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Chapter 14 – Fundamentals of Electrochemistry Homework - Due Friday, April 1 Problems: 14-4, 14-5, 14-8, 14-12, 14-15, 14-17, 14-18, 14-25, 14-26, 14-41, CHM 320 - Lecture 23 Chapt 14 Electrochemistry Review of the Basics • Oxidation • Reduction – Loss of electrons – Always occurs at the anode – Happens because of the action of a reducing agent – Gain of electrons (charge is reduced) – Always occurs at the cathode – Happens because of the action of the oxidizing agent Fe+3 + e- = CHM 320 - Lecture 23 Chapt 14 Fe+2 Redox Reaction ox1 + Example: red2 = red1 + ox2 2(e- + Fe+3 = Fe+2) + Sn+2 = Sn+4 + 2 e-----------------------------------------2 Fe+3 + Sn+2 = 2 Fe+2 + Sn+4 So… +2, red =Fe+2, ox1= Fe+3 CHM , 320red = Sn - Lecture 2 23 Chapt 14 1 ox =Sn+4 Electric Charge (in Coulombs) and Work • Voltage represents electrical • The charge in potential (potential to do coulombs (q) is equal work) to the number of moles of electrons (n) • If some total charge in coulombs (q) is moved times the Faraday through some electrical Constant (F) potential (E, in volts V) then work is done! q (coulombs) n (moles) x F (Faraday Constant) 4 Coulombs F 9.649E10 mole of e Work (joules) E (volts) x q (coulombs) CHM 320 - Lecture 23 Chapt 14 Ohm’s Law and Power • Ohm’s law relates electrical resistance, current and potential! • Power is the work done in some unit time (e.g. joules of work per second) • The units of Power are Watts (W) • Ohm’s law and power are related! E(potential) I (current) x R (resistanc e, in Ohms, ) Work (joules) E x q Power (in Watts) second s Exq q E x E xI s s CHM 320 - Lecture 23 Chapt 14 Let’s Work Some Problems • A 6.00V battery is connected across a 2.00 K resistor, how many electrons flow through the circuit per second? • How many joules of heat (heat is work) are produced per electron? • What voltage would the battery need to be to deliver a power at 100.0 Watts? CHM 320 - Lecture 23 Chapt 14 Electrochemical Cells • A complete cell contains: – anode – cathode – completed circuit (for electrons to flow) – a salt bridge (usually!) – an electrolyte solution – chemical species that undergo reaction. • There are two basic electrochemical cells: – A GALVANIC cell uses spontaneous chemical reactions to generate electricity – A ELECTROYLTIC cell requires an electrical potential to be applied to the cell to drive some reaction. CHM 320 - Lecture 23 Chapt 14 Galvanic Cell Cells and Cell Reactions Overall Cell Reaction Zn(s) + Cu+2(aq) ---> Zn+2(aq) + Cu(s) oxidation half reaction anode Zn(s) ---> Zn+2(aq) + 2 ereduction half reaction cathode Cu+2(aq) + 2 e- ---> Cu(s) CHM 320 - Lecture 23 Chapt 14 CHM 320 - Lecture 23 Chapt 14 CHM 320 - Lecture 23 Chapt 14 What is happening at the electrode(s) and how do we describe the cell? Anode half - rxn : Zn 0(s) Zn 2 (aq) 2 e 2 Cathode half - rxn : Cu(aq) 2 e - Cu0(s) Complete Cell reaction : Zn (s) Cu2 (aq) Zn 2 (aq) Cu0 (s) in shorthand, we use symbols! a single vertical line marks the phase difference a double vertical line marks the salt bridge Anode on the left, cathode on the right Including the counter - ions tells us something about the solutions Zn (s) | ZnSO 4(aq) || CuSO4(aq) | Cu(s) CHM 320 - Lecture 23 Chapt 14 The Standard Hydrogen Electrode (SHE) • The basis by which all other measurements are made. • Assigned a potential of zero by definition! • Not practical for regular use Hydrogen Half-Cell H2(g) = 2 H+(aq) + 2 ereversible reaction SHE consists of a platinum electrode covered with a fine powder of platinum around which H2(g) is bubbled. Its potential is defined as zero volts. CHM 320 - Lecture 23 Chapt 14 CHM 320 - Lecture 23 Chapt 14 CHM 320 - Lecture 23 Chapt 14 Standard Potentials • Standardized potentials (Eo), listed as reductions, for all half-reactions • Measured versus the S.H.E (0) • Used in predicting the action in either a galvanic cell or how much energy would be needed to force a specific reaction in a non-spontaneous cell • Assumes an activity of one for the species of interest (usually a fair approximation) at a known temperature in a cell with the S.H.E. • Assumes that the cell of interest is connected to the (+) terminal of the potentiometer (voltmeter) and the S.H.E. is connected to the (-) terminal CHM 320 - Lecture 23 Chapt 14 Better Oxidizing Agents in upper left hand corner. CHM 320 - Lecture 23 Chapt 14 Better Reducing Agents in lower Right hand corner