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 reductant (reducing agent) and which is an oxidant (oxidizing agent)? A.PbS, reductant; O 2, oxidant B.PbS, reductant; SO 2, oxidant C.Pb 2+, reductant; S 2- oxidant D.PbS, reductant; no oxidant E.PbS, oxidant; SO 2, reductant

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

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) E º = 0.77 V Cu +2 (aq)+2e -  Cu(s) E º = 0.34 V Cu(s)  Cu +2 (aq)+2e - E º = V 2Fe +3 (aq) + 2e -  2Fe +2 (aq) E º = 0.77 V

Reduction potential More negative E º – more easily electron is added – More easily reduced – Better oxidizing agent More positive E º – more easily electron is lost – More easily oxidized – Better reducing agent

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.

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

In a galvanic cell, the electrode that acts as a source of electrons to the solution is called the __________; the chemical change that occurs at this electrode is called________. a. cathode, oxidation b. anode, reduction c. anode, oxidation d. cathode, reduction

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+

Galvanic Cell l The reaction always runs spontaneously in the direction that produced a positive cell potential. l Four things for a complete description. 1)Cell Potential 2)Direction of flow 3)Designation of anode and cathode 4)Nature of all the components- electrodes and ions

Practice l Completely describe the galvanic cell based on the following half-reactions under standard conditions. MnO H + +5e -  Mn H 2 O E º=1.51 V Fe +3 +3e -  Fe(s) E º=0.036V

Standard Cell Potentials l Electric Potential - (E) difference between the potential energy at the anode and the potential energy at the cathode. – Also called cell voltage or cell potential – measured in volts – this is a measurement of the spontaneous flow of electrons - redox requires no outside energy to move electrons (redox - 2 half reactions) – Is like a water current - from high potential to low potential

Cell Potential l If a galvanic cell has a cell potential of 0 V, how many electrons are going to flow? l 0 electrons - there is no potential difference between electrodes (between anode and cathode)

Calculating Cell Potentials l Each half-reaction is written as a reduction. The half-cell potential for a reduction half- reaction is called a reduction potential l The numerical values of half-cell potentials depend on various conditions. So, these values are listed when ions and molecules are in standard states (just as it is for tables of standard enthalpy changes).

Standard State means... – Standard concentrations are 1 mol/L – Standard temperature is 298 K. – Standard reduction potentials are designated by the symbol E°

Calculating Standard Cell Potentials l METHOD 1: E° cell = E° cathode - E° anode l METHOD 2: E° cell = E° redox + E° ox

Sample Problem V Calculate the standard cell potential for the galvanic cell in which the following reaction occurs: