14.2a Voltaic Cells

1. 14.2a Voltaic Cells Basic Function

2. Voltaic Cell Basics Electrodes and electrolytes chemically react to form ions that move in or out of solution. Anode = electrode where electrons are transferred out of the solution (ox. occurs) Cathode = electrode where electrons are transferred into the solution (red. occurs) Electrolytes can be a liquid or a paste Each half-cell contains one electrode and one electrolyte

3. Voltaic Cells device that changes chemical energy into electrical energy involves a redox reaction reducing agent transfers electrons to the oxidizing agent oxidation: loss of electrons (  ox. state) reduction: gain of electrons (  ox state) <---- e- ----- cathode -- anode red/SOA ox/SRA

4. Voltaic Cells if the two half-rxtns are combined in the same container, the electron exchange occurs directly as work and released as heat (not ideal) to harness the energy, keep each half-rxtn in a separate container so the electron transfer occurs through a wire (half-cells) when the metal solid of the metal ion is not a possibility then an inert electrode is used usually carbon or platinum

5. Half-cells A salt bridge or porous disk is used to allow for unrelated ions to move between cells, allowing balance of charge porous cup is also used

6. Particle movement which direction will the electrons flow in? from reducing agent to oxidizing agent Locations of each half-rxtn anode: oxidation/SRA cathode: reduction/SOA cations ---> <--- anions

7. Cell Notation short hand for describing cells anode is on L and cathode is on R separate anode and cathode half-rxtn with || separate phases in one half-rxtn with | electrodes go on far ends of notation Mg(s) | Mg2+(aq) || Al3+(aq) | Al(s) Pt(s) | ClO3-(aq), ClO4-(aq) || MnO4-(aq), Mn2+(aq) | Pt(s)

8. |------------------------- e- flow -----------------------> anode (-) | electrolyte || electrolyte | cathode (+) cations ---> <--- anions

9. Basic Concepts of Electrochemical Cells Anode Cathode

10. CHEMICAL CHANGE --->ELECTRIC CURRENT Zn is oxidized and is the reducing agent Zn(s) ---> Zn2+(aq) + 2e- Cu2+ is reduced and is the oxidizing agentCu2+(aq) + 2e- ---> Cu(s) With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”

11. To obtain a useful current, we separate the oxidizing and reducing agents so e- transfer occurs thru an external wire. CHEMICAL CHANGE --->ELECTRIC CURRENT http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf This is accomplished in a GALVANIC or VOLTAIC cell. A group of such cells is called a battery.

12. •Electrons travel thru external wire Salt bridge allows anions and cations to move between half cells Zn --> Zn2+ + 2e- Cu2+ + 2e- --> Cu Reduction Cathode Positive Oxidation Anode Negative <--Anions Cations-->

14. Cell description Describe a voltaic Cell: balanced chemical eqtn. give the direction of electron flow assign the anode and cathode give cell notation

15. Example Describe the Galvanic cell based on the following half-reactions: Ag+ + e- Ag Fe3+ + e- Fe2+ Write balanced equation Fe half-rxtn is reversed since Fe2+ is SRA Ag+ + Fe2+ Ag + Fe3+

16. Example Give the direction of electron flow oxidation: Fe2+ Fe3+ + e- reduction: Ag+ + e- Ag electrons flow from Fe2+ half-cell to Ag+ half-cell Assign cathode and anode anode: oxidation: Fe2+ Fe3+ + e- cathode: reduction: Ag+ + e- Ag

17. Cell Notation Pt(s) | Fe2+(aq), Fe3+(aq) || Ag+(aq) | Ag(s)

18. Homework • Read p.622 - 626 in textbook • Questions p.626 #1-8 • LSM 14.2 C & D