Topic: Intro to Redox: Assigning Oxidation Number

Do Now:What is oxidation number for the following Metals in the ionic compounds below1. Fe2O3

2. FeO3. AgCl4. K2S5. Mn2O7

Redox Reactions = oxidation-reduction reaction

• Electrons transferred from 1 atom to another

• Redox reactions: – All single-replacement rxns– All combustion rxns

• Double Replace is never Redox

OIL RIG - memorize O – Oxidation I - isL - loss of electron

R - ReductionI - isG – gain of electron

REDOXOxidation & Reduction reactions are complementary reactions• Something is oxidized - __________ electrons• While something is reduced - ________ electrons• They occur every day – Respiration– Photosynthesis– combustion

losesgains

To understand these Redox Rxns we must first understand Oxidation

numbers

Rules to Assigning Oxidation Numbers( oxidation # =what an atoms charge would be if it owned all it’s electron

exclusively (not sharing)

#1: individual atom (monatomic, diatomic, polyatomic , but only 1

type of atom) = 0

#2for an ion

oxidation number = charge

Ca+2 = +2 Cl-1 = -1 Al+3 = +3

# 3• When Fluorine is in a compound F is

always ____________

CF4 C= ___ F = _-1_

OF2 O= ___ F = _-1_

-1

+4

+2

# 4

• When in a Oxygen is in a compound, O is AMLOST always ________– EXCEPT when with Fluorine• OF2 O= ____ F = _-1_

– Except when O2 is bonded with group 1 or 2 metal• Then it’s a ____________________–K2O2 = K = ____O = ____

-2

Peroxide

+2

+1 -1

Don’t forget O2 by itself has an

oxidation number of 0

#5

H is nearly always +1 • except if bonded to metal then -1• H2O H = ____O = ____

• H2SO4 H = ____SO4 = ____• LiH Li = ____H = ____• CaH2 Ca = ____H = ____

• NH3 N = ____H = ____

+1 -2+1 -2+1 -1+2 -1

+1-3

#6

• For neutral compounds, sum of oxidation numbers = 0

• H2O 2H = +1 = +2 1O= -2 = -2 +2-2 =0

• H2SO4 2H = +1 = +2 1SO4= -2 = -2 +2-2 =0• LiH 1Li = +1 = +1 1H= -1 = -1

+1-1 =0• CaH2 1Ca = +2 = +2 2H= -1 = -2 +2-2 =0

#7• For polyatomic ions, sum of oxidation numbers = charge of

ion

• SO4-2 1S = ___= ___ 4O= ___ = ____

___ + ___ =-2

• NO3-1 1N = ___= ___ 3O= ___ = ____

___ + ___ =-1

-2

-2

-8

-8+6

+6+6+6 -2

-6

-6+5

+5+5+5 -2

# 8

• If covalent compound – both elements are nonmetal, like to gain electrons (both negative?! – NO)–Assign negative to the more

electronegative element–SiCl4: Si = +4, Cl = -1

Assign Oxidation #’s• KCl

• CaBr2

• CO

• CO2

• Al(NO3)3

• Na3PO4

• H2S

• NH4+1

• SO3-2

K = +1, Cl = -1

Ca = +2, Br = -1

C = +2, O = -2

C = +4, O = -2

Al = +3, O = -2, N = +5

Na = +1, O = -2, P = +5

H = +1, S = -2

N = -3, H = +1

S = +4, O = -2