CHM Lecture 23 Chapt 14 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 Lecture 23 Chapt 14 Fe +3 + e - = Fe +2 Electrochemistry Review of the Basics Oxidation –Loss of electrons –Always occurs at the anode –Happens because of the action of a reducing agent Reduction –Gain of electrons (charge is reduced) –Always occurs at the cathode –Happens because of the action of the oxidizing agent

CHM Lecture 23 Chapt 14 2(e- + Fe +3 = Fe +2 ) + Sn +2 = Sn e Fe +3 + Sn +2 = 2 Fe +2 + Sn +4 So… ox 1 = Fe +3, red 2 = Sn +2, red 1 =Fe +2, ox 2 =Sn +4 Redox Reaction ox 1 + red 2 = red 1 + ox 2 Example:

CHM Lecture 23 Chapt 14 Electric Charge (in Coulombs) and Work The charge in coulombs (q) is equal to the number of moles of electrons (n) times the Faraday Constant (F) Voltage represents electrical potential (potential to do work) If some total charge in coulombs (q) is moved through some electrical potential (E, in volts V) then work is done! 4

CHM 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!

CHM 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 Watts?

CHM 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 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 anodeZn (s) ---> Zn +2 (aq) + 2 e - reduction half reaction cathodeCu +2 (aq) + 2 e - ---> Cu (s)

CHM Lecture 23 Chapt 14

What is happening at the electrode(s) and how do we describe the cell?

CHM 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 H 2(g) = 2 H + (aq) + 2 e - reversible reaction SHE consists of a platinum electrode covered with a fine powder of platinum around which H 2(g) is bubbled. Its potential is defined as zero volts.

CHM Lecture 23 Chapt 14

Standard Potentials Standardized potentials (E o ), 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 Lecture 23 Chapt 14 Better Oxidizing Agents in upper left hand corner. Better Reducing Agents in lower Right hand corner