electrochemical cells
DESCRIPTION
Description of and mathematics of Galvanic and Voltaic cells suitable for regular and AP high school chemistry.TRANSCRIPT
Electrochemical Cells
Separating Oxidation-Reduction to do work
HistoryWe attribute the discovery of electrochemical cells to Luigi
Galvani (1737-1798), a citizen of the Papal States
"Therefore having noticed that frog preparations which hung by copper hooks from the iron railings surrounding a balcony of our house contracted not only during thunder storms but also in fine weather, I decided to determine whether or not these contractions were due to the action of atmospheric electricity....Finally....I began to scrape and press the hook fastened to the back bone against the iron railing to see whether by such a procedure contractions might be excited, and whether instead of an alteration in the condition of the atmospheric electricity some other changes might be effective. I then noticed frequent contractions, none of which depended on the variations of the weather."
What He DidTraditional metals used: Zn & Cu
▪Galvani discovered that if two metals were immersed in a saline solution and connected by a wire or touched to a muscle, an electrical current made the muscle twitch
We know now that a small amount of each metal dissolved to make the corresponding Zn and Cu ions, forming an oxidation-reduction reaction.
Zn0 → Zn2+ + 2e-
Cu2+ + 2e- → Cu0
–Electrons transferred from Zn to Cu2+ through the muscle, which twitched from the stimulus
AlternativelyTypically, we see an electron-transfer reaction,
Zn0 + Cu2+ → Zn2+ + Cu0
▪The reaction is the same: Zn oxidized & Cu2+ reduced (cf. P. 677)
▸But in the case above, the Zn comes directly into contact with the copper ions in solution
▸And electrons pass directly from the metal to the ion
▸In Galvani’s reaction, he separated the reaction into two half-reactions using salt water and the animal muscle.
–So the electrons had to do some work first!
Activity SeriesMetals & Nonmetals
▪Recall from the STAAR guide▸Elements above will react with the ions of
elements below
▸Which means they will give electrons to the ions of elements below them
▸Thus Ca will react with H+
▸And Cu with Ag+
▸Can we quantify this?
Schoolisland.com
Alessandro VoltaToward a quantifying of electric potential
▪The Voltaic cell▸Alternate copper, water-soaked
cardboard, and zinc▸The more “cells” of
copper/cardboard/zinc, the higher the electric potential or driving force
▸We call this driving force “voltage”
Electrochemical Cell
Robert Asato, Ph.D, Leeward Community College
Attach voltmeter here
How It WorksOxidation: Zn → Zn2+ + 2e-
Electrons are produced at the Zn/Zn2+ interface
This electrode is called the anode because oxidation occurs
How It WorksWork
Electrons are pushed through the conductive wire and through the “work” device--a motor, a light bulb or a voltmeter
How It WorksReduction: Cu2++ 2e- → Cu
Electrons at the cathode collide with Cu2+ ions attracted to this locus of negative charge, and reduce them to metallic Cu
How It WorksIon migration--the salt bridge
Sulfate (SO42-)
migrates through the salt bridge and Zn2+ in the other direction to keep charges balanced
Experimental ResultsEach half-cell, measured against a standard half-cell, gives a
predictable voltage (standard 298K, 1 M solutions)
▪Standard: H2 gas/H+ (aq) 0.00 V
▸Fe2+/Fe -0.44 V
▸PbSO4/Pb -0.36 V
▸Pb2+/Pb -0.13 V
▸Cu2+/Cu +0.34 V
▸Ag+/Ag +0.80 V
▸Br2/Br- +1.07 V
▸PbO2/Pb2+ +1.46 V
Your TurnGiven this table, calculate the standard voltage of the
following full cells
▪Fe2+/Fe // Ag+/Ag▸1.24 V
▪Pb2+/Pb // Br2/Br-
▸1.20 V
▪Cu2+/Cu // PbO2/Pb2+
▸1.12 V
▪Thus we can predict the “force” with which electrons are driven from one half-cell to the other
PotentialCell-0.44 VFe2+/Fe -0.36 VPbSO4/Pb-0.13 VPb2+/Pb0.00 VH2 gas/H+ (aq)+0.34 VCu2+/Cu +0.80 VAg+/Ag +1.07 VBr2/Br-+1.46 VPbO2/Pb2+
How to Write ItThe shorthand diagram
▪Fe2+/Fe // Ag+/Ag PotentialCell-0.44 VFe2+/Fe -0.36 VPbSO4/Pb-0.13 VPb2+/Pb0.00 VH2 gas/H+ (aq)+0.34 VCu2+/Cu +0.80 VAg+/Ag +1.07 VBr2/Br-+1.46 VPbO2/Pb2+
Oxidation reaction first with product on left and oxidized species on right
How to Write ItThe shorthand diagram
▪Fe2+/Fe // Ag+/Ag PotentialCell-0.44 VFe2+/Fe -0.36 VPbSO4/Pb-0.13 VPb2+/Pb0.00 VH2 gas/H+ (aq)+0.34 VCu2+/Cu +0.80 VAg+/Ag +1.07 VBr2/Br-+1.46 VPbO2/Pb2+
Reduction reaction second with reduced species on the left and product on the right
BatteriesHow do “Dry Cells” Work?
Inner half-cell: e.g. Zn/Zn2+ connected to central post
Electrons exit here
BatteriesHow do “Dry Cells” Work?
Outer half-cell: e.g. Cu/Cu2+ connected to outer post
Electrons enter here
Electrolyte inside case is a solid-like gel
Cutaway of Dry Cell Battery