ch19 MSJ jlm - Electrochemistry Electrochemistry and Its...

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1 Electrochemistry Electrochemistry and Its Applications and Its Applications Chapter 19 Chapter 19 Humphry Davy 1778-1829. Prepared metallic K, Na, Sr, Ca, B, Ba, Mg, Li by electrolysis.
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2 Oxidation-Reduction Reactions Oxidation-Reduction Reactions OXIDATION = loss of electrons Examples: Na Na + + e - Al Al 3+ + 3e - S 2- S + 2e - OXIDATION = increasing the oxidation number (more positive) Example: NO NO 2 -2 +2 -2 +4 change = +2 +2 to +4 : N is oxidized LEO the lion goes GER Oxid. Nos:
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3 Oxidation-Reduction Reactions Oxidation-Reduction Reactions REDUCTION = gain of electrons Examples: N + 3e - N 3- Fe 3+ +e - Fe 2+ REDUCTION = decreasing the oxidation number (more negative) Example: MnO 4 - Mn 2+ Oxid. Nos: +7 +2 change = -5 +7 to +2 : Mn is reduced LEO the lion goes GER
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4 Review of Oxidation Numbers Review of Oxidation Numbers O.N. = 0 for atom in element form (C, Ag, O 2 , H 2 , P 4 , etc.) = charge for any monoatomic ion (e.g., +1 for Na + , -2 for S 2- , +3 for Al 3+ ) = -2 for oxygen in compound or ion (except peroxides and when bonded to fluorine) = +1 for hydrogen when bonded to non-metals (e.g., H 2 O, CH 4 , H 3 N, HCl) = -1 for hydrogen when bonded to metals (e.g., NaH) = -1 for oxygen in peroxides (e.g., HOOH) Sum of O.N. = 0 for neutral compound (e.g., for Na 2 CO 3 , Na = +1, O = -2, C = +4) Sum of O.N. = ion charge for polyatomic ion (e.g., for CO 3 -2 , O = -2, C = +4)
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5 Oxidation-Reduction Reactions = “REDOX” Oxidation-Reduction Reactions = “REDOX” What we have looked at are called half-reactions . Half-reactions are fully balanced with respect to both mass (atoms) and electrons (net charge) and are either reduction or oxidation (but not both), e.g., Sn 2+ + 2e - Sn (Sn is reduced) FeCl 3 + e - FeCl 2 + Cl - ( Fe is reduced ) Mn 2+ ( aq ) + 4 H 2 O ( l ) 8H + ( aq ) + MnO 4 - ( aq ) + 5e - ( Mn is oxidized) But half-reactions do not occur by themselves in the real world; reduction cannot occur without oxidation (and vice versa). Hence, we have REDOX occurring (both oxidation and reduction) in a chemical reaction.
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6 Oxidation-Reduction Reactions Oxidation-Reduction Reactions Half reactions are combined to get REDOX reactions. When combining, • charges must balance (this is conservation of electrons) and • atoms must balance (this is conservation of mass) . Half-reaction method of balancing REDOX equations Consider the (unbalanced) reaction: MnO 4 - ( aq ) + C 2 O 4 2- ( aq ) Mn 2+ ( aq ) + CO 2 ( g ) (1) Write out separate reduction and oxidation half-reactions; (2) Add H 2 O and/or H + as needed for mass balance, and add electrons (e - ) for charge balance; (3) Multiply each half reaction by a common denominator so that electrons (e - ) will cancel; (4) Add the half reactions and (mathematically) simplify.
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7 1. The two incomplete half reactions are: MnO 4 - ( aq ) Mn 2+ ( aq ) C 2 O 4 2- ( aq ) 2CO 2 ( g ) 2a. Balance 1 st half-reaction : MnO 4 - ( aq ) Mn 2+ ( aq ) 5e - + 8H + + MnO 4 - Mn 2+ + 4H 2 O 2b. Balance 2 nd half-reaction: C 2 O 4 2- ( aq ) CO 2 ( g ) C 2 O 4 2- 2CO 2 + 2e - MnO 4 - ( aq ) + C 2 O 4 2- ( aq ) Mn 2+ ( aq ) + CO 2 ( g ) Balancing Equations by the Method of Half-Reactions Balancing Equations by the Method of Half-Reactions
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8 Balancing Equations by the Method of Half-Reactions
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ch19 MSJ jlm - Electrochemistry Electrochemistry and Its...

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