Electrochemistry Lesson 6 Electrochemical Cells

Electrochemical cells are Batteries

Alkaline Batteries “dry cells” (Zn/MnO2) KOH

Car Batteries Pb-Acid H2SO4

Mitsubishi iMiEV - Pure Electric Car $ 36,000 US $ 50,000 Can Powered by a 330 v Li-Ion Rechargeable battery Plugs into your house and takes 14 hours to charge -100 km for $ 0.60 http://www.electriphobia.com/?gclid=CK-L-f7SpL4CFYdrfgodRlQAKg#/INTRO Top Speed 155 km/h

Cell Phone batteries Lithium Ion Rechargeable battery/secondary cells

Lithium ion or Ni-Cadmium cells

Space Ship Batteries Powered by Radioisotopes

Ni-Metal Hydride

Standard Electrochemical Cells  Definitions  Electrochemical cell – A device which converts chemical energy into electrical energy through a spontaneous redox reaction; also known as galvanic or voltaic cells. Electrode – A conductor (usually a metal) at which a half-cell reaction(oxidation or reduction) occurs. The metal electrode is immersed in a solution that contains ions of that same metal. Anode - The electrode at which oxidation occurs (A & O are both vowels) LEO-A Cathode – The electrode at which reduction occurs (R & C are both consonants) GER-C

Notes on Electrochemical Cells   An electrochemical cell – a system of electrodes, electrolytes, and salt bridge that allow oxidation and reduction reactions to occur and electrons to flow through an external circuit. 1. Spontaneous redox reaction       2. Produces electricity from chemicals 3. Is commonly called a battery

Analyzing Electrochemical Cells The reaction that is higher on the reduction chart is the reduction and the lower is oxidation and is written in reverse. In other words, it is a spontaneous redox reaction between the strongest oxidizing agent and the strongest reducing agent. Oxidation always occurs at the anode and reduction at the cathode

For any cell  Electrons flow through the wire and go from anode to cathode Anions (- ions) migrate to the anode and cations (+ions) migrate towards the cathode through the salt bridge The salt bridge allows ions to migrate from one half-cell to the other without allowing the solutions to mix.

The salt bridge- where ion migration occurs to prevent a charge build-up. How? the reduction half cell would become more negative, while the oxidation half-cell would become more positive…this leads to polarization that would stop electron flow…a salt bridge prevents that! Salt bridge keeps the neutrality of the cell by ion migration so the reaction does not reach equilibrium fast and stop!!!

1. Draw and completely analyze a Cu+2/Sn electrochemical cell. voltmeter e- e- Higher Greatest Electron Affinity Reduction Cathode Cu2+ + 2e- → Cu(s) 0.34 v Gains mass(called plating) + 0.48 v Lower Oxidation Anode Sn(s) → Sn2+ + 2e- +0.14 v Loses mass - Salt bridge Na-+ NO3- Sn Cu Cu2+ Sn2+ NO3- NO3- Overall Reaction: Cu2+ + Sn → Sn2+ + Cu(s) 0.48 v

Final notes… Be able to tell which two half-reactions would occur ( which one is the reduction one, which is the oxidation one) Where do electrons flow from-to? Through? Where do ions migrate from-to? Through? Immerse the electrodes in solutions that contain their ions. A salt bridge can be any soluble salt such as NaNO3, KNO3, etc…

Final notes… A salt bridge can be any soluble salt such as NaNO3, KNO3, etc… Spectator ions Na+, K+, Cl-, Ca+2, Mg+2, SO4-2(in neutral solutions) They have such low tendencies to either reduce or oxidize that they are spectators under most circumstances…

Homework Page 217 #34,35