Chemistry 1011 Introductory Chemistry II http www mi

Chemistry 1011 Introductory Chemistry II http: //www. mi. mun. ca/~pfisher/chemistry. html Password for final exams Midgley Chemistry 1011 Slot 5 1

Chemistry 1011 TOPIC Electrochemistry TEXT REFERENCE Masterton and Hurley Chapter 18 Chemistry 1011 Slot 5 2

18. 2 Standard Voltages YOU ARE EXPECTED TO BE ABLE TO: • Define the standard electrode potential of a half cell • Order species according to their ease of oxidation or reduction based on a table of standard reduction potentials • Calculate the net cell voltage, Eo, of a combination of half cells from standard electrode potential data • Determine whether a given redox reaction will be spontaneous or non-spontaneous Chemistry 1011 Slot 5 3

Cell Voltage • The force that pushes the electrons through the external circuit of a cell is known as the – Potential difference, or – Electromotive force (emf), or – Voltage • It is measured in volts • The magnitude of the voltage depends on – The nature of the redox reaction – The concentrations of the ions in solution, (or pressures of any gases) Chemistry 1011 Slot 5 4

Standard Voltage • In order to compare the voltages of different cells, or to calculate the expected voltage of a given cell, measurements are taken under standard conditions: – Current flow is almost zero – All ions and molecules in solution are at a concentration of 1. 0 mol/L – All gases are at a pressure of 1. 0 atm Chemistry 1011 Slot 5 5

The Zinc – Hydrogen Voltaic Cell Chemistry 1011 Slot 5 6

The Standard Voltage of the Zinc – Hydrogen Voltaic Cell Zn(s) + 2 H+(aq) 1. 0 mol/L Zn 2+(aq) + H 2(g) 1. 0 mol/L 1. 0 atm Zn | Zn 2+ || H+ | H 2 | Pt • Cell voltage with no current flowing is +0. 762 V • This is the standard voltage for this cell Zn(s) + 2 H+(aq, 1. 0 M) Zn 2+(aq, 1. 0 M) + H 2(g, 1. 0 atm) Eo = +0. 762 V Chemistry 1011 Slot 5 7

Standard Half Reaction Voltages • Each half reaction has a standard voltage • Eoox (standard oxidation voltage) • Eored (standard reduction voltage) Eo = Eoox + Eored • Only Eo can be measured - the standard voltage of a half reaction cannot be measured directly Chemistry 1011 Slot 5 8

Obtaining Values for Standard Half Reaction Voltages • Standard half reaction voltages are determined by arbitrarily assigning the value of zero to the standard reduction half reaction for hydrogen ions to give hydrogen gas 2 H+(aq, 1. 0 M) + 2 e- H 2(g, 1. 0 atm) Eored (H+ H 2) = 0. 000 V Since Eo = Eoox + Eored Eoox (Zn Zn 2+) = +0. 762 V Chemistry 1011 Slot 5 9

Obtaining Values for Standard Half Reaction Voltages • Once one half reaction standard voltage is established, others can be deduced: • For: Zn(s) + Cu 2+(aq) Zn 2+(aq) + Cu(s) standard cell voltage is +1. 101 V Zn(s) Zn 2+(aq, 1. 0 M) + 2 e. Cu 2+(aq, 1. 0 M) + 2 e- Cu(s) the Eoox (Zn Zn 2+) = +0. 762 V Eored (Cu 2+ Cu) = ? ? V Since Eo = Eoox + Eored +1. 101 V = +0. 762 V + Eored = +0. 339 V Chemistry 1011 Slot 5 10

Standard Reduction Potentials • Standard half cell voltages are found in tables of standard potentials • These are the values for reduction half reactions based upon the convention that 2 H+(aq, 1. 0 M) + 2 e- H 2(g, 1. 0 atm) Eored (H+ H 2) = 0. 000 V • Standard reduction potential = Eored Chemistry 1011 Slot 5 11

Standard Reduction Potentials Oxidizing Agent Li+(aq) + e. Na+(aq) + e. Zn 2+(aq) + 2 e. Ni 2+(aq) + 2 e 2 H+(aq) + 2 e. Cu 2+(aq) + 2 e. Ag+(aq) + e. NO 3 -(aq) + 4 H+(aq) + 3 e. Mn. O 4 -(aq) + 8 H+(aq) + 5 e. F 2(g) + 2 e- Reducing Agent Li(s) Na(s) Zn(s) Ni(s) H 2(g) Cu(s) Ag(s) NO(g) + 2 H 2 O Mn 2+(aq) + 4 H 2 O 2 F-(aq) Chemistry 1011 Slot 5 Eored (V) -3. 040 -2. 714 -0. 762 -0. 236 0. 000 +0. 339 +0. 799 +0. 964 +1. 512 +2. 889 12

Standard Reduction Potentials • Elements above hydrogen in the table of standard reduction potentials will react with a solution of hydrogen ions to produce hydrogen gas M(s) + 2 H+(aq) M 2+(aq) + H 2(g) M 2+(aq) + 2 e- M(s) Eored = negative M(s) M 2+(aq) + 2 e. Eoox = positive 2 H+(aq) + 2 e- H 2(g) Eored = 0. 000 V M | M 2+ || H+ | H 2 | Pt Eocell = positive • Elements below hydrogen in the table of standard reduction potentials will NOT react with a solution of hydrogen ions to produce hydrogen gas Chemistry 1011 Slot 5 13

Standard Voltages for Voltaic Cells • The table of standard reduction potentials gives standard voltages for reduction half reactions • Standard voltages for oxidation half reactions are obtained by reversing these reactions and changing the sign of the Eored value • If: Zn 2+(aq) + 2 e Zn(s) Eored = -0. 762 • Then: Zn(s) Zn 2+(aq) + 2 e. Eoox = +0. 762 Chemistry 1011 Slot 5 14

Computing Standard Cell Potential • The standard voltage of a cell is the sum of the standard potentials for the two half reactions • For the cell: Zn | Zn 2+ || Cu 2+ | Cu Zn(s) Zn 2+(aq) + 2 e. Eoox = +0. 762 V Cu 2+(aq) + 2 e- Cu(s) Eored = +0. 339 V Zn(s) + Cu 2+(aq) Zn 2+(aq) + Cu(s) • Eocell = Eoox + Eored = + 0. 762 + 0. 339 = 1. 101 V Chemistry 1011 Slot 5 15

Oxidizing Agents • An oxidizing agent is a species that can gain electrons – The strongest oxidizing agents are the species that gain electrons most readily – They have the largest positive Eored values – Oxidizing strength increases moving down the left column of the table of standard reduction potentials – Oxidizing agents in the table of standard reduction potentials can oxidize any species above Chemistry 1011 Slot 5 16

Reducing Agents • A reducing agent is a species that readily loses electrons – The strongest reducing agents are the species that lose electrons most readily – They have the largest negative Eored values (The largest positive Eoox values) – Reducing strength increases moving up the right column of the table of standard reduction potentials – Reducing agents in the table of standard reduction potentials can reduce any species below Chemistry 1011 Slot 5 17

Strong Reducing and Oxidizing Agents Reducing agent causes another species to be reduced - it is oxidized Li(s) Li+(aq) + e. Eoox = +3. 040 V Oxidizing agent causes another species to be oxidized - it is reduced F 2(g) + 2 e- R 2 F-(aq) Eored = +2. 889 V Table of Standard Reduction Potentials R = strongest reducing agent O = strongest oxidizing agent O Chemistry 1011 Slot 5 18

Spontaneity of Redox Reactions • In order for a redox reaction to occur spontaneously, the calculated cell potential MUST BE POSITIVE • Questions: – Will copper metal be oxidized to Cu 2+ ions by dilute hydrochloric acid? – Will copper metal be oxidized to Cu 2+ ions by dilute nitric acid? Chemistry 1011 Slot 5 19

Reaction of Copper with Dilute Hydrochloric Acid? ? • Possible oxidation half reaction: Cu(s) Cu 2+(aq) + 2 e. Eoox = -0. 339 V • Possible reduction half reaction (H+ and Cl- ions are present - Cl- ions cannot be reduced): 2 H+(aq) + 2 e- H 2(g) Eored = 0. 000 • Net possible reaction: Cu(s) + 2 H+(aq) Cu 2+(aq) + H 2(g) • Net calculated cell voltage Eocell = Eoox + Eored = - 0. 339 + 0. 000 = - 0. 339 V • Reaction will not be spontaneous i. e no reaction Chemistry 1011 Slot 5 20

Reaction of Copper with Dilute Nitric Acid? ? • Possible oxidation half reaction: Cu(s) Cu 2+(aq) + 2 e- Eoox = -0. 339 V • Possible reduction half reactions (H+ and NO 3 - ions are present): 2 H+(aq) + 2 e- H 2(g) Eored = 0. 000 V NO 3 -(aq) + 4 H+(aq) + 3 e- NO(g) + 2 H 2 O Eored = +0. 964 V • Net spontaneous reaction (Add multiples of the two half reactions so that same #electrons (6) in each half): 3 Cu(s) + 2 NO 3 -(aq) + 8 H+(aq) 3 Cu 2+(aq) + 2 NO(g) + 4 H 2 O • Net calculated cell voltage: Eocell = Eoox + Eored = - 0. 339 + 0. 964 = + 0. 629 V • Reaction will be spontaneous i. e reaction takes place Chemistry 1011 Slot 5 21

Voltaic Cells with Inert Electrodes • Half cells will frequently be constructed with inert electrodes (often carbon or platinum) • The Hydrogen half cell is one example: H+ | H 2 | Pt • A cell with two inert electrodes might be: Pt | Fe 2+(aq) | Fe 3+(aq) || Cl -(aq) | Cl 2(g) | Pt Chemistry 1011 Slot 5 22

The Leclanché Cell • The Leclanché cell is the ordinary commercial flashlight battery Zn | Zn 2+ ||Mn. O 2 | Mn 2 O 3 | C • Anode half reaction: Zn(s) Zn 2+(aq) + 2 e- Eoox = +0. 762 V • Cathode half reaction (complex): 2 Mn. O 2(s) + 2 NH 4+(aq) + 2 e- Mn 2 O 3(s) + 2 NH 3(aq) + H 2 O Eored = +0. 7 V • Net cell voltage Eocell = 1. 5 V Chemistry 1011 Slot 5 23

Chemistry 1011 Slot 5 24