ch 22sec2
DESCRIPTION
assigning oxidation numbersTRANSCRIPT
22.2
Oxidation Numbers
Assigning Oxidation Assigning Oxidation NumbersNumbers
• An “oxidation number” is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.
• Generally, a bonded atom’s oxidation number is the charge it would have if the electrons in the bond were assigned to the atom of the more electronegative element
Rules for Assigning Oxidation Rules for Assigning Oxidation NumbersNumbers1)The oxidation number of any
uncombined element is zero.
2)The oxidation number of a monatomic ion equals its charge.
11
2
00
22
ClNaClNa
Rules for Assigning Oxidation Rules for Assigning Oxidation NumbersNumbers3)The oxidation number of oxygen in
compounds is -2, except in peroxides, such as H2O2 where it is -1.
4)The oxidation number of hydrogen in compounds is +1, except in metal hydrides, like NaH, where it is -1.
2
2
1
OH
Rules for Assigning Oxidation Rules for Assigning Oxidation NumbersNumbers
5) The sum of the oxidation numbers of the atoms in the compound must equal 0.
2
2
1
OH2(+1) + (-2) = 0 H O
2
122
)(
HOCa(+2) + 2(-2) + 2(+1) = 0 Ca O H
Rules for Assigning Oxidation Rules for Assigning Oxidation NumbersNumbers6) The sum of the oxidation numbers in
the formula of a polyatomic ion is equal to its ionic charge.
3
2?
ONX + 3(-2) = -1N O
24
2?
OS
X = +5 X = +6
X + 4(-2) = -2S O
Point to note
• Oxidation numbers are written with the charge first and number second
• Ex. HCl H oxidation number is +1
Cl oxidation nmber is -1
Ionic charge though is written with the charge after the number: H charge is 1+, Cl charge is 1-
Problem:
• What is the oxidation number of each element in SO2
Do oxygen first because you know that is -2.
x -4 = 0 S O2, so, S is +4
S=+4 O=-2
Reducing Agents and Reducing Agents and Oxidizing AgentsOxidizing Agents
• An increase in oxidation number = oxidation
• A decrease in oxidation number = reduction
eNaNa10
10 CleCl
Sodium is oxidized – it is the reducing agent
Chlorine is reduced – it is the oxidizing agent
Trends in Oxidation and ReductionTrends in Oxidation and Reduction
Active metals: Lose electrons easily Are easily oxidized Are strong reducing agents
Active nonmetals: Gain electrons easily Are easily reduced Are strong oxidizing agents
Problem
• Use oxidation numbers to decide which elements are oxidized and which reduced in the following reaction:
C + O2 ------CO2
0 0 +4 -2
C + O2 ------CO2
Notice C goes from 0 to +4 so it is oxidized ( reducing agent)
O goes from 0 to -2,so it is reduced (oxiding agent)