E na electrolytic cell a cell that uses electrical

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) + e Na ( ) electrolytic cell = a cell that uses electrical energy to drive a nonspontaneous reaction Diagram of an Electrolytic Cell 70
Sodium reacts spontaneously with chlorine, 2Na (s ) + Cl 2 ( g ) 2 NaCl( s ) An external source of energy is required to push a ball uphill, and to drive a chemical reaction in a nonspontaneous direction. 2NaCl( s ) 2Na( s ) + Cl 2 ( g ) The electrolytic cell on the previous slide can written as (excluding inert electrodes), Cl ( ) Cl 2 ( g , 1atm) Na + ( ) , Na( ) 2Cl ( ) Cl 2 ( g ) + 2e 2Na + ( ) + 2e 2Na ( ) 2NaCl( ) 2Na( ) + Cl 2 ( g ) Example: Electrolysis of NaCl 71
A. Competing Reactions Q: In the preceding cell, why was molten NaCl used rather than aqueous NaCl? Ans: Aqueous electrolytic cells introduce “complications”. 2Cl ( aq ) Cl 2 ( g ) + 2e (oxidation) 2H 2 O ( ) + 2e H 2 ( g ) + 2OH ( aq ) (reduction) 2H 2 O ( ) + 2Cl ( aq ) H 2 ( g ) + 2OH ( aq ) + Cl 2 ( g ) Q: Why were the Na + ions not reduced? Ans: It is easier to reduce H 2 O than Na + and mother nature chose the easier path. Electrolytic Cell Complications 72
E H 2 O/H 2 > E Na + /Na H 2 O is easier to reduce Note: A higher E implies an easier reduction. A lower E implies an easier oxidation. Q: How could we have predicted that H 2 O would be easier to reduce? Ans: Compare their standard electrode potentials . 2H 2 O ( ) + 2e H 2 ( g ) + 2OH ( aq ) E = 0.83 V 2Na + ( aq ) + 2e 2Na ( s ) E = 2.71 V BUT, it’s not quite this simple. These are standard electrode potentials . Aqueous NaCl is not likely to have [OH ] = 1.0 M Electrolytic Cell Complications: Competing Reactions (cont.) 73
Even More Electrolytic Cell Complications B. Overvoltage (NOT Required for Exam 3) Calculated electrolytic cell potentials (for nonstandard conditions) often underestimate the voltage required for electrolysis. overvoltage = the difference between the actual cell potential required for appreciable electrolysis, and the calculated cell potential. Example: Electrolysis of aqueous KBr yields Br 2 at the anode and not the predicted O 2 . 2 Br ( aq ) Br 2 ( g ) + 2 e E = 1.07 V 2 H 2 O( ) O 2 ( g ) + 4 H + ( aq ) + 4 e E = 0.82 V 0.82 < 1.07 H 2 O should be easier to oxidize, but it is not (0.82 V is an underestimation). 74 Reddish- brown Br 2
Electrolysis Application: Electroplating 75 See Fig. 20.29 A thin layer of metal is deposited on another metal. Reasons: Prevent corrosion Improve beauty Limit an expensive metal to outer layer
It should be apparent, that 1 mole of electrons are required to electroplate 1 mole of silver (Ag). Ag + ( aq ) + e Ag( s ) Q: How do we measure moles of electrons? Exercise: How many grams of silver will be electroplated using a current of 5.3 A for 1.2 hr? 1 mole e = 9.65 10 4 coulombs second coulombs s C A amperes Solution: Example: The Stoichiometry of Electrolysis 76 Ag g 26 Ag mol Ag g 107.9 - e mol 1 Ag mol 1 C 10 9.65 - e mol 1 s C 5.3 hr 1 s 3600 hr 1.2 4

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