Values and Reaction Tendency

E 0 Values and Reaction Tendency

Recall that half-reactions can be combined to give whole reactions and that half- reactions can occur in separate half-cells of an electrochemical cell. The inherent tendency of a half-reaction to occur is expressed by a characteristic E 0 value. In favorable cases, E 0 s of half-reactions can be measured directly versus a standard hydrogen electrode by a cell such as the one shown in Figure 8.13 , or they can be calculated from thermodynamic data. Several half-reactions and their E 0 values are given below:

Cl 2 + 2e- ←→ 2Cl-

E 0 = 1.359 volt

E 0 = 1.229 volt

(8.10.2) Fe 3+ + e-

+ e- 0 ←→ Ag E = 0.7994 volt

(8.10.6) Cu 2+ + 2e- ←→ Cu

Fe ←→ 2+

E 0 = 0.771 volt

(8.10.7) 2H +

E 0 = 0.337 volt

+ 2e- 0 ←→ H

2 E = 0.00 volt

(8.10.8) Zn 2+ + 2e- ←→ Zn

Pb 0 2+ + 2e- ←→ Pb E = -0.126 volt

(8.10.9) Basically, the E 0 values of these half-reactions express the tendency for the reduction

E 0 = -0.763 volt

half-reaction to occur when all reactants and products are present at unit activity; the more positive the value of E 0 , the greater the tendency of the reduction half-reaction to proceed. (In a simplified sense, the activity of a substance is 1 when its concentra- tion in aqueous solution is 1 mole/liter, its pressure as a gas is 1 atm, or it is present

as a solid.) With these points in mind, examination of the E 0 values above show the following:

• The highest value of E 0 shown above is for the reduction of Cl 2 gas to Cl- ion. This is consistent with the strong oxidizing tendency of Cl 2 ; chlorine

much “prefers” to exist as chloride ion rather than highly reactive Cl 2 gas. • The comparatively high E 0 value of 0.7994 volt for the reduction of Ag +

ion to Ag metal indicates that silver is relatively stable as a metal, which is consistent with its uses in jewelry and other applications where resistance to oxidation is important.

• The value of exactly E 0 = 0.000 volt is assigned by convention for the half-reaction in which H + ion is reduced to H 2 gas; all other E 0 s are rela- tive to this value.

• The lowest (most negative) E 0 value shown above is -0.763 volt for the half-reaction Zn 2+ + 2e- ←→ Zn. This reflects the strong tendency for • The lowest (most negative) E 0 value shown above is -0.763 volt for the half-reaction Zn 2+ + 2e- ←→ Zn. This reflects the strong tendency for

Half-reactions and their E 0 values can be used to explain observations such as the following: A solution of Cu 2+ flows through a lead pipe and the lead acquires a layer of copper metal through the reaction

Cu 2+ + Pb → Cu + Pb 2+ (8.10.10) This reaction occurs because the copper(II) ion has a greater tendency to acquire

electrons than the lead ion has to retain them. This reaction can be obtained by subtracting the lead half-reaction, Equation 8.10.8, from the copper half-reaction, Equation 8.10.7:

Cu 2+ + 2e- ←→ Cu

E 0 = 0.337 volt

-(Pb 2+ + 2e- ←→ Pb

E 0 = -0.126 volt)

Cu 2+ + Pb ←→ Cu + Pb 2+

E 0 = 0.463 volt

(8.10.10) The appropriate mathematical manipulation of the E 0 s of the half-reactions enables

calculation of an E 0 for the overall reaction, the positive value of which indicates that Reaction 8.10.10 tends to go to the right as written. This is in fact what occurs when lead metal directly contacts a solution of copper(II) ion. Therefore, if a waste solution containing copper(II) ion, a relatively innocuous pollutant, comes into contact with lead in plumbing, toxic lead may go into solution.

Based on the above calculation, if the electrochemical cell shown in Figure 8.14 were set up with activities of both Cu 2+ and Pb 2+ at exactly 1 (approximated by con- centrations of 1 mole/liter), the potential registered by “E,” a meter that measures voltage, but does not allow any current to flow, would be 0.463 volts. The lead electrode would be negative because the reaction tendency is for Pb metal to give up negative electrons to the external circuit and go into solution as Pb 2+ ion, whereas Cu 2+ ions tend to remove electrons from the copper electrode, giving it a + charge and coming out of solution as Cu metal. The effects of different concentrations on the potential that would be measured in such a cell are discussed in the following section.

Salt bridge

Figure 8.14 Electrochemical cell in which the tendency for the reaction Cu 2+ + Pb ←→ Cu + Pb 2+ can be measured. In this configuration, “E” has a very high resistance and current cannot flow.