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Oxidation-Reduction Reactions REDOX Reactions Oxidation State • Oxidation numbers are very similar to charge. • There are some different rules for assigning them. Rules for Oxidation State • 1. The oxidation number for any uncombined element is zero • Ex. The oxidation state for Na is zero. Rule 2 • The oxidation number for a monatomic ion equals the charge on the ion. • Ex. Cl-1 has an oxidation of -1. Rule 3 • The more electronegative element in a binary compound is assigned the number equal to the charge it would have if it were an ion. • Ex. The oxidation number of O in NO is -2 because oxygen is more electronegative than nitrogen. Rule 4 • The oxidation number of fluorine in a compound is always -1. • Ex. The oxidation number of F in LiF is -1. Rule 5 • Oxygen has an oxidation number of -2 unless it is combined with fluorine, when it is +2, or it is in a peroxide such as H2O2, when it is 1. • Ex. The oxidation of O in NO2 is -2. Rule 6 • The oxidation state of hydrogen in most of its compounds is +1 unless it is combined with a metal, in which case it is -1. • Ex. The oxidation of H in LiH is -1. The oxidation of H in HCl is +1. Rule 7 • In compounds, Group 1 and 2 elements and aluminum have oxidation numbers of +1, +2, and +3, respectively. • Ex. The oxidation number of Ca in CaCO3 is +2. Rule 8 • The sum of the oxidation numbers of all atoms in a neutral compound is 0. • Ex. The oxidation number of each atom in CaCO3 can be found by knowing the rules above and knowing the compound is neutral • Ca = +2 (rule 7) • O = -2 x 3 atoms = -6 (rule 5) • C = +4 (rule 8—balances out the charge) Rule 9 • The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge of the ion. • Ex. The oxidation of each element in H2PO42- can be determined by knowing the overall charge is -2 • H = +1 x 2 atoms = +2 (rule 6) • O = -2 x 4 atoms = -8 (rule 5) • P = +4 (rule 9) Types of Reactions 5 types plus a new one Type of Reactions Chemical reactions are classified into five general types Combination Decomposition Single Replacement Double Replacement Combustion Combination (Synthesis) Two or more elements or simple compounds combine to form (synthesize) one product • A + B AB • 2Mg + O2 2MgO • 2Na + Cl2 2NaCl • SO3 + H2O H2SO4 Decomposition One substance is broken down (split) into two or more simpler substances. • AB A + B • 2HgO 2Hg + O2 • 2KClO32KCl + 3 O2 Learning Check R1 Classify the following reactions as 1) combination or 2) decomposition: ___A. H2 + Br2 2HBr ___B. Al2(CO3)3 Al2O3 + 3CO2 ___C. 4 Al + 3C Al4C3 Solution R1 Classify the following reactions as 1) combination or 2) decomposition: _1_A. H2 + Br2 2HBr _2_B. Al2(CO3)3 Al2O3 + 3CO2 _1_C. 4 Al + 3C Al4C3 Single Replacement One element takes the place of an element in a reacting compound. A + BC AB + C Zn + 2HCl ZnCl2 + H2 Fe +CuSO4 FeSO4 + Cu Learning Check R2 Classify the following reactions as 1) single replacement 2) double replacement __A. 2Al + 3H2SO4 Al2(SO4)3 + 3H2 __B. Na2SO4 + 2AgNO Ag2SO4 + 2NaNO3 __C. 3C + Fe2O3 2Fe + 3CO Solution R2 Classify the following reactions as 1) single replacement 2) double replacement 1_A. 2Al + 3H2SO4 Al2(SO4)3 + 3H2 2_B. Na2SO4 + 2AgNO3 Ag2SO4 + 2NaNO3 1_C. 3C + Fe 2Fe + 3CO Combustion A reaction in which a compound (often carbon) reacts with oxygen C + O2 CH4 + 2O2 CO2 C3H8 + 5O2 CO2 + 2H2O 3CO2 + 4H2O C6H12O6 + 6O2 6CO2 + 6H2O Learning Check R3 Balance the combustion equation ___C5H12 + ___O2 ___CO2 + ___H2O Solution R3 Balance the combustion equation 1 C5H12 + 8 O2 5 CO2 + 6 H2O Oxidation-Reduction Reactions REDOX Rxns Oxidation and Reduction Reactions that involve a loss or gain of electrons Occurs in many of the 4 types of reactions and combustion Important in food metabolism, batteries, rusting of metals Requirements for Oxidization-Reduction Electrons are transferred Two processes occur Oxidation = Loss of electrons (LEO) Zn Zn2+ + 2e- Reduction = Gain of electrons (GER) Cu2+ + 2e- Cu Balanced Red-Ox Equations Combine the oxidation and reduction reactions to make Loss of electrons = Gain of electrons Zn + Cu2+ + 2e- Zn2+ + 2e- + Cu Zn + Cu2+ Zn2+ + Cu Gain/Loss of Hydrogen In organic and biological reactions oxidation = Loss of H reduction = Gain of H Half-Reaction • The part of a reaction involving oxidation or reduction alone. • The overall reaction is the sum of the two half reactions • Ex. • 3Cu 3Cu2++ 6 e• 2NO3-1 + 6 e- + 8H+ 2NO + 4 H2O • 3Cu + 2NO3-1 + 8 H+ 3Cu2+ + 2NO + 4H2O Learning Check R3 Identify the following as an 1) oxidation or a reduction process: __A. Sn Sn4+ + 4e- __B. Fe3+ + 1e- Fe2+ __C. Cl2 + 2e- 2Cl- Solution R3 Identify the following as an 1) oxidation or a reduction process: 1_ A. Sn Sn4+ + 4e- 2_ B. Fe3+ + 1e- Fe2+ 2_ C. Cl2 + 2e- 2Cl- Learning Check R4 In light-sensitive sunglasses, UV light initiates an oxidationreduction reaction Ag+ + ClAg + Cl A. Which reactant is oxidized 1) Ag+ 2) Cl3) Ag B. Which reactant is reduced? 1) Ag+ 2) Cl3) Cl Solution R4 In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction Ag+ + ClAg + Cl A. Which reactant is oxidized 2) ClClCl + eB. Which reactant is reduced? 1) Ag+ Ag+ + eAg Oxidizing and Reducing Agents Reducing Agents • A substance that has the potential to cause another substance to be reduced. • Lose electrons (gain higher charge) Oxidizing Agent • Substance that has the potential to cause another substance to be oxidized • Gains electrons (become more negative) Summary Chart of Terminology Term Change in oxidation number Change in electron population Oxidation Increases Loss of electrons Reduction Decreases Gain of electrons Oxidizing agent Decreases Gains electrons Reducing agent Increases Loses electrons Autooxidation • A process in which a substance acts as both an oxidizing agent and a reducing agent Electrochemistry Electrodes, half cells, anodes, cathodes, voltaic and electrolytic cells Electrochemistry • The branch of chemistry that deals with electricity-related applications of oxidation-reduction A bright idea! • As redox rxns occur, electrons are transferred which also produces heat. • But if the substances are separated by a porous barrier then they are able to transfer electricity instead of heat Electrode •Conductor used to establish electrical contact with a nonmetallic part of a circuit such as an electrolyte Half-cell • A single electrode immersed in a solution of its ions. Anode • One half cell is the anode which is the electrode where oxidation takes place. Cathode • Other half cell where reduction takes place. Electrochemical cell • A system of electrodes and electrolytes in which either chemical reactions produce electrical energy of an electric current produces chemical change. Voltaic Cells •If the redox rxn in an electrochemical cell occurs spontaneously and produces electrical energy then it is a voltaic cell. Types of Dry Cell Batteries Zinc-Carbon Dry Cells Alkaline Batteries Mercury batteries Zinc-Carbon Dry Cells •Ex. Flashlight batteries •Have Zn container which is the anode •Filled with moist paste of MnO2, graphite, and NH4Cl Alkaline Batteries •Ex. Batteries found in small, portable devices such as CD players •Very similar to the Zn-C but it does not contain the C which allows them to be smaller •Uses paste of Zn and KOH instead of a solid metal anode. Mercury Batteries •Ex. Calculator batteries •The anode half rxn is the same as the alkaline but the cathode involves HgO Rechargeable cells •Combination of voltaic and electrolytic cells •Acts as a voltaic cell during use and as an electrolytic cell during charging. •Ex. Car battery