Galvanic Cells (Voltaic Cells) Lesson 4 Text 10.1

Galvanic Cells (Voltaic Cells) a device that spontaneously converts chemical energy to electrical energy electrons flow from one reactant to the other through an external circuit a porous barrier (or a salt bridge) separates the two half-cells the oxidation half-reaction occurs in one half-cell Zn(s)  Zn2+ + 2e- the reduction half-reaction occurs in the other half-cell Cu2+(aq) + 2e-  Cu(s)

Galvanic Cells (Voltaic Cells)

ROLE OF SALT BRIDGE Sodium sulfate is an ideal electrolyte because it is very soluble and its ions do not react with the electrodes or the electrolyte solutions. The cotton plugs are the ends of the tube are porous enough to allow ions to follow in and out of the tube, but they prevent the solution from pouring out. Sulfate ions migrate into the zinc cell to offset the buildup of zinc ions Sodium ions migrate into the copper half-cell to offset the loss of copper ions. As a result each half-cell remains electrically neutral.

Electrochemistry Terms ANODE (-): electrons are released by oxidation rxn CATHODE (+): electrons are used in reduction rxn Memory Tips Anode = An Ox Cathode = Red Cat

Shorthand Notation SHORTHAND NOTATION: Zn | Zn2+ || Cu2+ | Cu Anode is always shown on the left (Zn) Cathode is always shown on the right (Cu) || represents the salt bridge

Example Problem 1 a) Write the oxidation half-reaction, the reduction half-reaction, and the overall cell reaction for the following galvanic cell. Identify the anode and the cathode. Note: platinum is present as an inert electrode. Cd(s) | Cd2+(aq) || Cu2+(aq) | Cu(s) | Pt(s) Oxidation reaction / Anode Cd  Cd2+ + 2 e- Reduction reaction / Cathode Cu2+ + 2 e-  Cu Overall cell reaction Cd + Cu2+  Cd2+ + Cu

Cd(s) | Cd2+(aq) || Cu2+(aq) | Cu(s) | Pt(s) Example Problem 1 b) Draw a diagram of the complete cell. Label the cathode and anode. Show the directions of electron flow through the external circuit (arrows). Show each half-reaction taking place in the appropriate location. Cd(s) | Cd2+(aq) || Cu2+(aq) | Cu(s) | Pt(s)

Example Problem 2 For the Ag+/Ag and Ni2+ /Ni voltaic cell the products are Ag(s) and Ni2+ (aq). Write the appropriate half-reactions, write the balanced overall cell reaction, and write the short-hand notation of the cell. Oxidation reaction / Anode Ni  Ni2+ + 2 e- Reduction reaction / Cathode Ag+ + 1 e-  Ag Overall cell reaction to cancel out the electrons multiply the cathode reaction by 2. 2Ag+ + 2 e-  2Ag 2Ag+ + Ni  Ni2+ + 2Ag Short-hand notation: Ni(s) | Ni2+(aq) || Ag+(aq) | Ag(s)

Homework Answer Pg. 641 #1-7 Go to http://www.energizer.com/learning/ to learn more about batteries!