Oxidation-Reduction Reactions (REDOX reactions) Oxidation – the loss of electrons by a species; often, this involves a reaction with oxygen Reduction – the gain of electrons by a species To determine which species is oxidized and which species is reduced, we need to look at oxidation numbers.

Oxidation Numbers Oxidation number – the charge or effective charge of a species When a substance is oxidized, the oxidation number for one of its elements increases. When a substance is reduced, the oxidation number for one of its elements decreases (it’s reduced!). Reduction and oxidation occur at the same time. It is not possible to have one without the other!

Oxidation Numbers Oxidation numbers are applied to individual elements, either by themselves, or within compounds and polyatomic ions. SO42- does not have an oxidation number. The S has an oxidation number. The O has an oxidation number.

Rules for assigning oxidation numbers Elements The oxidation number is zero Monatomic ions The oxidation number is the charge of the ion Compounds and ions The sum of the oxidation numbers equal the charge of the species

Rules for assigning oxidation numbers Oxygen In compounds, the oxidation number of oxygen is usually -2 (exception is peroxide, where O is -1) Hydrogen Oxidation number is usually +1, but can be -1if acting as a negative ion (ex. NaH). Halogens Oxidation number is usually -1, but can be positive if combined with more electronegative elements like oxygen. The oxidation numbers for other elements can be determined using deductive reasoning.

Determine the oxidation number for each atom/ion in these neutral compounds. Ga2O3 PbO2 KBrO4 K2MnO4 Fe2(CrO4)3

Determine the oxidation number for each atom/ion in these polyatomic ions. NH2- IO3- Al(OH)4- H2PO4- S2O32-

BASIC TYPES OF REDOX REACTIONS Synthesis (or Combination ) Ex. 2 H2 + O2  2 H2O __________ is oxidized and each one ________ ____ electron(s) __________ is reduced and each one ________ ____ electron(s)

Decomposition Ex. 2 H2O2  2 H2O + O2 __________ are oxidized and each one ________ ____ electron(s) __________ are reduced and each one ________ ____ electron(s) Note: Not all decomposition reactions are redox (if products are both compounds, usually not redox). Check the oxidation #’s!

Single replacement (or displacement) Ex. 2 Al + 3 CuCl2  3 Cu + 2 AlCl3 __________ is oxidized and each one ________ ____ electron(s) __________ is reduced and each one ________ ____ electron(s)

Combustion (rapid reaction with oxygen) Many combustion reactions fall into other categories above. We will concern ourselves with the combustion of organic compounds (those compounds containing C and H or C, H, and O). These reactions always result in products of CO2 and H2O. Ex. CH4 + 2 O2  CO2 + 2 H2O