Introduction to Electrochemistry Quick Video

LEO GER Lose Electron – Oxidation Gain Electron – Reduction

Oxidation – Reduction Reactions Redox (oxidation – reduction reactions) Electrons are transferred in reaction Oxidation is always paired with reduction   Oxidation Atom has lost an electron LEO Occurs at the anode Reduction Atom has gained an electron GER Occurs at the cathode Oxidation Numbers Also called oxidation states Used to keep track of electrons

Rules for Assigning Oxidation Numbers   Atoms in elemental form always have oxidation number of zero For any monatomic ion the oxidation number equals the charge on the ion Oxidation number of oxygen is usually –2 Oxidation number of hydrogen is +1 when bonded to nonmetals Oxidation number of hydrogen is –1 when bonded to metals

Oxidation number of fluorine is always –1 Oxidation number of halogens is –1 in binary compounds The sum of the oxidation numbers in neutral compounds must be 0 The sum of the oxidation numbers in an ion equals the net charge on the ion.

P2O5 NaH Cr2O72- SnBr2 Examples Assign the oxidation numbers for each of the elements   P2O5 NaH Cr2O72- SnBr2

20.3 – Voltaic Cells (Galvanic Cells) Electrons move from one cell to another to produce current Anode the electrode where oxidation occurs (loss of electrons) always assigned “–“ charge   Cathode the electrode where reduction occurs (gain of electrons) always assigned the “+” charge

Current or electrons always flows from the anode to the cathode Half – cell – compartment where oxidation or reduction happens Salt bridge – porous disc which allows migration of ions so solutions remain electrically neutral  

A video overview of Voltaic (galvanic cells)

Identify what is being oxidized and what is being reduced   Cd(s) + NiO2 (s) + 2H2O (l)  Cd(OH)2(s) + Ni(OH)2 (s) 2 H2O (l) + Al (s) + MnO4- (aq)  Al(OH)4- (aq) + MnO2 (s)

  Cr2O72- (aq) + Cl- (aq)  Cr3+ (aq) + Cl2 (g)

Cu (s) + NO3- (aq)  Cu2+ (aq) + NO2 (g)

NO2- (aq) + Al (s)  NH3 (aq) + Al(OH)4-

ClO3- (aq) + 6H+ (aq) + 6 e-  Cl-(aq) + 3 H2O (l) Example The two half reaction in a voltaic cell are:   Zn (s)  Zn2+ (aq) + 2e- ClO3- (aq) + 6H+ (aq) + 6 e-  Cl-(aq) + 3 H2O (l) Indicate which reaction occurs at the anode and at the cathode. Which electrode is consumed in the cell reaction? Which electrode is positive?