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Ch. 19 Oxidation and Reduction

Ch. 19 Oxidation and Reduction. REDOX. 3 ways of looking at oxidation and reduction. 1. oxidation is a gain of oxygen atoms, reduction is a loss of oxygen atoms 2. oxidation is a loss of hydrogen atoms, reduction is a gain of hydrogen atoms

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Ch. 19 Oxidation and Reduction

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  1. Ch. 19Oxidation and Reduction REDOX

  2. 3 ways of looking at oxidation and reduction • 1. oxidation is a gain of oxygen atoms, reduction is a loss of oxygen atoms • 2. oxidation is a loss of hydrogen atoms, reduction is a gain of hydrogen atoms • 3. oxidation is a loss of electrons, reduction is a gain of electrons • Most fundamental explanation, what we will be dealing with the most

  3. Oxygen Hydrogen e-

  4. Leo the Lion! • LEO the lion says GER • Loss of Electrons is Oxidation • Gain of Electrons is Reduction • OIL RIG • Oxidation Is the Lost of electrons • Reduction Is the Gain of electrons

  5. Examples • Is the reactant oxidized or reduced? • Pb  PbO3 • SnO2  SnO • KClO3 KCl • C2H6O  C2H4O • C2H2  C2H6

  6. Pertaining to LEO… • Mg + S  MgS • Mg + S  Mg2+ + S2- • Magnesium is oxidized • Said to be the reducing agent • Substance in the reaction that loses electrons • Sulfide sulfur atom is reduced • Said to be the oxidizing agent • Substance in the reaction that gains electrons

  7. Oxidation Numbers • A count of the electrons transferred or shared in the formation or breaking of chemical bonds • You must assign each element in the reaction an oxidation number • Follow a set of rules…

  8. Oxidation Number Rules 1. The total of the oxidation numbers of all the atoms in a neutral molecule, an isolated atom, or a formula unit is 0 2. In their compounds, the Group 1A metals all have an oxidation number of +1, and the Group 2A metals have an oxidation number of 2+

  9. Rules Con’t • In its compounds, hydrogen has an oxidation number of +1 (except in metal hydrides such as NaH, where it is -1) 4. In its compounds, oxygen has an oxidation number of -2 (except in peroxides such as H2O2, where it is -1) 5. In their binary compounds with metals, Group7A elements have an oxidation number of -1. Group 6A elements have an oxidation number of -2, and Groups 5A elements have an oxidation number of -3.

  10. Problems • What is the oxidation number of each element? • I2 • Cr2O3 • AlCl3 • Na2SO4 • CaH2

  11. Identifying Redox Reactions 0 +3 -2 0 +3 -2 • 2 Al + Fe2O3 2 Fe + Al2O3 • Al increases from 0 to +3, it is ______ • Oxidized! • Fe decreases from +3 to 0, it is _______ • Reduced!

  12. Oxidizing and Reducing Agents • Now the confusing part… • CuO + H2 Cu + H2O • Cu goes from +2 to 0 • Cu is reduced, therefore it is called an oxidizing agent because it causes some other substance to be oxidized • H goes from 0 to +1 • H is oxidized, therefore it is called a reducing agent because it causes some other substance to be reduced.

  13. Identifying Agents in an Equation • CuO + H2 Cu + H2O Reduction: CuO is the oxidizing agent Oxidation: H2 is the reducing agent

  14. Electrochemical Cells • An apparatus that allows a redox reaction to occur by transferring electrons through an external connector. • Product favored reaction ---> voltaic or galvanic cell ----> electric current • Reactant favored reaction ---> electrolytic cell ---> electric current used to cause chemical change. Batteries are voltaic cells

  15. Basic Concepts of Electrochemical Cells Anode Cathode

  16. 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.”

  17. GOOD AFTERNOON!  PRE-AP: Please turn in your redox problems to the bin. Grab the sheets from the corner. Homework DUE: FRIDAY REGULAR Please grab a worksheet from the corner Take out the homework from last night – on desk REMINDERS: Project Description and Outline DUE MAY 2nd. Project DUE May 23rd (or by May 9th for +5 pts)

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

  19. Standard reduction Potential • E° = standard potential of the cell. • Represents the voltage of the cell when the electrons create a current by passing through the wire. • E°= E (reduction) + E (oxidation)

  20. Electrolysis of Water • Electrolysis of water is the breaking apart of water from H20 into its ions by running an electrical charge through it. • Example of the first use and type of a electrochemical cell

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

  22. Terms Used for Voltaic Cells

  23. Balancing Equations for Redox Reactions Some redox reactions have equations that must be balanced by special techniques. MnO4- + 5 Fe2+ + 8 H+ ---> Mn2+ + 5 Fe3+ + 4 H2O Mn = +7 Fe = +2 Mn = +2 Fe = +3

  24. Balancing Equations Consider the reduction of Ag+ ions with copper metal. Cu + Ag+ --give--> Cu2+ + Ag

  25. Step 1: Divide the reaction into half-reactions, one for oxidation and the other for reduction. Ox Cu ---> Cu2+ Red Ag+ ---> Ag Step 2: Balance each element for mass. Already done in this case. Step 3: Balance each half-reaction for charge by adding electrons. Ox Cu ---> Cu2+ + 2e- Red Ag+ + e- ---> Ag

  26. Step 4: Multiply each half-reaction by a factor so that the reducing agent supplies as many electrons as the oxidizing agent requires. Reducing agent Cu ---> Cu2+ + 2e- Oxidizing agent 2 Ag+ + 2 e- ---> 2 Ag Step 5: Add half-reactions to give the overall equation. Cu + 2 Ag+ ---> Cu2+ + 2Ag The equation is now balanced for both charge and mass.

  27. Balancing Equations Balance the following in acid solution— VO2+ + Zn ---> VO2+ + Zn2+ Step 1: Write the half-reactions Ox Zn ---> Zn2+ Red VO2+ ---> VO2+ Step 2: Balance each half-reaction for mass. Ox Zn ---> Zn2+ Red 2 H++ VO2+ ---> VO2+ + H2O Add H2O on O-deficient side and add H+ on other side for H-balance.

  28. Balancing Equations Step 3: Balance half-reactions for charge. Ox Zn ---> Zn2+ + 2e- Red e- + 2 H+ + VO2+ ---> VO2+ + H2O Step 4: Multiply by an appropriate factor. Ox Zn ---> Zn2+ +2e- Red 2e-+ 4 H+ + 2 VO2+ ---> 2 VO2+ + 2 H2O Step 5: Add balanced half-reactions Zn + 4 H+ + 2 VO2+ ---> Zn2+ + 2 VO2+ + 2 H2O

  29. Tips on Balancing Equations • Never add O2, O atoms, or O2- to balance oxygen. • Never add H2 or H atoms to balance hydrogen. • Be sure to write the correct charges on all the ions. • Check your work at the end to make sure mass and charge are balanced. • PRACTICE!

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