Electrochemistry Applications of Redox

Review l Oxidation reduction reactions involve a transfer of electrons. l OIL- RIG l Oxidation Involves Loss l Reduction Involves Gain l LEO-GER l Lose Electrons Oxidation l Gain Electrons Reduction

Solid lead(II) sulfide reacts with oxygen in the air at high temperatures to form lead(II) oxide and sulfur dioxide. Which substance is a reducing agent and which is an oxidizing agent? A.PbS, ra; O 2, oa B.PbS, ra; SO 2, oa C.Pb 2+, ra; S 2- oa D.PbS, ra; no oa E.PbS, oa; SO 2, ra

Applications l Moving electrons is electric current. 8H + +MnO Fe +2 +5e -  Mn Fe +3 +4H 2 O l Helps to break the reactions into half reactions. 8H + +MnO e -  Mn +2 +4H 2 O 5(Fe +2  Fe +3 + e - ) l In the same mixture it happens without doing useful work, but if separate

H + MnO 4 - Fe +2 l Connected this way the reaction starts l Stops immediately because charge builds up. e-e- e-e- e-e- e-e- e-e-

H + MnO 4 - Fe +2 Galvanic Cell Salt Bridge allows current to flow

H + MnO 4 - Fe +2 e-e- l Electricity travels in a complete circuit

H + MnO 4 - Fe +2 Porous Disk l Instead of a salt bridge

Reducing Agent Oxidizing Agent e-e- e-e- e-e- e-e- e-e- e-e- AnodeCathode

Cell Potential l Oxidizing agent pulls the electron. l Reducing agent pushes the electron. The push or pull (“driving force”) is called the cell potential E cell l Also called the electromotive force (emf) l Unit is the volt(V) l = 1 joule of work/coulomb of charge l Measured with a voltmeter

Zn +2 SO M HCl Anode M ZnSO 4 H + Cl - H 2 in Cathode

1 M HCl H + Cl - H 2 in Standard Hydrogen Electrode l This is the reference all other oxidations are compared to E º = 0 l º indicates standard states of 25ºC, 1 atm, 1 M solutions.

Cell Potential Zn(s) + Cu +2 (aq)  Zn +2 (aq) + Cu(s) l The total cell potential is the sum of the potential at each electrode. E º cell = E º Zn  Zn +2 + E º Cu +2  Cu l We can look up reduction potentials in a table. l One of the reactions must be reversed, so change it sign.

Cell Potential l Determine the cell potential for a galvanic cell based on the redox reaction. Cu(s) + Fe +3 (aq)  Cu +2 (aq) + Fe +2 (aq) Fe +3 (aq) + e -  Fe +2 (aq) Eº = 0.77 V Cu +2 (aq)+2e -  Cu(s) Eº = 0.34 V Cu(s)  Cu +2 (aq)+2e - Eº = V 2Fe +3 (aq) + 2e -  2Fe +2 (aq) Eº = 0.77 V l E cell 0 = 0.43 V

Line Notation solid  Aqueous  Aqueous  solid Anode on the left  Cathode on the right l Single line different phases. l Double line porous disk or salt bridge. l If all the substances on one side are aqueous, a platinum electrode is indicated. solid  Aqueous  Aqueous  Aqueous/Pt

Cu 2+ Fe +2 l For the last reaction Cu(s)  Cu +2 (aq)  Fe +2 (aq),Fe +3 (aq)  Pt(s)

Under standard conditions, which of the following is the net reaction that occurs in the cell? Cd|Cd 2+ || Cu 2+ |Cu a. Cu 2+ + Cd → Cu + Cd 2+ b. Cu + Cd → Cu 2+ + Cd 2+ c. Cu 2+ + Cd 2+ → Cu + Cd d. Cu + Cd 2+ → Cd + Cu 2+

18 Practice: Write in cell notation and diagram the cell to show e - movement and ion migration. 1. Pb + 2Fe 3+ Pb Fe 2+ Pb/Pb 2+ //Fe 3+ /Fe 2+ /Pt anode cathode oxidation reduction

19 Practice: Write in cell notation and diagram the cell to show e - movement and ion migration. 4Au + 16Cl - + 3O 2 +12H + 4AuCl H 2 O Au/AuCl 4 - //O 2 /H 2 0/Pt anode cathode oxidation reduction

20 Practice: Write in cell notation and diagram the cell to show e - movement and ion migration. Fe + 2H 2 O Fe(OH) 2 + H 2 Fe/Fe(OH) 2 //H 2 O/H 2 /Pt anode cathode oxidation reduction