Class Note for CHEM 112 at UMass(15)
Class Note for CHEM 112 at UMass(15)
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This 8 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Massachusetts taught by a professor in Fall. Since its upload, it has received 13 views.
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Date Created: 02/06/15
Balancing OxidationReduction Reactions 1Assign oxidation numbers 2 Separate into oxidation and reduction half reactions 3 Balance each halfreaction using the following steps a Balance all elements except oxygen or hydrogen b Balance oxygen by adding HZO c Balance hydrogen by adding H d Balance charge by adding electrons Electrons go on the RIGHT product side for OXIDATION reactions Electrons go on the LEFT reactant side for REDUCTION reactions e In BASIC solu ion do this additional step For every H add OHto BOTH sides ofthe reaction Combine H OH into H20 Cancel out any waters that appear on both sides You should now have a balanced half reaction 4 Multiply balanced half reactions so an equal number of electrons are consumed and produced 5 Add together halfreactions 6 Clean up Combine identical substances and reduce coef cients to the lowest terms 7 CHECK Atom and charge must balance RCdUCtiOH of V02 with Zn The voginn is yelan in acid Zn added with line the with time the blue v0 inn Finally green v3 ion is seldlien yelldw vogidn is reduced is further reduced to green redueed tn vidlel v inn tn blue v0 inn v ien Add Zn Balancing Equations Balance the following in acid solution V02 Zn gt VO2 Zn2 Step 1 Write the halfreactions Ox Red Step 2 Balance each halfreaction for mass Ox Red Add H20 on Odeficient side and add H on other side for Hbalance Balancing Equations Step 3 Balance halfreactions for charge Ox Red Step 4 Multiply by an appropriate factor Ox Red Step 5 Add balanced halfreactions Balancing Equations Never add 02 O atoms or 0239 to balance oxygen Never add H2 or H atoms to balance hldroven Be sure to write the correct charges on all the ions Check your work at the end to make sure mass and charge are balanced I PRACTICE Question The following skeletal oxidationreduction reaction occurs under acidic conditions Write the balanced REDUCTION half reaction HNo3 Cr3 a NO CrO4239 1 Identify the species being reduced HNO3 or Cr Conclusion N is being Cr is being Balance each half reaction using the following steps a Balance all elements except oxygen or hydrogen b Balance oxygen by adding HZO c Balance hydrogen by adding H d Balance charge by adding electrons Electrons go on the RIGHT product side for OXIDATION reactions Electrons go on the LEFT reactant side for REDUCTION reactions e In BASIC solution do this additional step For every H add OH to BOTH sides of the reaction Combine H OH into HZO Cancel out any waters that appear on both sides Skeleton Reaction HNO3 Cr3 9 NO CrO4239 Oxidation Reaction Cr3 9 CrO4239 4 amp 5 Combine Half Reactions HNO3 3H 3e39 9 NO 2H20 7 CHECK Atom and charge must balance HNo3 Cr3 2H20 a NO CrO4239 5H Reactants Products H moles H moles N mole N mole O moles O moles Cr mole Cr moe Net Charge Net Charge The Masses amp Charges Balance Electrochemistry Alessandro Volta 1745 1827 Italian scientist and Inventor Luigi Galvani 17371798 Italian scientist and inventor CHEMICAL CHANGE gt ELECTRIC CURRENT 2quot metal With time Cu plates out onto Zn metal strip and Zn strip disappears Electrons are transferred from Zn to Cu but there is no useful electric current Oxidation Reduction 11 CHEMICAL CHANGE gt ELECTRIC CURRENT To obtain a useful current we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire wire elect rons gt saIl bride Zn ions This is accomplished in a GALVANIC or VOLTAIC cell A group of such cells is called a battery 12 Terms Used for Voltaic Cells Direction urelemnn new Eleztmde Electrnie 7 Salt bridge Eledvulym Reduced nxidized a ramquot spades species mink lt y 39 iAniuns Dxidiled L h Reduned ll speuies a quot species 77 r ANUDE onipartment CATHDDE Compartment OXIDATIUN octms REDUCTION ocrws 13 Electrochemical Cell Zn gt Zn2 2e wire Cu2 2e gt Cu electrons gt Oxidation Reduction salt Anode brid e Cathode Negative Positive ltAnions Cationsgt EZri Zt ions Electrons travel through ISalt bridge allows to move between electrode compartments The Cu l Cu2 and Ag l Ag Cell Net veactinn Eus 2 Agaq 4 En aq 2 Ags 15
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