3. Chapter Menu Electrochemistry Section 20.1Section 20.1Voltaic Cells Section 20.2Section 20.2 Batteries Section 20.3Section 20.3 Electrolysis Exit Click a hyperlink or folder tab to view the corresponding slides.

4. Section 20-1 Section 20.1 Voltaic Cells Describe a way to obtain electrical energy from a redox reaction. oxidation: the loss of electrons from the atoms of a substance; increases an atom’s oxidation number Identify the parts of a voltaic cell, and explain how each part operates. Calculate cell potentials, and determine the spontaneity of redox reactions. reduction: the gain of electrons by the atoms of a substance; decreases the atom’s oxidation number

5. Section 20-1 Section 20.1 Voltaic Cells (cont.) salt bridge electrochemical cell voltaic cell half-cell In voltaic cells, oxidation takes place at the anode, yielding electrons that flow to the cathode, where reduction occurs. anode cathode reduction potential standard hydrogen electrode

6. Section 20-1 Redox in Electrochemistry Electrochemistry is the study of the redox processes by which chemical energy is converted to electrical energy and vice versa. Redox reactions involve a transfer of electrons from the species that is oxidized to the species that is reduced.

7. Section 20-1 Redox in Electrochemistry (cont.) An electrochemical cell is an apparatus that uses a redox reaction to produce electrical energy or uses electrical energy to cause a chemical reaction.electrochemical cell

8. Section 20-1 Redox in Electrochemistry (cont.) A voltaic cell is a type of electrochemical cell that converts chemical energy to electrical energy by a spontaneous redox reaction.voltaic cell

9. Section 20-1 Chemistry of Voltaic Cells An electrochemical cell has 2 half-cells, where the separate oxidation and reduction reactions take place.half-cells The electrode where oxidation takes place is called the anode.anode The cathode is the electrode where reduction occurs.cathode A salt bridge is a pathway to allow movement of ions from one side to another, so that ions do not build up around the electrodes.salt bridge

10. The below picture is a voltaic cell using Zinc and Copper Anode: Oxidation Zn  Zn +2 + 2e - Cathode: Reduction 2e - + Cu +2  Cu

11. Section 20-1 Chemistry of Voltaic Cells (cont.) Electric potential energy is a measure of the amount of current that can be generated from a voltaic cell. In order for a flow of electric charge there must be a difference in the electric potentials of 2 points. A volt is a unit used to measure cell potential

12. Section 20-1 Calculating Electrochemical Cell Potentials The tendency of a substance to gain electrons is its reduction potential.reduction potential When two half-reactions are coupled, the voltage generated corresponds to the difference in potential between the half- reactions.

13. Section 20-1 Calculating Electrochemical Cell Potentials (cont.) The standard hydrogen electrode consists of a small sheet of platinum immersed in a 1 M HCl solution. Hydrogen gas (H 2 ), at a pressure of 1 atm, is bubbled in. This is donestandard hydrogen electrode at 25°C.

14. Section 20-1 Calculating Electrochemical Cell Potentials (cont.) The standard hydrogen electrode is the standard against which all other reduction potentials are measured.

15. Section 20-1 Calculating Electrochemical Cell Potentials (cont.) Formula for cell potential E 0 cell = E 0 reduction – E 0 oxidation

16. Section 20-1 Use Standard Reduction Potentials Cell potentials can be used to determine if a proposed reaction under standard conditions will be spontaneous. If the calculated potential is positive, the reaction is spontaneous. If the calculated potential is negative, the reaction is not spontaneous. ****E 0 cell = spontaneous -E 0 cell = not spontaneous****

17. Section 20-1 Calculating Electrochemical Cell Potentials (cont.) For a voltaic cell the half-reaction with the larger reduction potential is being reduced (cathode) and the smaller reduction potential will be oxidized (anode).

18. Line notation is a shorthand for showing what is happening in the voltaic cell. In line notation, the anode is written before the cathode Line Notation: Zn │ Zn +2 ║ Cu +2 │ Cu Anode (oxidation) Zn  Zn +2 + 2e - Cathode (reduction) Cu +2 + 2e -  Cu

19. A.A B.B C.C D.D Section 20-1 Section 20.1 Assessment In electrochemistry, the site where oxidation occurs is called ____. A.electrode B.anode C.cathode D.ion

20. A.A B.B C.C D.D Section 20-1 Section 20.1 Assessment The standard potential of a voltaic cell is the difference between the: A.electrode voltage B.standard reduction potential of the cell and hydrogen C.standard reduction potentials of the half-cell reactions D.half-cell reactions and the salt bridge

21. End of Section 20-2

22. Section 20-3 Section 20.3 Electrolysis Describe how it is possible to reverse a spontaneous redox reaction in an electrochemical cell. redox reaction: an oxidation-reduction reaction electrolysis electrolytic cell Compare the reactions involved in the electrolysis of molten sodium chloride with those in the electrolysis of brine. Discuss the importance of electrolysis in the smelting and purification of metals. In electrolysis, a power source causes nonspontaneous reactions to occur in electrochemical cells.

23. Section 20-3 Reversing Redox Reactions The use of electrical energy to bring about a chemical reaction is called electrolysis.electrolysis An electrochemical cell in which electrolysis occurs is called an electrolytic cell.electrolytic cell

24. Section 20-3 Applications of Electrolysis Electrolysis of water is one method of obtaining hydrogen gas for commercial use.

25. Section 20-3 Applications of Electrolysis (cont.) In the decomposition of brine, an aqueous solution of NaCl, electrolysis is used to produce hydrogen gas, chlorine gas, and sodium hydroxide.

26. Section 20-3 Applications of Electrolysis (cont.) Objects can be electroplated with a metal such as silver. The cathode is the object to be electroplated, where reduction occurs. The anode is a bar of silver, where silver is oxidized and silver ions transferred to the cathode.