CHAPTER 12: Redox Reactions And Electrochemistry

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CHAPTER 12: Redox Reactionsand Electrochemistry Recall “GERtrude and LEO”Gain of Electrons ReductionLoss of Electrons OxidationGoals of Chapter:– Understand redox reactions in detail– Review oxidation numbers– Learn electrochemical techniques Application of Redox Chemistry – extracting metals fromores, e.g. 2 400 4Cu2CO3(OH)2(s) C(s) Æ 2Cu(s) 2CO2(g) H2O(g)“azurite” Need to learn to balance tricky redox reactionsCHEM 1310 A/B Fall 2006

Balancing Redox Equations Book provides a very schematic, step-bystep approach. Take a look at it. We’ll take a more freestyle approach. Let’s do the first example in the book. Balance:S2O62- (aq) HClO2 (aq) Æ SO42- (aq) Cl2(g)CHEM 1310 A/B Fall 2006

Strategies for Balancing RedoxEquationsS2O62- (aq) HClO2 (aq) Æ SO42- (aq) Cl2(g) General Strategy––––Divide equation into two half-reactionsOne reaction for reductionOne reaction for oxidationBalance each separately then recombine Another Trick– Assuming reactions in aqueous solution, H2O can bethrown in to the equation when needed (might not begiven!!)– H can be helpful for acidic solutions– OH- can be of use in basic solutionsCHEM 1310 A/B Fall 2006

Back to the ExampleS2O62- (aq) HClO2 (aq) Æ SO42- (aq) Cl2(g) First break into half-reactions Whatelement is reduced? What is oxidized?Reduced:Oxidized:CHEM 1310 A/B Fall 2006

Begin the Balancing ActS2O62- (aq) HClO2 (aq) Æ SO42- (aq) Cl2(g) Now balance the non-H, non-O atoms foreach SO42-CHEM 1310 A/B Fall 2006

Balance H and O’sS2O62- (aq) HClO2 (aq) Æ SO42- (aq) Cl2(g) Now throw in H2O, H (if acidic), OH- (ifbasic) as needed to balance the H and Oatoms. Here acidic (HClO2).Reduction:Oxidation:2HClO2 Æ Cl2S2O6 Æ 2SO42-CHEM 1310 A/B Fall 2006

OverviewS2O62- (aq) HClO2 (aq) Æ SO42- (aq) Cl2(g)Break into half-reactions: 1 3 -2Reduced:Oxidized:0HClO2 Æ Cl2 5 -2S2O62- 6 -2Æ SO42-Balance Cl, S:2HClO2 Æ Cl2S2O62- Æ 2SO42Balance H, O:6H 2HClO2 Æ Cl2 4H2O2H2O S2O62- Æ 2SO42 4H Combine the halves balance electrons first!CHEM 1310 A/B Fall 2006

Balancing Electrons Add and subtract electrons to make charge balance onboth sides of equation (seem strange? Don’t worry, justtemporary for book keeping!)6H 2HClO2 Æ Cl2 4H2OH2O S2O62- Æ 2SO42 4H Multiply one of the equations to obtain equal number ofelectrons. Then, add to cancel out electrons.Reduction:Oxidation:6H 2HClO2 Æ Cl2 4H2OH2O S2O62- Æ 2SO42 4H Final: Check: everything balanced?CHEM 1310 A/B Fall 2006

Another Practice ProblemAsO33- (aq) Br2 (aq) Æ AsO43- (aq) Br- (aq)I.II.III.IV.V.**assume basicIdentify what’s oxidized and what’s reducedSplit oxidation and reduction reaction,balance for all atoms but O,HAdd H2O, H , OH- to balance H,OAdd electrons to balance charge for halfreactionAdd half-reactions together to cancelelectronsCHEM 1310 A/B Fall 2006

Another Practice ProblemAsO33- (aq) Br2 (aq) Æ AsO43- (aq) Br- (aq)CHEM 1310 A/B Fall 2006**assume basic

Disproportionation The same chemical species is bothoxidized and reduced. e.g.,Cl2 (aq) Æ ClO3- (aq) Cl- (aq)[unbalanced] In these cases, a single species is allowedto appear in both half-reactions.Cl2 Æ ClO3Cl2 ÆCl-CHEM 1310 A/B Fall 2006[unbalanced]

Application to Batteries Batteries work by using redox reactions. Example of an electrochemical cell:Cu(s) 2Ag (aq) Æ Cu2 (aq) 2Ag(s) Above equation is balanced – recallexample of deposition of Ag(s) on copperwire in AgNO3 solution. Batteries harness the flow of electrons inredox reactions to perform electrical work.CHEM 1310 A/B Fall 2006

A Look Inside a Battery Electrons are produced atthe anode by oxidation.They flow to the ( )cathode, where theypromote reduction. Salt bridge allows flow ofions to keep chargeneutrality of solutions. Amount of charge flowcan be measured by:I Q/tcurrent charge/timeamperes (A) Coulombs/secCHEM 1310 A/B Fall 2006

Example Problem How many amps would be needed to reduce 1 mol ofAg ions in one hour? I Q/t To reduce a mol of Ag , one mol of e- is needed.CHEM 1310 A/B Fall 2006

Example Problem 2 A galvanic cell generates an average current of 0.121 Afor 15.6 min. The cathode half-reaction in the cell isPb2 (aq) 2e- Æ Pb(s). What mass of lead is depositedat the cathode?CHEM 1310 A/B Fall 2006

Electrometallurgy Electrochemical methods to producemetals from compounds (often ores) Uses redox reactions. e.g.,2Al2O3 3C Æ 4Al 3CO2MgCl2(l) Æ Mg(l) Cl2(g)Dangerous process!!Converted to HCl.CHEM 1310 A/B Fall 2006

Electrorefining Purify metals byelectrochemistry. Metals leave anode(where they’re oxidized)as ions and re-deposit oncathodes. Impurities are more likelyto stay in solution.CHEM 1310 A/B Fall 2006

Electroplating Use of electrochemistry to deposit a thinfilm of a metal (like Ag, Au) on top ofanother substance.CHEM 1310 A/B Fall 2006

CHAPTER 12: Redox Reactions . Goals of Chapter: - Understand redox reactions in detail - Review oxidation numbers - Learn electrochemical techniques Application of Redox Chemistry - extracting metals from ores, e.g. Need to learn to balance tricky redox reactions Cu 2CO 3(OH) 2(s) C(s) Æ2Cu(s) 2CO 2(g) H 2O(g .

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