Chapter 18 Oxidation-Reduction Reactions & Electrochemistry

Learning Goals: I will be able to… 1.Identify the “givers” and “takers” of electrons in a chemical reaction: oxidized vs. reduced elements; 2.Assign an oxidation state to any element in a chemical equation. 2

3 Oxidation-Reduction Reactions  Involve a transfer of electrons between metals, nonmetals, or metal and nonmetal  “Redox” reactions include/are used in: automobile and other engines natural gas burning to heat buildings coal burning to make electricity battery-powered calculators, digital watches, radios, MP3 players, etc.  Provide most of the energy for work and play!  Oxidation is a loss of electrons. Ex.: Na  Na +  Reduction is a gain of electrons. Ex.: Cl  Cl -

The Lion Method: “Leo Ger” 4 Loss of Electrons is Oxidation Gain of Electrons is Reduction

Oxidation-Reduction Reactions – Which element is oxidized? – Which element is reduced? Na (loses 1 e-) Cl (gains 1 e-)

Copyright© by Houghton Mifflin Company. All rights reserved. 6 Oxidation-Reduction Reactions  When metal + nonmetal  ionic compound, the metal is oxidized and the nonmetal is reduced. Ex.: 2Na(s) + Cl 2 (g)  2NaCl(s) Na is oxidized to Na +, Cl is reduced to Cl - Na is the reducing agent – it donates electrons Cl 2 is the oxidizing agent – it accepts electrons

Copyright© by Houghton Mifflin Company. All rights reserved. 7 Oxidation States  Numbers/charges used to keep track of electrons in a redox reaction: 2Mg(s) + O 2 (g)  2MgO(s) ↑↑ ↑ ↑ Mg is oxidized, O is reduced Reducing agent: Mg, Oxidizing agent: O 2 CH 4 (g) + 2O 2  CO 2 (g) + 2H 2 O(g) + energy ↑ ↑ ↑ ↑ ↑ ↑ ↑ C oxidized, O reduced Reducing agent: CH 4, Oxidizing agent: O 2

Copyright© by Houghton Mifflin Company. All rights reserved. 8 F > O > N > Cl

B. Oxidation States For elements in compounds, Start by asking if the compound is ionic or covalent: Ionic compound  use p. table/ion charges Covalent compound (including polyatomic ions)  use the list of rules

Learning Goal: I will be able to… Design a voltaic cell (battery) and describe the oxidation and reduction reactions separately using the half-reaction method. 10

Copyright© by Houghton Mifflin Company. All rights reserved. 11 Balancing Oxidation-Reduction Reactions by the Half-Reaction Method  Write and balance a reduction half-reaction – electrons shown on reactant side: 2e - + Cl 2  2Cl -  Write and balance an oxidation half-reaction – electrons shown on product side: 2Na  2Na + + 2e -  Check: no. electrons gained = no. electrons lost  Add the half-reactions and cancel electrons to give the overall balanced equation: 2e - + Cl 2  2Cl - 2Na  2Na + + 2e - 2e - + Cl 2 + 2Na  2Cl - + 2Na + + 2e - Cl 2 + 2Na  2Cl - + 2Na +

Copyright© by Houghton Mifflin Company. All rights reserved. 12 Balanced half-reactions – more examples:  The reduction of Br 2 (l) to Br - (aq) Br 2 (l) + 2e -  2Br - (aq)  The oxidation of Zn(s) to Zn 2+ (aq) Zn(s)  Zn 2+ (aq) + 2e -

Copyright© by Houghton Mifflin Company. All rights reserved. 13 Electrochemistry  Study of the interchange of chemical and electrical energy  Involves two types of processes: Production of an electric current from a chemical reaction Use of an electric current to produce a chemical reaction  When a redox reaction occurs in solution, no useful work is obtained from the chemical energy  To harness the chemical energy from the reaction, the oxidizing agent (e - acceptor) and reducing agent (e - donor) must be separated so electrons are transferred through a wire  The current flowing through the wire can be directed to a device, such as motor, to do useful work.

Schematic for Separating the Oxidizing and Reducing Agents in a Redox Reaction * check your activity series to find a pair of metals that could be used here; subtracting the potentials gives the potential for the battery/voltaic cell. * Copyright© by Houghton Mifflin Company. All rights reserved. 14 Less active metal in ionic solution More active metal in ionic solution ZnCu Zn 2+ SO 4 2- Cu 2+

Electron flow Copyright© by Houghton Mifflin Company. All rights reserved. 15

Ion flow keeps the charge neutral. Copyright© by Houghton Mifflin Company. All rights reserved. 16

The salt bridge contains a strong electrolyte and allows for ion flow. Ion flow is needed to keep the charges neutral on both sides. Copyright© by Houghton Mifflin Company. All rights reserved. 17

The porous disk also allows ion flow. Copyright© by Houghton Mifflin Company. All rights reserved. 18

Schematic of a Battery or Galvanic Cell Electrons from one electrode to another: anode  cathode Copyright© by Houghton Mifflin Company. All rights reserved. 19

Schematic of one cell of the lead battery used in automobiles Copyright© by Houghton Mifflin Company. All rights reserved. 20

A common dry cell battery Copyright© by Houghton Mifflin Company. All rights reserved. 21

A mercury battery Alkaline batteries (basic medium) do not corrode the steel as quickly and last longer. Copyright© by Houghton Mifflin Company. All rights reserved. 22

Corrosion  Most metals are found in nature in compounds with nonmetals (O, S, etc.)  Corrosion - oxidation of a metal; returns metal to its “natural state”  Most metals react with O 2 and develop a thin oxide coating – this protects the metal from further oxidation/corrosion:  Aluminum – Al 2 O 3 (very effective)  Iron – FeO (not very effective)  Gold – shows no significant corrosion in air Copyright© by Houghton Mifflin Company. All rights reserved. 23

Mg donates the electrons and the iron pipe is preserved.  Copyright© by Houghton Mifflin Company. All rights reserved. 24 Corrosion Protection Methods  Coating – paint or metal plating; chromium and tin are often used  Alloy use - metal mixture instead of pure metal; stainless steel contains chromium and nickel, which form a protective coating  Cathodic protection – a metal that donates electrons more easily is connected by a wire (picture)

Copyright© by Houghton Mifflin Company. All rights reserved. 25 Electrolysis  Forcing a current through a cell (eg., battery) to produce a chemical change that would not occur otherwise  Used to reverse redox reactions; allows recharging of lead and other batteries  Electrolysis of water: 2H 2 O(l)  2H 2 (g) + O 2 (g) Occurs whenever a current is forced through an aqueous solutions – a nonreacting electrolyle is needed.  Used in the production of metals from oxides – aluminum from aluminum ore (bauxite) Price of 1 lb. aluminum: 1855, $100,000; 1890, $2; 1990, $0.74

Electrolysis of Aluminum Oxide 26 Electric current provides electrons to reduce Al ions: Al e -  Al (pure metal) Purified Al 2 O 3 dissolved in molten cryolite (NaF, AlF 3 ) at about 950 o C

CHROME PLATING BY ELECTROLYSIS 27

Silver Plating by Electrolysis 28