14.2a Voltaic Cells Basic Function

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

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

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)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)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 usedwhen the metal solid of the metal ion is not a possibility then an inert electrode is used usually carbon or platinumusually carbon or platinum

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

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

Cell Notation short hand for describing cellsshort hand for describing cells anode is on L and cathode is on Ranode is on L and cathode is on R separate anode and cathode half-rxtn with ||separate anode and cathode half-rxtn with || separate phases in one half-rxtn with |separate phases in one half-rxtn with | electrodes go on far ends of notationelectrodes go on far ends of notation Mg(s) | Mg 2+ (aq) || Al 3+ (aq) | Al(s)Mg(s) | Mg 2+ (aq) || Al 3+ (aq) | Al(s) Pt(s) | ClO 3 - (aq), ClO 4 - (aq) || MnO 4 - (aq), Mn 2+ (aq) | Pt(s)Pt(s) | ClO 3 - (aq), ClO 4 - (aq) || MnO 4 - (aq), Mn 2+ (aq) | Pt(s)

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

AnodeCathode Basic Concepts of Electrochemical Cells

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

To obtain a useful current, we separate the oxidizing and reducing agents so e - transfer occurs thru an external wire.To obtain a useful current, we separate the oxidizing and reducing agents so e - transfer occurs thru an external wire. CHEMICAL CHANGE ---> ELECTRIC CURRENT This is accomplished in a GALVANIC or VOLTAIC cell. A group of such cells is called a battery.

Electrons travel thru external wire Salt bridge allows anions and cations to move between half cellsSalt bridge allows anions and cations to move between half cells Electrons travel thru external wire Salt bridge allows anions and cations to move between half cellsSalt bridge allows anions and cations to move between half cells Zn --> Zn e - Cu e - --> Cu <--AnionsCations--> OxidationAnodeNegativeOxidationAnodeNegative Reduction Cathode Positive Reduction Cathode Positive

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

Example Describe the Galvanic cell based on the following half-reactions:Describe the Galvanic cell based on the following half-reactions:  Ag + + e -  Ag  Fe 3+ + e -  Fe 2+ Write balanced equationWrite balanced equation  Fe half-rxtn is reversed since Fe 2+ is SRA  Ag + + Fe 2+  Ag + Fe 3+

Example Give the direction of electron flowGive the direction of electron flow  oxidation: Fe 2+  Fe 3+ + e -  reduction: Ag + + e -  Ag  electrons flow from Fe 2+ half-cell to Ag + half- cell Assign cathode and anodeAssign cathode and anode  anode: oxidation: Fe 2+  Fe 3+ + e -  cathode: reduction: Ag + + e -  Ag

Cell NotationCell Notation Pt(s) | Fe 2+ (aq), Fe 3+ (aq) || Ag + (aq) | Ag(s)

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