23.2. Cell Potentials http://www.ck12.org
Lithium metal (Li) is the strongest reducing agent. It is capable of reducing any of the reactants above it on the table.
For example, lithium will reduce water according to the following reaction:
2Li(s)+2H 2 O(l)→2Li+(aq)+2OH−(aq)+H 2 (g)Using the table above (Table23.2) will allow you to predict whether reactions will occur or not. For example, nickel
metal is capable of reducing copper(II) ions, but it is not capable of reducing zinc ions. Nickel (Ni) is below Cu^2 +
but above Zn^2 +on the table. This means Ni^2 +can outcompete Zn^2 +for electrons, but not Cu^2 +. As a result, Cu^2 +
can pull electrons away from neutral Ni, but Zn^2 +cannot. In order for two species to react spontaneously, they must
be in an upper-left to lower-right diagonal orientation on a table of standard reduction potentials, as shown below.
Lesson Summary
- The ability of a particular electrochemical cell to generate an electric current is called its electrical potential.
Reduction potentials measure the tendency of a substance to be reduced in a redox reaction. - The standard hydrogen electrode is arbitrarily assigned a standard reduction potential of 0.00 V, and it serves
as a reference by which all other half-cell potentials are measured. - The standard cell potential for any electrochemical cell can be determined by finding the difference in reduc-
tion potentials between the two half-cells. The cell potential must be positive for the overall reaction to be
spontaneous. - Reduction potentials can be used to make predictions about whether reactions will occur and whether a
particular oxidizing or reducing agent is strong enough for a given purpose.
Lesson Review Questions
Reviewing Concepts
- How can the standard reduction potential of a half-cell be determined?
- The reduction potential of A+
- Use the table above (Table23.2) to rank the following reducing agents from strongest to weakest: Pb, Cl−,
Ca, Fe^2 +, Au, and Cs. - Rank the following oxidizing agents from strongest to weakest: NO 3 −, Al^3 +, Na+, Br 2 , MnO 4 −, and H+.
Problems
- Determine whether the following redox reactions will occur spontaneously or not. Calculate the standard cell
potential in each case.
a. Co(s) + Ni^2 +(aq)→Co^2 +(aq) + Ni(s)
b. 3Sn^2 +(aq) + 2Cr^3 +(aq)→3Sn^4 +(aq) + 2Cr(s)