Quantitative Analysis CHEM 3000
Weber State University
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This 17 page Class Notes was uploaded by Alfonso Reynolds on Wednesday October 28, 2015. The Class Notes belongs to CHEM 3000 at Weber State University taught by Kyle Ashby in Fall. Since its upload, it has received 27 views. For similar materials see /class/230794/chem-3000-weber-state-university in Chemistry at Weber State University.
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Date Created: 10/28/15
Weber State University H CHEM 3000 Quantitative Analysis H Electrochemistry Reading Chapter 14 Electrochemistry is the branch of chemistry concerned With the interconversion of chemical and electrical energy Via redox reactions Redox reactions involve a transfer of electrons from one species to another The species that gains the electrons is said to be reduced oxidizing agent and the species that loses the electrons is said to be oxidized reducing agent Consider the mineral nutrient iron in cereals Fes 2HClaq FeC12aq H2g Fes is oxidized Fes Fe2aq 2e39 Haq is reduced 2H 616 2639 H2g The reaction between iron metal and acid is spontaneous AG is negative Free Energy gt V potent1al Reaction progress gt This decrease in potential free energy offers us the capacity to do something purposeful outside the system In this case push or pull electrons Thus a difference in electrical potential E is de ned to match the difference in chemical potential AG This electrical potential electromotive force is the means to move electrons in other words a voltage some number of joules of energy to push some number of coulombs of electrons work E charge F 9649gtlt104 Cmol n 2 moles of e le 2 160217733x103919 C Electrical current ampere is the quantity of charge owing each second Current is proportional to the rate of the redox reaction ifslow current is low A couple more relationships Ohm s Law Power W R reSiStanCC Otha 9 Power generated by a circuit appears Larger E greater current as heat in a resistor being generated Larger R smaller current at a certain rate Now consider the mineral nutrient copper in cereals Cus 2Haq Cu2aq H2g Some species are very Willing to be oxidized or reduced other species are not so Willing We can represent reactions of many species relative to each other to compare their oxidizingreducing strengths By convention The value of E is a a way to rank one half reaction against another as to the tendency of a particular species to be reduced relative to H Table 141 Ordered redox potentials Oxidizing agent Reducing agent E V F2g 2e 213 2890 03g 2H 2e 02g H20 2075 o I 5 MnO 8H 5e Mn2 4H20 a 1507 Ag e Ags m 0799 5 Cu2 Ze Cus g 0339 O E 2 0 a 2H 2e H2g 0000 O I 3 Cd2 2e Cds 0402 r E K e Ks 2936 O Li e Lis 3040 Tab Quantitative Chemical Analysis Seventh Edilian 2007 w H Freeman and Company E 141 Think of E0 as a disguised equilibrium constant A reduction process cannot occur without an oxidation occurring simultaneously Thus two reactions from the table must be coupled together a reduction written as the forward process and an oxidation written as the reverse process The potential or voltage E for the overall process is the Let s look at the iron metal reaction With acid Fes 2HClaq FeC12aq H2g Fes Fe2aq 2e39 BO 2 044 V 2Haq 2e39 2g E 000 V The E for the overall spontaneous process is the difference in the reduction potentials Does a positive E for the overall spontaneous process make sense Remember AG 2 nFE If we wanted to utilize the electricity electrons produced from this reaction we would have to separate the two half reaction into separate cells and connect them together Via a wire and salt bridge This is called a Under standard conditions PH2 1 atm H l M Fe2 l M Pt is an inert metal electrode Line notation E0 is the work done per charge So if you multiply a reaction by a Whole number to balance the number of electrons gained and lost you do not multiply the E0 value by that Whole number THE E0 REMAINS THE SAME Consider a rechargeable battery Zns ZnC12aq Cl39aq C12l Cs Write half reactions for each electrode From Which electrode Will electrons ow from the battery into a circuit if the electrode potentials are not too different from E0 values If the battery delivers a constant current of l00gtlt103 A for 100 h how many kilograms of C12 Will be consumed As an electrochemical or redox reaction proceeds it Will proceed toward its equilibrium position just as any other reaction Will Reactants get used up amp products form Concentrations change Remember at equilibrium AG 2 0 w I Thus the amount of free energy available decreases as the redox reaction proceeds Once the available energy free energy decreases to zero there is no longer any quotpushquot or quotpullquot available to transfer electrons The electrochemical cell is dead at equilibrium Energy Available Free If conditions are not standard conditions What is E0611 Remember LeChatelier s principle If R increases reaction is driven to the right If P increases reaction is driven to the left The N ernst equation allows us to determine the net driving force Eceu under non standard conditions Derived from the equation AG 2 AGO RT an substituting nFE for the respective AGs and rearranging At equilibrium E 0 14 17 a Write the line notation for the cell shown in the diagram pg 294 b Calculate the potential of each half cell and the overall cell voltage E In Which direction Will electrons ow through the circuit Write the spontaneous net cell reaction Latimer Diagrams Shows standard reduction potentials for an element With various oxidation states In acid solution l 1318 1589 I 1154 14301 0535v 3902 3903 039 392 39 Oxidation 7 5 1 0 1 state of iodine 1210 Unnumbered figure pg 282 unlitutive hemicalAnaIysl39S Seventh Edition 2007 w HFreeman and Company 1154 103 103 5H 4639 e HOI H20 E 1154 V l I 1318 1589 I 1154 1430 1 0535v IO4 gt I03 gt HOI gt2 l Oxidation 7 5 1 0 1 lt State I T of iodine 1210 Unnumbered figure pg 282 Quantitative Chemiml Analysis Seventh Edilian c 2007 w HFreeman and Company What about the dashed equation We use AG values to determine the E value for the unknown reaction We pick reactions from the diagram that start With the same species and add up to get to the same product Use 103 e 12 amp 12 e 1 Now let s probe a little further There s equilibria that exits between two half cells and equilibria that exits within each half cell As long as a cell has a nonzero voltage equilibrium has not been established between the two half cells But equilibrium can be established within each half cell given enough time The voltage of a cell then senses the equilibrium concentrations of each species in each half cell Thus we can determine unknown equilibrium concentrations by measuring the voltage of a cell 14 37 Given the following cell calculate Kb for the organic base RNH2 if the cell voltage is 0490 V Pts I H2100 bar I RNH2aq 010 M RNH3C1aq 0050 M II SHE
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