Download Oxidation Numbers

Oxidation
Numbers
Oxidation state:
the net charge that an atom
would have if the electrons in a
covalent bond belonged entirely
to the more electronegative atom
Oxidation number:
+ or − value (integer or fraction)
representing an atom’s oxidation
state
Common Oxidation Numbers:
a) Any element = 0
b) H (in compounds) = +1
c) O (in compounds) = −2
d) Any monoatomic ion = its charge
Assign oxidation numbers to each atom/ion
shown below
HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)
Zn(s) + CuCl2 (aq) → Cu(s) + ZnCl2 (aq)
Two Exceptions to the Common
Oxidation Numbers:
In metal hydrides (ex. LiH), H = −1
In peroxides (ex. H2O2), O = −1
Rules for Assigning Other
Oxidation Numbers:
All of the oxidation numbers in a
molecule/ion must add up to equal the
charge on the molecule/ion
In compounds with two unknown
oxidation numbers, the more
electronegative atom is given an
oxidation number equal to the
negative charge it usually has as a
simple anion
Assign an oxidation number to
each atom/ion:
(a) HClO4
(b) Cr2O72−
(c) SiBr4
Identifying Redox
Reactions With
Oxidation Numbers
A chemical’s oxidation number changes
when it gains or loses electrons
oxidation: an increase in the oxidation
number
Fe(s) → Fe2+(aq) + 2 e
2Cl(aq) → Cl2(g) + 2 e
(oxidation # becoming more positive
or less negative)
reduction: a decrease in the oxidation
number
Cl2(g) + 2e− → 2Cl−(aq)
Sn4+(aq) + 2e− → Sn2+(aq)
(oxidation # becoming less positive or
more negative)
Change in the oxidation number
oxidation
-3
-2
-1
0
+1
reduction
+2
+3
In a redox reaction, the OA is always
reduced.
the OA is the reactant whose
oxidation number has decreased
In a redox reaction, the RA is always
oxidized.
the RA is the reactant whose
oxidation number has increased
A chemical reaction is not a redox
reaction when there is no change in
any of the oxidation numbers
Is the combustion of methane a redox
reaction? If so, identify the OA and RA.
Assign oxidation numbers and identify the
OA and RA.
Cr2O72−(aq) + H+ (aq) + C2H5OH(aq) →
Cr3+(aq) + H2O(l) + CH3COOH(aq)
In disproportionation reactions, only
one type of atom is changing its
oxidation number
The same reactant acts as both the OA
and RA
HS2O3−(aq) + H+(aq) →
S8(s) + H2O(l) + HSO4−(aq)