Electrochemistry (Ch 17) - AP Chemistry

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4/1/2011 A GALVANIC CELL A GALVANIC CELL • Unfortunately, this set up won’t work. – Electrons will flow from Fe 2+ to MnO 4- , but charge will build up in each compartment and stop the flow of electrons • A simple solution is to connect the solutions so that ions can flow to keep the net charge in each compartment zero. – A salt bridge is a U-tube filled with an electrolyte that will allow charges to flow from one compartment to another to keep the net charge zero – A porous disk (or cup in LAB 17) can also be used to do the same thing • A galvanic cell is a device in which chemical energy is changed to electrical energy – Uses a spontaneous redox reaction to produce a current that can be used to do work – The reverse non-spontaneous process (converting electrical energy into chemical energy) is called electrolysis (Section 17.7) • You remember LEO goes GER, but do you remember what color he is Ch 17 - Electrochemistry 7 Ch 17 - Electrochemistry 8 A GALVANIC CELL CELL POTENTIAL • “LEO is a RED CAT” – The electrode compartment in which reduction occurs is called the cathode – The electrode compartment in which oxidation occurs is called the anode • Reduction occurs at the cathode. Oxidation occurs at the anode. – Electrons are pulled from the anode to the cathode • The “pull” on the electrons is called the cell potential (E cell ) or the electromotive force (emf) of the cell. – Unit of electric potential is the volt (V) • 1 V = 1 J/C (joule per coulomb) – Measured with a voltmeter, but frictional heating of the resisters wastes some of the useful energy of the cell, so the reading will always measure less than the maximum cell potential – A potentiometer measures the cell potential under conditions of zero current with a variable-voltage device powering the reverse reaction – Modern digital voltmeters draw only a negligible amount of current Ch 17 - Electrochemistry 9 Ch 17 - Electrochemistry 10 STANDARD REDUCTION POTENTIALS Section 17.2 STANDARD REDUCTION POTENTIALS • A galvanic cell’s redox reaction can always be broken down into two halfreactions. The sum of the potential (E ) of each half-reaction equals the cell potential (E cell ). – For example, this cell reaction is: 2 H + (aq) + Zn(s) Zn 2+ (aq) + H 2 (g) – The anode’s reaction (oxidation) is: Zn Zn 2+ + 2 e - – The cathode’s reaction (reduction) is: Ch 17 - Electrochemistry 11 2 H + 2 e - H 2 • This cathode uses a Pt electrode in contact with 1 M H + bathed in H 2 (g) at 1 atm and is called the standard hydrogen electrode Ch 17 - Electrochemistry 12 2
4/1/2011 STANDARD REDUCTION POTENTIALS • The total potential (E cell ) can be measured (0.76 V), but there is no way to measure the individual electrode potentials. – To calculate potentials for each half-reaction, the reaction 2 H + 2 e - H 2 • where [H + ] = 1 M and P H2 = 1 atm – has been assigned a potential of 0V – E anode can be calculated because E° cell = E° anode + E° cathode • As a result, the reaction, ZnZn 2+ + 2 e - has a potential of 0.76 V • Using the standard hydrogen potential as a reference, all other potentials can be determined experimentally. Ch 17 - Electrochemistry 13 STANDARD REDUCTION POTENTIALS • What is the potential of the cathode – Anode: Zn Zn 2+ + 2 e - – Cathode: Cu 2+ + 2 e - Cu • Since E° cell = E° anode + E° cathode – E° cat = 1.10 V – 0.76 V = 0.34 V • By convention, the potentials of half reactions are given as reduction potentials and are tabulated in standard states in a list of standard reduction potentials Ch 17 - Electrochemistry 14 STANDARD REDUCTION POTENTIALS STANDARD REDUCTION POTENTIALS • Combining two half-reactions to obtain a balanced oxidationreduction reaction often requires two manipulations: – One of the reduction half-reactions must be reversed (since redox reactions must involve a substance being oxidized and a substance being reduced). • The half-reaction with the largest positive potential will run as written (as a reduction) • The other half-reaction will be forced to run in reverse (as an oxidation) • The net potential of the cell will be the difference between the two: E° cell = E° cathode – E° anode – Or “change the sign and add” (by changing the sign of the oxidation reaction) Ch 17 - Electrochemistry 15 STANDARD REDUCTION POTENTIALS • Combining two half-reactions to obtain a balanced oxidationreduction reaction often requires two manipulations: – Since the number of electrons lost must equal the number gained, the halfreactions must be multiplied by integers as necessary to achieve the balanced equation. However, the value of E° is not changed when a halfreaction is multiplied by an integer. • Since a standard reduction potential is an intensive property, the potential is not multiplied by the integer. STANDARD REDUCTION POTENTIALS • Consider a galvanic cell based on the redox reaction Fe 3+ (aq) + Cu(s) Cu 2+ (aq) + Fe 2+ (aq) – The half reactions are: Fe 3+ + e - Fe 2+ E° = 0.77 V (1) Cu 2+ + 2 e - Cu E° = 0.34 V (2) – To balance the cell reaction and calculate the standard cell potential, reaction (2) must be reversed and multiply reaction (1) by two: Cu Cu 2+ + 2 e - – The cell reaction becomes: -E° = -0.34 V 2 Fe 3+ + 2 e - 2 Fe 2+ E° = 0.77 V Cu(s) + 2 Fe 3+ (aq) Cu 2+ (aq) + 2 Fe 2+ (aq) E° cell = 0.43 V Ch 17 - Electrochemistry 18 3
Page 1: 4/1/2011 ELECTROCHEMISTRY Chapter 1
Page 5 and 6: 4/1/2011 CELL POTENTIAL AND WORK
Page 7 and 8: 4/1/2011 CALCULATING E FOR NON-STAN
Page 9 and 10: 4/1/2011 NICKEL-CADMIUM BATTERY •
Page 11: 4/1/2011 ELECTROLYSIS OF MIXTURES O

Electrochemistry (Ch 17) - AP Chemistry

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