23.1. Electrochemical Cells http://www.ck12.org
of all sorts, including those used to power a flashlight, a calculator, or an automobile, rely on chemical reactions
to generate electricity. Electricity can also be used to plate objects with decorative metals like gold or chromium.
Electrochemistry is also relevant to the transmission of nerve impulses in biological systems. Redox chemistry, the
transfer of electrons, is the underlying force behind all electrochemical processes.
Direct Redox Processes
When a strip of zinc metal is placed into a blue solution of copper(II) sulfate, a reaction immediately begins as the
zinc strip begins to darken. If left in the solution for a longer period of time, the zinc will gradually decay as it
is oxidized to zinc ions, which enter the solution. Meanwhile, the copper(II) ions from the solution are reduced to
copper metal, which eventually causes the blue copper(II) sulfate solution to become colorless.
The process that occurs in this redox reaction is shown below as two separate half-reactions, which can then be
combined into the full redox reaction.
Oxidation: Zn(s)→Zn^2 +(aq)+2e−
Reduction: Cu^2 +(aq)+2e−→Cu(s)
Full Reaction: Zn(s)+Cu^2 +(aq)→Zn^2 +(aq)+Cu(s)As you know, the oxidation and reduction processes occur simultaneously. Breaking the process apart into separate
oxidation and reduction half-reactions is helpful for analyzing the overall reaction.
Why does this reaction occur spontaneously? In the chapterChemical Reactions, you learned about the activity
series, which is a list of elements in descending order of reactivity. An element that is higher in the activity series
is capable of displacing an element that is lower on the series in a single-replacement reaction. Now that you have
learned about oxidation and reduction, we can look at the activity series in another way. It is a listing of elements in
order of ease of oxidation. The elements at the top are the easiest to oxidize, while those at the bottom are the most
difficult to oxidize. The table below (Table23.1) shows the activity series together with each element’s oxidation
half-reaction.
TABLE23.1: Activity Series of Metals (in Order of Reactivity)
Element Oxidation Half Reaction
Most active or most easily oxidized Lithium Li(s)→Li+(aq) + e−
Potassium K(s)→K+(aq) + e−
Barium Ba(s)→Ba^2 +(aq) + 2e−
Calcium Ca(s)→Ca^2 +(aq) + 2e−
Sodium Na(s)→Na+(aq) + e−
Magnesium Mg(s)→Mg^2 +(aq) + 2e−
Aluminum Al(s)→Al^3 +(aq) + 3e−
Zinc Zn(s)→Zn^2 +(aq) + 2e−
Iron Fe(s)→Fe^2 +(aq) + 2e−
Nickel Ni(s)→Ni^2 +(aq) + 2e−
Tin Sn(s)→Sn^2 +(aq) + 2e−
Lead Pb(s)→Pb^2 +(aq) + 2e−
Hydrogen H 2 (g)→2H+(aq) + 2e−
Copper Cu(s)→Cu^2 +(aq) + 2e−
Mercury Hg(l)→Hg^2 +(aq) + 2e−
Silver Ag(s)→Ag+(aq) + e−
Platinum Pt(s)→Pt^2 +(aq) + 2e−