Oxidation - Reduction Reactions aka “Redox” corresponding to chapter 20 in text

Oxidation - Reduction reactions Redox reactions involve –Complete or partial transfer of electrons –Change in oxidation numbers Oxidation numbers are pseudo charges

Rules for Oxidation Numbers Usually is the charge the atom would have if it was an ion Monatomic ion = ionic charge H is always 1+ unless bound to metal (1-) O is always 2- unless in peroxide (1-) Free elements are always 0 (HONClBrIF too) Σ of n eutral compounds constituent oxidation numbers = 0 Σ of polyatomic constituent oxidation numbers = charge

Assigning Oxidation Numbers NaClOH 2 SO 4 H 2 ONaCl NaCrO 2 NaOH H 2 O 2 Na 2 CrO 4 OH -1 Ag HNO 3 AgNO 3 NOCa(OH) 2 Cl 2 CaCl 2 Ca(ClO 3 ) 2 KNO 3

LEO the lion says GER Can’t have a growl without a lion –Can’t have oxidation without reduction –Paired reaction

LEO the lion… L = Lose E = Electrons O = Oxidation Oxidation: Loss of electrons

LEO the lion… Fe +2  Fe +3 + e - –Oxidation number becomes more positive –Gained oxygen –Shifts electrons away

…says GER G = Gain E = Electrons R = Reduction Reduction: Gain of electrons

…says GER Fe +3 + e -  Fe +2 –Oxidation number becomes more negative –Loss of oxygen –Shifts electrons towards

Why do Lions Growl? Oxidation and reduction occur together –Why? Presence of oxidizing and reducing agents

Agents Oxidizing Agent –Substance that is being reduced Hence, causing the oxidation Reducing Agent –Substance that is being oxidized Causing the reduction

Balancing Redox Reactions 2 methods –H–Half reaction (preferred by chemists) –O–Oxidation number change (preferred by lazy students and can lead you astray)

Half Reaction (from book)

Half Reaction (basics) Separate the equation into the oxidation portion and the reduction portion Balance both separately for e- gained or lost Multiply each ½ rxn to yield a balanced overall e- shift Put coefficients back up into the original eq Balance “spectators” for atoms

Try It! Write the balanced redox reaction for the following: Na(s) + FeCl 2 (aq)  NaCl(aq) + Fe(s)

Oxidation Number Change Method Step 1: assign oxidation numbers Step 2: ID atoms being oxidized and reduced Step 3: use brackets to connect the oxidation (loss of e-) and reduction (gain in e-)

Same eq… but using ox number change method Na(s) + FeCl 2 (aq)  NaCl(aq) + Fe(s)