241section4-10-6

241section4-10-6 - Chem 241 Summer 2010 6/9/2010...

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Chem 241 Summer 2010 6/9/2010 1 Definitions Electrochemistry is the use of electricity to: »make chemical measurement 1 »make chemical measurements »drive chemical reactions Definitions - Electricity Basics of electricity »unit of charge - coulomb (C) blf h i t i 2 »symbol for charge in an equation - q »1 electron = 1.60217733 x 10 -19 C »1 mole of electrons = a Faraday (F) »F = 9.6485309 x 10 4 C/mole »q = nF Definitions - Electricity Current - quantity of charge flowing through a circuit per second »C/second = ampere (amp A 3 »C/second = ampere (amp, A) Difference in electrical potential (E) is measured in volts (V) moving a charge from 1 electrical potential to a higher potential = work Definitions - Electricity Work = Eq »Work (joules, J) E( lt 4 »E (volts) »q (coulombs) Work = - G = Eq G = -nFE Definitions - Electricity Ohm’s law »V = IR V potential difference (volts 5 V - potential difference (volts) I - current (amps) R - resistance (ohms) P = VI = I 2 R P = Power (watts, J/s) Definitions - Galvanic Cell A galvanic cell uses a spontaneous chemical reaction to generate electricity 6 one reactant must be oxidized, i.e. lose an electron(s) one reactant must be reduced, i.e. gain an electron(s)
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Chem 241 Summer 2010 6/9/2010 2 Definitions - Galvanic Cell The reduced species is an oxidant or oxidizing agent The oxidized species is a reductant or 7 The oxidized species is a reductant or reducing agent Definitions - Galvanic Cell Fe 3+ + V 2+ Fe 2+ + V 3+ Break reaction down into two hl ti 8 half-reactions Fe 3+ + e - Fe 2+ V 2+ V 3+ + e - ____________________________________________________________________________________________________ Fe 3+ + V 2+ Fe 2+ + V 3+ Definitions - Galvanic Cell V - + potentiometer e - 9 Cd AgCl Ag Cd 2+ + 2Cl - electrode electrode Definitions - Galvanic Cell Chemical species that might react: »Cd »Ag 10 »AgCl 2+ »Cl - What species change oxidation state? Definitions - Galvanic Cell Find half-reactions in Appendix H Cd(s) Cd 2+ (aq) + 2e - 2 - + 2AgCl(s) 2Ag(s) + 2C - (aq 11 2e + 2AgCl(s) 2Ag(s) + 2Cl (aq) ______________________________________________________________________________________ Cd + 2AgCl(s) Ag(s) + 2Cl - (aq) + Cd 2+ electrons flow from Cd electrode to Ag/AgCl electrode Definitions - Galvanic Cell Cd(s) Cd 2+ (aq) + 2e - 2e - + 2AgCl(s) 2Ag(s) + 2Cl - (aq) ______________________________________________________________________________________ Cd + 2AgCl(s) 2Ag(s) + 2C - (aq) + Cd 2+ 12 Cd + 2AgCl(s) 2Ag(s) + 2Cl (aq) + Cd electrode where oxidation occurs is the anode electrode where reduction occurs is cathode
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Chem 241 Summer 2010 6/9/2010 3 Definitions - Galvanic Cell V - + Cd Cd 2+ + 2e - 2e - + 2Ag + 2Ag __________________________________________________________________________________ 2Ag + +Cd 2Ag+Cd 2+ e - 13 Cd Ag Cd 2+ + 2Cl - + Ag + + NO 3 - + Cd 2Ag + Cd Ag + reacts at Cd electrode Definitions - Galvanic Cell V - + e - 14 Cd Ag Cd 2+ + Cl - Ag + + NO 3 - Excess Cd 2+ , positive charge attracts electrons Excess NO 3 - , negative charge repels electrons Definitions - Galvanic Cell V - + e - Salt Bridge 15 Cd Ag Cd 2+ + Cl - Ag + + NO 3 - Cl - K + Definitions - Galvanic Cell V - + e - Cl - K + 16 Cd Ag Cd 2+ + Cl - Ag + + NO 3 - Cd(s) |Cd 2+ ,Cl - (aq) | |Ag + , NO 3 - (aq) | Ag(s) Definitions - Galvanic Cell Shorthand to describe galvanic cells » | = phase boundary || ltbid 17 » | | = salt bridge Previous cell Cd(s) |Cd 2+ ,Cl - (aq) | |Ag + , NO 3 - (aq) | Ag(s) Typically write anode on left Standard Reduction Potential To determine the potential difference we need a reference potential - electrical ground 18
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241section4-10-6 - Chem 241 Summer 2010 6/9/2010...

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