Class Note for CHEM 112 at UMass(47)
Class Note for CHEM 112 at UMass(47)
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This 11 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 15 views.
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Date Created: 02/06/15
Turn on Camtasia Recording Today we start chapter 20 The final chapter of the semester Why Study Electroc Ba enes Corrosion Industrial production of chemicals such as Cl2 NaOH F2 and Al Biological redox reactions Useful way to see chemical energy produced to do work Become a better hostess The heme group Terminology for Redox Reactions I oss of electrons by a species increase in oxidation number I gan ot electroms decrease In oxidation number I eectron acceptor species is reduced I eectron donor s ecies is oxidized What is the reducing agent in this skeletal reaction MnO4391 re2 39 Mn2 1 re3 AMnO439 B Fe2 CMn2 DFe3 OXIDATIONREDUCTION REACTIONS Direct Redox Reaction Oxidizing and reducing agents in direct contact CUS 2AgaQgt CU2aQ 2Ag8 OXIDATIONREDUCTION REACTIONS Indirect Redox Reaction A battery transfers electrons through an external wire Trom tne reducrng agent to the oxrdrzrng agent movie Oxidation States of Atoms in Elements and Compounds IN ELEMENTS Oxidation Number Examples Atoms in their 139 elemental state 390 Fer Hz 02 2 Monatomic ions charge Fl39 Na Fe3t IN COMPOUNDS 3 Group 1A 1 NaCl KNO3 4 Group 2A 2 M90 5 Fluorine 1 HF CIF 6 Hydrogen 1 H20 7 Oxygen 2 02 HCIOA 8 Group 7A 1 HCI 9 Group 6A 2 Pbs2 The sum of all oxidation numbers of all elements charge on substance What is the Charge on the Mn ion in MnO439 gt Mn2 1 re3 MnO439 1 re A Mn B Mn CMn DMn E Mn 7 6 4 2 1 Balancing OxidationReduction Reactions 1 Assign 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 d Balance charge by adding electrons Balance hydrogen by adding H Electrons go on the RIGHT product side for OXIDATION reactions Electrons go on the LEFT reactant side for REDUCTION reactions In BASIC solu ion do this additional step For every HY add OH to BOTH sides ofthe reaction Combine H OH into H20 Cancel out any waters that appear on both sides You should now have a balanced halfreaction 4 Multiply balanced halfreactions so an equal number of electrons are consumed and produced 5 Add together half reactions 6 Clean up Combine identical substances and reduce coef cients to the lowest terms 7 CHECK Atom and charge must balance 10 Balancing Equations Skeletal reaction Cu Agt gt Cu2 Ag 11 Balancing Equations Steal Assign Oxidation Numbers Step 2 Divide the reaction into halfreactions one for oxidation and the other for reduction OX Red Step 3ac Balance each for mass Step 3d Balance each halfreaction for charge by adding electrons OX Red 12 Balancing Equations Step 4 Multiply each halfreaction by a factor so that the reducing agent supplies as many electrons as the oxidizing agent requires Reducing agent Oxidizing agent Step 5 Add halfreactions to give the overall equation Step 6 amp 7 Check The equation is now balanced for both charge and mass 13 Balancing Equations for Redox Reactions MnO439 Fe2 v Mn2 Fe3 Mn Fe Mn Fe MnO439 5Fe2 8H gt Mn2 5Fe3 4H20 14 RCdUCtiOH of V02 with Zn The voginn isyelluw in acid Zn added Witli time the With time Hid blue v02 inn Finally gVeEi v ion is solution yellow vogidn is ieddced is fwthev ieddced to green reduned tn violet v2 inn tn blue v0 inn v inn 15 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 Ode cient side and add H on other side for Hbalance 16 Balancing Equations Step 3 Balance halfreactions for charge Ox Red Step 4 Multiply by an appropriate factor Ox Red Step 5 Add balanced halfreactions 17 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 18 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 19 Balance each half reaction using the following steps a b Balance oxygen by adding H20 c 1 Balance charge by adding electrons Balance all elements except oxygen or hydrogen Balance hydrogen by adding H Electrons go on the RIGHT product side for OXIDATION reactions Electrons go on the LEFT reactant side for REDUCTION reactions In BASIC solution do this additional step For every H add OH to BOTH sides of the reaction Combine H OH into H20 Cancel out any waters that appear on both sides 20 Skeleton Reaction HNO3 Cr3 9 NO CrO4239 Oxidation Reaction Cr3 9 CrO4239 amp 5 Combine Half Reactions HNO3 3H 3e39 9 NO 2H2O 21 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 22
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