Chemistry Chapter 19 Oxidation and Reduction 1

Oxidation-Reduction Involves movement of electrons Oxidation: ▫Effective loss of electrons Reduction: ▫Effective gain of electrons ▫Both must occur simultaneously! ▫To know what’s happening, we must be able to assign oxidation numbers

Rules for Assigning Oxidation Numbers Oxidation number of uncombined element is zero ▫Ex: in O 2, oxidation number of O is zero Oxidation number of monatomic ion is the charge ▫Ex: in NaCl, oxidation number of Na is +1 and of Cl is -1 More electronegative element in compound assigned oxidation number equal to the charge it would have as ion ▫Ex: in NO, oxidation number of O is -2

Rules Continued Oxidation number of fluorine is always -1 ▫Example: oxidation number of F in HF is -1 Oxygen’s oxidation number is usually -2 ▫Exceptions: when oxygen combines with F, its oxidation number is +2 ▫Exceptions: when oxygen is in a peroxide, its oxidation number is -1 ▫Example: H2O: oxidation number of O is -2

Rules Continued Oxidation number of hydrogen is usually +1, unless H combines with active metal ▫Example: in H2O, oxidation number of H is +1 ▫Example: in NaH, oxidation number of H is -1 Oxidation numbers of group 1 metals are +1 Oxidation numbers of group 2 metals are +2 Oxidation number of aluminum is +3

Rules Continued Sum of oxidation numbers in a compound is zero ▫Example: in H2O hydrogen is +1 and oxygen is -2 Sum of oxidation numbers in a polyatomic ion equals the charge of the ion ▫Example: in CO3 -2 carbon has oxidation number +4 and oxygen has oxidation number -2

Oxidation Oxidation defined: ▫Reaction in which atoms or ions of an element increase oxidation number ▫Note: This does NOT mean the substance has to have a positive oxidation number! ▫Substance whose oxidation number is increasing is oxidized Example: ▫2Na (s) + Cl 2(g)  2NaCl (s)

Reduction Reduction defined: ▫Reaction in which an atom or element experiences a decrease in oxidation number ▫Note: this does NOT mean the oxidation number must be negative! ▫Substance whose oxidation number decreases is reduced Example: ▫2Na (s) + Cl 2(g)  2NaCl (s)

Redox Reaction Oxidation-Reduction reaction: ▫Chemical process where there is a change in oxidation numbers ▫Called redox reactions Half reaction: ▫Separate part of the reaction where oxidation or reduction is occurring Example: ▫Copper reacts with nitric acid to produce copper (II) nitrate, nitrogen monoxide, and water

Assignment: Read pages ! Worksheet Assigning Oxidation numbers