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 NaClNaCrO2 NaOHH2O2 Na2CrO4

OH-1 AgHNO3 AgNO3

NO Ca(OH)2

Cl2 CaCl2Ca(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 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

– Half reaction • (preferred by chemists)

– 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) + 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)