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Oxidation - Reduction Reactions. a ka “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.
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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 neutral compounds constituent oxidation numbers = 0 • Σ of polyatomic constituent oxidation numbers = charge
Assigning Oxidation Numbers NaClO H2SO4 H2O NaCl NaCrO2 NaOH H2O2 Na2CrO4 OH-1 Ag HNO3 AgNO3 NO Ca(OH)2 Cl2 CaCl2 Ca(ClO3)2 KNO3
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 oxidizingand 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 • Half reaction • (preferred by chemists) • Oxidation number change • (preferred by lazy students and can lead you astray)
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) + FeCl2(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 0 +2-1 +1 -1 0 • Na(s) + FeCl2(aq) NaCl(aq) + Fe(s)