Oxidation

Process in which atoms or ions attain a more + oxidation stateEx.

Na → Na+ + eCl- → Cl + eCa+ → Ca2+ + e

Reduction

Atoms or ions of elements attain a more negative oxidation stateEx. Cl + e → Cl-

Mnemonic: “OIL RIG”Oxidation involves loss, reduction involves gain

Oxidation NumberUncombined elements have oxidation # of 0Monoatomic ions have oxidation # equal to their ion chargeF is always –1 in a compoundO is almost always –2 in a compoundH is +1 in all compounds except those with metals (then its –1)More electronegative element is assigned a number equal to its ion chargeAlgebraic sum of oxidation numbers in a compound is always zero

Practice

Determine the oxidation numbers for each element in the compounds below

NaClH2SO4

NO3-

CO2

SF6

AgNO3

Redox Reactions

Redox reactions are among the most common of all chemical processesCombustion, synthesis, decomposition, and single replacements are all redox reactions

Definition

Reaction in which a change in oxidation number occursOxidation – becomes more positiveReduced – becomes more negativeOxidation and reduction always come in pairs Electrons are transferred from the substance being oxidized to the substance being reduced

Example

In the reactions below, identify what is oxidized and what is reduced2PH3 + 4O2 P2O5 + 3H2O

2H2O + Al + MnO4- Al(OH)4

- + MnO2