Majors analytical chemistry, note set 5
Majors analytical chemistry, note set 5 2154
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This 2 page Class Notes was uploaded by Thomas Salazar on Sunday October 9, 2016. The Class Notes belongs to 2154 at Virginia Polytechnic Institute and State University taught by Dr. Amanda Morris in Fall 2016. Since its upload, it has received 13 views. For similar materials see Majors Analytical Chemistry in Chemistry at Virginia Polytechnic Institute and State University.
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Date Created: 10/09/16
Note Set #5 – week 7 Analytical Chemistry ELECTROCHEMISTRY Gibbs Free Energy: ΔG = ΔH – TΔS (units in kJ, kJ/mol, K, and J/K) o ΔH: change in enthalpy of rxn. o ΔS: change in entropy of rxn. 1 Joule = 1 Volt•1 Coulomb ΔS = S productsS reactants o T: temperature in Kelvin Work (W) = ΔE • q where “ΔE” is change in electric potential in volts (V) and “q” is charge in Coulombs (C) o q = nF where “n” is moles of electrons and “F” is Faraday Constant: 96,485 C/mol e o Therefore; W = nFΔE o Also, W = -ΔG; so ΔG = -nFΔE Power (P) o Measured in (watts): 1 watt = 1 Joule/sec P = (ΔE • q) / sec ΔE ???? ΔE • I Power = Voltage x Current ???????????? P = ΔE • I Resistance/Ohm’s Law o V = IR : electric potential (volts) = current (amps) x resistance (ohms) o Rearranging eqns. we know, we get a power eqn. as well; P = I R Reduction potentials @ standard conditions: o Make sure when balancing half rxns. that elements, electrons, and charges all balance when applied to the net reaction o When writing standard reduction potentials (E°), always write half rxns as the reduction and use the value given. Do not change any signs, or flip any numbers. *** E°cell E° red- E°ox *** (Read as) The standard cell potential of a galvanic cell [a cell that produces a spontaneous electrochemical rxn] is calculated as: the std. reduction potential of the species being reduced minus the std. reduction potential of the species being oxidized. o This eqn. for a galvanic cell should always have the difference of the oxidant and the reductant be the largest, most positive value possible Reduction potentials @ non-standard conditions: o Follow the same rules as std. state reactions o –RTln(K) = -nFE° cell ** E° cell???????? ln ???? ** ???????? o Nernst Equation: ???????? E cell E°cell ???????????????? ????) Note Set #5 – week 7 Analytical Chemistry Forming Galvanic Cells: half cells must be separated and connected by a salt bridge to maintain charge balance, and allow electron transfer o Notation for galvanic cells 3+ e.g.) A rxn of Al (aq) in 0.01M Al(NO ) wi3 3Br (l) i2 0.1M HBr using a Pt(s) electrode: Al(s)l(NO ) 3 30.01M)//HBr, (0.1M)/Br 2(l)Pt(s) o A / represents a phase change, // represents a salt bridge separating the two half rxns. Solid phase elements always go on the outside. o In galvanic cells, the left side is the species being oxidized, and acts as the anode o The right side species is being reduced and acts as the cathode Calculating cell potentials in cells at non-std. conditions: ????.???????????? o Base 10 form of Nernst eqn: E cell E° cell ???????????? ????) ???? 1. Write out two half rxn eqns, and calculate the equilibrium quotient “Q” 2. Using std. reduction potentials of the half rxns, calculate the half-cell reduction potentials (E 1/2)for each half rxn at given conditions using one of Nernst’s eqns 3. Determine which rxn is the reduction and which is the oxidation 4. Using the calculated half-cell potentials, plug the values into the formula, E cell E red- E ox to calculate the cell potential 5. Using the E cell one can calculate the Gibb’s free energy of the cell, and determine if it is galvanic or electrolytic (spontaneous or non-spontaneous)
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