \<0.5em> extCathode;(reduction): & extCu^2+(aq);+;2 exte^- & extCu(s) \<0.5em> hline \<-0.25em> extOverall: & extCu^2+(aq);+; extH_2(g) & 2 extH^+(aq);+; extCu(s) endarray

The traditional reduction potential deserve to be determined by subtracting the typical reduction potential for the reaction arising at the anode from the conventional reduction potential for the reaction developing at the cathode. The minus sign is vital bereason oxidation is the reverse of reduction.


+0.34; extV = E_ extCu^2+/ extCu^circ;-;E_ extH^+/ extH_2^circ = E_ extCu^2+/ extCu^circ;-;0 = E_ extCu^2+/ extCu^circ
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Figure 2. A galvanic cell have the right to be supplied to determine the traditional reduction potential of Cu2+.

Using the SHE as a referral, various other typical reduction potentials deserve to be established. Consider the cell displayed in Figure 3, where


extPt(s)mid extH_2(g ext,;1; extatm)mid extH^+(aq ext,;1;M)parallel extAg^+(aq ext,;1;M)mid extAg(s)
longrightarrow l} extanode;(oxidation): & extH_2(g) & 2 extH^+(aq);+;2 exte^- \<0.5em> extcathode;(reduction): & 2 extAg^+(aq);+;2 exte^- & 2 extAg(s) \<0.5em> hline \<-0.25em> extoverall: & 2 extAg^+(aq);+; extH_2(g) & 2 extH^+(aq);+;2 extAg(s) endarray

The traditional reduction potential can be determined by subtracting the standard reduction potential for the reaction emerging at the anode from the conventional reduction potential for the reactivity developing at the cathode. The minus sign is necessary because oxidation is the reverse of reduction.


+0.80; extV = E_ extAg^+/ extAg^circ;-;E_ extH^+/ extH_2^circ = E_ extAg^+/ extAg^circ;-;0 = E_ extAg^+/ extAg^circ

It is essential to note that the potential is not doubled for the cathode reaction.

The SHE is fairly dangerous and also rarely offered in the laboratory. Its primary definition is that it establiburned the zero for standard reduction potentials. Once figured out, conventional reduction potentials deserve to be used to determine the standard cell potential, E_ extcell^circ, for any type of cell. For example, for the cell shown in Figure 2 in Chapter 17.2 Galvanic Cells,


extCu(s)mid extCu^2+(aq ext,;1;M)parallel extAg^+(aq ext,;1;M)mid extAg(s)
longrightarrow l} extanode;(oxidation): & extCu(s) & extCu^2+(aq);+;2 exte^- \<0.5em> extcathode;(reduction): & 2 extAg^+(aq);+;2 exte^- & 2 extAg(s) \<0.5em> hline \<-0.25em> extoverall: & extCu(s);+;2 extAg^+(aq) & extCu^2+(aq);+;2 extAg(s) endarray
E_ extcell^circ = E_ extcathode^circ;-;E_ extanode^circ = E_ extAg^+/ extAg^circ;-;E_ extCu^2+/ extCu^circ = 0.80; extV;-;0.34; extV = 0.46; extV

Aacquire, note that as soon as calculating E_ extcell^circ, typical reduction potentials always remajor the same also when a half-reaction is multiplied by a factor. Standard reduction potentials for schosen reduction reactions are shown in Table 2. A more finish list is gave in Appendix L.

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Figure 3. A galvanic cell can be offered to identify the typical reduction potential of Ag+. The SHE on the left is the anode and assigned a standard reduction potential of zero.Half-ReactionE° (V)
extF_2(g);+;2 exte^-;longrightarrow;2 extF^-(aq)+2.866
extPbO_2(s);+; extSO_4^;;2-(aq);+;4 extH^+(aq);+;2 exte^-;longrightarrow; extPbSO_4(s);+;2 extH_2 extO(l)+1.69
extMnO_4^;;-(aq);+;8 extH^+(aq);+;5 exte^-;longrightarrow; extMn^2+(aq);+;4 extH_2 extO(l)+1.507
extAu^3+(aq);+;3 exte^-;longrightarrow; extAu(s)+1.498
extCl_2(g);+;2 exte^-;longrightarrow;2 extCl^-(aq)+1.35827
extO_2(g);+;4 extH^+(aq);+;4 exte^-;longrightarrow;2 extH_2 extO(l)+1.229
extPt^2+(aq);+;2 exte^-;longrightarrow; extPt(s)+1.20
extBr_2(aq);+;2 exte^-;longrightarrow;2 extBr^-(aq)+1.0873
extAg^+(aq);+; exte^-;longrightarrow; extAg(s)+0.7996
extHg_2^;;2+(aq);+;2 exte^-;longrightarrow;2 extHg(l)+0.7973
extFe^3+(aq);+; exte^-;longrightarrow; extFe^2+(aq)+0.771
extMnO_4^;;-(aq);+;2 extH_2 extO(l);+;3e^-;longrightarrow; extMnO_2(s);+;4 extOH^-(aq)+0.558
extI_2(s);+;2 exte^-;longrightarrow;2 extI^-(aq)+0.5355
extNiO_2(s);+;2 extH_2 extO(l);+;2 exte^-;longrightarrow; extNi(OH)_2(s);+;2 extOH^-(aq)+0.49
extCu^2+(aq);+;2 exte^-;longrightarrow; extCu(s)+0.337
extHg_2 extCl_2(s);+;2 exte^-;longrightarrow;2 extHg(l);+;2 extCl^-(aq)+0.26808
extAgCl(s);+;2 exte^-;longrightarrow; extAg(s);+; extCl^-(aq)+0.22233
extSn^4+(aq);+;2 exte^-;longrightarrow; extSn^2+(aq)+0.151
2 extH^+(aq);+;2 exte^-;longrightarrow; extH_2(g)0.00
extPb^2+(aq);+;2 exte^-;longrightarrow; extPb(s)−0.126
extSn^2+(aq);+;2 exte^-;longrightarrow; extSn(s)−0.1262
extNi^2+(aq);+;2 exte^-;longrightarrow; extNi(s)−0.257
extCo^2+(aq);+;2 exte^-;longrightarrow; extCo(s)−0.28
extPbSO_4(s);+;2 exte^-;longrightarrow; extPb(s);+; extSO_4;2-(aq)−0.3505
extCd^2+(aq);+;2 exte^-;longrightarrow; extCd(s)−0.4030
extFe^2+(aq);+;2 exte^-;longrightarrow; extFe(s)−0.447
extCr^3+(aq);+;3 exte^-;longrightarrow; extCr(s)−0.744
extMn^2+(aq);+;2 exte^-;longrightarrow; extMn(s)−1.185
extZn(OH)_2(s);+;2 exte^-;longrightarrow; extZn(s);+;2 extOH^-(aq)−1.245
extZn^2+(aq);+;2 exte^-;longrightarrow; extZn(s)−0.7618
extAl^3+(aq);+;3 exte^-;longrightarrow; extAl(s)−1.662
extMg^2+(aq);+;2 exte^-;longrightarrow; extMg(s)−2.372
extNa^+(aq);+; exte^-;longrightarrow; extNa(s)−2.71
extCa^2+(aq);+;2 exte^-;longrightarrow; extCa(s)−2.868
extBa^2+(aq);+;2 exte^-;longrightarrow; extBa(s)−2.912
extK^+(aq);+; exte^-;longrightarrow; extK(s)−2.931
extLi^+(aq);+;2 exte^-;longrightarrow; extLi(s)−3.04
Table 2. Schosen Standard Reduction Potentials at 25 °C

Tables like this make it feasible to identify the traditional cell potential for many oxidation-reduction reactions.


Example 1

Cell Potentials from Standard Reduction PotentialsWhat is the typical cell potential for a galvanic cell that is composed of Au3+/Au and Ni2+/Ni half-cells? Identify the oxidizing and also reducing agents.

SolutionUsing Table 2, the reactions affiliated in the galvanic cell, both created as reductions, are


extAu^3+(aq);+;3 exte^-;longrightarrow; extAu(s);;;;;;;E_ extAu^3+/ extAu^circ = +1.498; extV
extNi^2+(aq);+;2 exte^-;longrightarrow; extNi(s);;;;;;;E_ extNi^2+/ extNi^circ = -0.257; extV

Galvanic cells have actually positive cell potentials, and also all the reduction reactions are reversible. The reaction at the anode will be the half-reactivity via the smaller or even more negative traditional reduction potential. Reversing the reaction at the anode (to present the oxidation) yet not its typical reduction potential gives:


longrightarrow ll} extAnode;(oxidation): & extNi(s) & extNi^2+(aq);+;2 exte^- & E_ extanode^circ = E_ extNi^2+/ extNi^circ = -0.257; extV \<0.5em> extCathode;(reduction): & extAu^3+(aq);+;3 exte^- & extAu(s) & E_ extcathode^circ = E_ extAu^3+/ extAu^circ = +1.498; extV endarray
3 extNi(s);+;2 extAu^3+(aq);longrightarrow;3 extNi^2+(aq);+;2 extAu(s)

The reduction potentials are not scaled by the stoichiometric coefficients as soon as calculating the cell potential, and also the unmodified conventional reduction potentials should be supplied.


E_ extcell^circ = E_ extcathode^circ;-;E_ extanode^circ = 1.498; extV;-;(-0.257; extV) = 1.755; extV

From the half-reactions, Ni is oxidized, so it is the reducing agent, and Au3+ is decreased, so it is the oxidizing agent.

Check Your LearningA galvanic cell is composed of a Mg electrode in 1 M Mg(NO3)2 solution and also a Ag electrode in 1 M AgNO3 solution. Calculate the conventional cell potential at 25 °C.


Answer:

extMg(s);+;2 extAg^+(aq);longrightarrow; extMg^2+(aq);+;2 extAg(s);;;;;;;E_ extcell^circ = 0.7996; extV;-;(-2.372; extV) = 3.172; extV


Key Concepts and Summary

Assigning the potential of the conventional hydrogen electrode (SHE) as zero volts enables the determicountry of traditional reduction potentials, , for half-reactions in electrochemical cells. As the name suggests, standard reduction potentials usage conventional states (1 bar or 1 atm for gases; 1 M for solutes, often at 298.15 K) and are composed as reductions (wright here electrons show up on the left side of the equation). The reduction reactions are reversible, so standard cell potentials can be calculated by subtracting the typical reduction potential for the reaction at the anode from the conventional reduction for the reactivity at the cathode. When calculating the typical cell potential, the conventional reduction potentials are not scaled by the stoichiometric coefficients in the well balanced in its entirety equation.

Key EquationsE_ extcell^circ = E_ extcathode^circ;-;E_ extanode^circ

Chemisattempt End of Chapter Exercises

For each reactivity noted, determine its conventional cell potential at 25 °C and whether the reactivity is spontaneous at conventional problems.

(a) extMg(s);+; extNi^2+(aq);longrightarrow; extMg^2+(aq);+; extNi(s)

(b) 2 extAg^+(aq);+; extCu(s);longrightarrow; extCu^2+(aq);+;2 extAg(s)

(c) extMn(s);+; extSn(NO_3)_2(aq);longrightarrow; extMn(NO_3)_2(aq);+; extSn(s)

(d) 3 extFe(NO_3)_2(aq);+; extAu(NO_3)_3(aq);longrightarrow;3 extFe(NO_3)_3(aq);+; extAu(s)

For each reaction provided, identify its conventional cell potential at 25 °C and whether the reactivity is spontaneous at conventional problems.

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(a) extMn(s);+; extNi^2+(aq);longrightarrow; extMn^2+(aq);+; extNi(s)

(b) 3 extCu^2+(aq);+;2 extAl(s);longrightarrow;2 extAl^3+(aq);+;2 extCu(s)

(c) extNa(s);+; extLiNO_3(aq);longrightarrow; extNaNO_3(aq);+; extLi(s)

(d) extCa(NO_3)_2(aq);+; extBa(s);longrightarrow; extBa(NO_3)_2(aq);+; extCa(s)

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

extCu(s)mid extCu^2+(aq)parallel extAu^3+(aq)mid extAu(s)

Determine the overall reaction and its typical cell potential at 25 °C for the reactivity entailing the galvanic cell made from a half-cell consisting of a silver electrode in 1 M silver nitprice solution and also a half-cell consisting of a zinc electrode in 1 M zinc nitprice. Is the reactivity spontaneous at standard conditions?Determine the overall reactivity and also its conventional cell potential at 25 °C for the reactivity entailing 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 nitprice solution. Is the reaction spontaneous at conventional conditions?Determine the as a whole reaction and its typical cell potential at 25 °C for these reactions. Is the reaction spontaneous at standard conditions? Assume the traditional reduction for Br2(l) is the very same as for Br2(aq). extPt(s)mid extH_2(g)mid extH^+(aq)parallel extBr_2(aq)mid extBr^-(aq)mid extPt(s)

Glossary

traditional cell potential f (E^circ_ extcell)the cell potential as soon as all reactants and assets are in their typical states (1 bar or 1 atm or gases; 1 M for solutes), usually at 298.15 K; have the right to be calculated by subtracting the conventional reduction potential for the half-reaction at the anode from the standard reduction potential for the half-reaction arising at the cathodestandard hydrogen electrode (SHE)the electrode is composed of hydrogen gas bubbling with hydrochloric acid over an inert platinum electrode whose reduction at standard problems is assigned a worth of 0 V; the recommendation allude for standard reduction potentialsconventional reduction potential (E°)the worth of the reduction under traditional problems (1 bar or 1 atm for gases; 1 M for solutes) normally at 298.15 K; tabulated worths used to calculate conventional cell potentials

Solutions

Answers to Chemistry End of Chapter Exercises

1. (a) +2.115 V (spontaneous); (b) +0.4626 V (spontaneous); (c) +1.0589 V (spontaneous); (d) +0.727 V (spontaneous)

3. 3 extCu(s);+;2 extAu^3+(aq);longrightarrow;3 extCu^2+(aq);+;2 extAu(s); +1.16 V; spontaneous

5. 3 extCd(s);+;2 extAl^3+(aq);longrightarrow;3 extCd^2+(aq);+;2 extAl(s); −1.259 V; nonspontaneous