OXIDATION-REDUCTION REACTIONS (REDOX)

A Change in Charge

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REDOX BASICS• transfer of electrons between two atoms

Na F

Oxidation Numbers

• the ox. # of free elements is ZERO (including diatomics like O2 and polyatomics like P4 or S8)

• the ox. # of a monatomic ion = its charge (like Na+1)• Other elements:

o O = -2 (except in peroxide, each O = -1)o H = +1 (except in metal hydrides, then H = -1)o Halogens = -1 (except when central atom of

polyatomic ions, like ClO3-1)

Identify the oxidation number for the underlined element

• H2O

• H2O2

• FeSO4

• FeSO4

• HClO• HClO2

• HClO3

• HClO4

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Is it Oxidation or Reduction or neither?

1. Silver ions change to raw (elemental) silver

2. Hydrogen gas changes to hydrogen ions

3. Blue CrCl2 changes to green CrCl3

4. Yellow K2CrO4 changes to orange K2Cr2O7

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Oxidation-Reduction ReactionsSingle Replacement: Zn(s) + HCl(aq) ZnCl2(aq) + H2(g)

Synthesis: Mg(s) + O2(g) MgO(s)

Decomposition: Fe2O3(s) Fe(s) + O2(g)

Combustion: CH4(g) + O2(g) CO2(g) + H2O(g)

These types of reactions can be balanced by inspection

Redox reactions in acid/base solutions

MnO4- + S2O3

-2 + H+ Mn+2 + SO4-2 + H2O

Br2 + OH- Br- + BrO3- + H2O

S8 + O2 + H2O SO4-2 + H+

These will be balanced using the method of half-reactions

Balancing with Half-Reactions1. Split the reaction into half reactions involving similar species

(ignore H+, OH-, and H2O)

2. Balance atoms of all elements EXCEPT O and H3. Balance O by adding H2O to the side that has fewer O atoms

4. Balance H by adding H+ to the side that has fewer H atomsa. If acidic solution: continue on to step 5b. If basic solution: add enough OH- to the same side that you

added H+ to. Change H+ and OH- to H2O. Add same amount of OH- to the other side.

5. Balance by CHARGEa. Determine total charge on each side of each half-reactionb. Add electrons to the more positive sidec. Use a multiplier to get electrons in each reaction to be equal

6. Add half reactions together and cancel any terms that are the same on opposite sides.

MnO4- + S2O3

-2 + H+ Mn+2 + SO4-2 + H2O

Br2 + OH- Br- + BrO3- + H2O

Fe+2 + Cr2O7-2 Fe+3 + Cr+3

(in acidic solution)

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S8 S2O3-2 + S-2

(in basic solution)

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