electrochemical cells

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