Honors Chemistry
Name: __________________________________ Date: ______________ Mods: _________
Electrochemistry – Intro to Redox Reactions
20.1 - Oxidation States
 Redox rxns occur when _____________________ are transferred between reactants
► Oxidation is a process which occurs when an atom, ion, or molecule
_________________ electrons (LEO)
► Reduction is a process which occurs when an atom, ion, or molecule
________________ electrons (GER)
LEO the lion says GER!!!
► Oxidation and reduction go hand in hand  one cannot occur without the other …
because every electron that is lost must be transferred and gained by something!
 Oxidation Numbers (aka: Oxidation States)
► To determine if a redox rxn has occurred, oxidation numbers are assigned to each
___________________ in a compound or ion
► These #s keep track of electrons lost and gained during a reaction
► The oxidation # of some atoms will _________________ in a redox rxn (one # on
reactant side, but different # on product side)
o Oxidation (LEO) = ______________________ in oxidation #
o Reduction (GER) = ______________________ in oxidation #
 Rules for Assigning Oxidation Numbers
(1) Elements in their elemental form (a single element or diatomic molecule) have an
oxidation # of zero
► ex: Mg (s), Fe (s), O2 (g), H2 (g)
(2) The oxidation number of a monatomic ion is the same as its charge
► ex: alkali metal ions = +1; alkali earth ions = +2; S2- = -2; Al3+ = +3; etc
(3) Nonmetals usually have negative oxidation #s (though they can be positive in certain
compounds or ions)
► Oxygen (typically assign oxygen’s oxidation # first)
o -2 almost always
o -1 when in the peroxide ion (O22-) (ex: CaO2  calcium peroxide)
► Hydrogen
o -1 when bonded to a metal (b/c metals are usually positive cations)
o +1 when bonded to a nonmetal (b/c nonmetals are usually negative
anions) (ex: HCl, H2S)
► Halogens
o Fluorine: -1 always
o Other Halogens: -1 (except if combined with oxygen  ex: ClO-, ClO2-,
ClO3-, ClO4- … then halogens have positive oxidation #s)
(4) In a neutral compound the sum of the oxidation #s of the atoms equals zero
► MgCl2  1(Mg) + 2(Cl) = 0
(5) In a polyatomic ion the sum of the oxidation numbers of the atoms equals the
charge on the ion
► PO43-  1(P) + 4(O) = -3
 Systematic Approach to Assigning Oxidation Numbers
► Is the substance elemental? if yes, oxid.# = 0
► Is the substance ionic?
o If so, does it contain monatomic ions?  alkali metals, alkali earth, Al3+ ???
o If yes, the oxid.# on the monatomic ions = charge of ion
► Does it contain elements which have specific rules?  H, O, halogens
► Use Steps 4 & 5 to figure out oxid.#s for remaining elements without specific rules
 Examples - Assigning Oxidation Numbers:
Substance: Show your
WORK in this box for full
Oxidation Numbers:
credit
A
Na2SO4
B
CO32-
C
H2S
Na =
S=
C=
O=
---------------------
H=
D
Co(ClO)2
E
NO3-
BaO2
G
P2O5
H
Cr2O72-
I
SCl2
S=
---------------------
Co =
Cl =
N=
O=
O=
--------------------Ba =
F
O=
O=
---------------------
P=
O=
---------------------
Cr =
O=
---------------------
S=
Cl =
---------------------
Assigning Oxidation Numbers WS
Substance: Show your
WORK in this box for full
Oxidation Numbers:
credit
Na =
1
NaCl
2
NH4+
--------------------N=
3
Na2Cr2O7
4
Ca(OH)2
Cl =
H=
---------------------
Na =
Cr =
O=
Ca =
O=
H=
I=
5
I2
6
CO2
--------------------C=
7
HNO2
8
KClO3
9
CoCrO4
10
FeCl3
11
Fe+3
12
SrO
13
AsO4-3
---------------------
O=
---------------------
H=
N=
O=
K=
Cl =
O=
Co =
Cr =
O=
Fe =
Cl =
---------------------
Fe =
--------------------Sr =
---------------------
O=
---------------------
As =
O=
---------------------
14
H=
S=
O=
C=
O=
Cl =
Na =
O=
H2SO3
15
COCl2
16
Na2O2
--------------------Ni =
17
Ni (s)
18
PO3-3
--------------------P=
19
H4P2O7
20
MnO2
21
AgNO3
22
CaH2
MgBr2
24
Ba(IO)2
O=
---------------------
H=
P=
Mn =
O=
O=
--------------------Ag =
N=
Ca =
H=
O=
--------------------Mg =
23
---------------------
Br =
---------------------
Ba =
I=
O=
 Redox Reactions
► Many redox rxns are _______________ ___________________________ rxns
► A + BX  AX + B
► This obviously has a relationship to the activity series of metals
o those metals which are easier to oxidize are higher up on the series
 Redox Reactions: Determining Oxidation & Reduction
Example: Use oxidation numbers to determine what is being oxidized and what is reduced.
2 Al (s) + 3 CuCl2 (aq)  2 AlCl3 (aq) + 3 Cu (s)
Net Ionic:
_______ is being oxidized;
__________ is the oxidizing agent;
_______ is being reduced
__________ is the reducing agent
(Note: the reactant which includes the oxidized atom is known as the
“reducing agent” or “reducer” and the reactant which includes the
reduced atom is known as the “oxidizing agent” or “oxidizer”)
Practice: Use oxidation numbers to determine what is being oxidized and what is reduced.
Mg (s) + CoSO4 (aq)  MgSO4 (aq) + 2 Co (s)
Net Ionic:
_______ is being oxidized;
__________ is the oxidizing agent;
_______ is being reduced
__________ is the reducing agent
Identifying Oxidation & Reduction in a Redox Rxn WS
Directions: Use oxidation numbers to determine which element is oxidized and which is
reduced in the following reactions as well as the oxidizing agents and reducing
agents.
1.
Ni (s) + Cl2 (g)  NiCl2 (s)
_______ is being oxidized;
__________ is the oxidizing agent;
2.
__________ is the reducing agent
3 Fe(NO3)2 (aq) + 2 Al (s)  3 Fe (s) + 2 Al(NO3)3 (aq)
_______ is being oxidized;
__________ is the oxidizing agent;
3.
_______ is being reduced
_______ is being reduced
__________ is the reducing agent
Cl2 (g) + 4 NaI (aq)  I2 (g) + 2 NaCl (aq)
_______ is being oxidized;
__________ is the oxidizing agent;
_______ is being reduced
__________ is the reducing agent
4.
4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s)
_______ is being oxidized;
__________ is the oxidizing agent;
5.
__________ is the reducing agent
Cu (s) + 2 AgClO2 (aq)  Cu(ClO2)2 (aq) + 2 Ag (s)
_______ is being oxidized;
__________ is the oxidizing agent;
6.
_______ is being reduced
_______ is being reduced
__________ is the reducing agent
2 I– (aq) + S2O82– (aq)  I2 (s) + 2 SO42– (aq)
_______ is being oxidized;
__________ is the oxidizing agent;
_______ is being reduced
__________ is the reducing agent
20.2 - Balancing Redox Equations
 When balancing any reaction, we must have equal types and #s of atoms on both side of
the equation to obey the law of conservation of matter…
► With redox rxns. There is an additional requirement: the gain and loss of
_______________ must be balanced
► Perhaps the easiest way to balance a redox rxn is via the __________________________ method
 Balancing Equations by Half-Reaction Method in ACIDIC solutions:
► On paper, the oxidation half and reduction half of the rxn are treated as two
separate processes
o First, balance each of these half reactions, and then combine them to attain
the balanced equation for the overall reaction
(1) Assign oxidation #s to determine what is being oxidized and what is reduced
(2) Write the oxidation and reduction half reactions
(3) Balance each half-reaction:
a. Balance elements other than H and O first
b. Balance O by adding H2O
c. Balance H by adding H+ (because this is an acidic solution we add H+ ions)
d. Balance charge by adding electrons
(4) Multiply the half-reactions by integers so that the electrons gained and lost are the
same in both the oxidation-half and the reduction-half
(5) Add the half-reactions, canceling things that appear on both sides
(6) Make sure the equation is balanced according to mass (equal atoms)
(7) Make sure the equation is balanced according to charge (equal electrons)
 Example – Balancing Redox Reactions in ACIDIC Solutions
____MnO4- (aq) + ____C2O42- (aq)  ____Mn2+ (aq) + ____CO2 (aq)
Reduction-Half:
Oxidation-Half:
________________________________________________________________________
 Consider the incomplete half-reactions below.
(a) Use oxidation #s to identify the reactions below as oxidation or reduction
(b) Place the correct # of electrons on the appropriate side of the reaction to complete
the half-reaction
1.
2.

I2 (s)  2 I– (aq)
2 Cr3+ (aq) + 7 H2O (l)  CrO72– (aq) + 14 H+ (aq)
 Balancing Redox Reactions in BASIC Solutions
► If a redox rxn occurs in a basic solution, the equation must be balanced in step (3)c
by using __________ and H2O instead of H+ and H2O (as we do in acidic solutions)
► This is difficult to balance because OH- has both hydrogen and oxygen…
► Therefore, a good way to balance these rxns is to balance the half rxn as if they
occurred in acidic solution (using H+ and H2O)… then count the # of H+ in each half
reaction and add the same # of OH- to each side of the half reaction (that way each
H+ and OH- come together to form H2O which can be canceled accordingly)
 Example – Balancing Redox Reactions in BASIC Solutions
CN- (aq) + MnO4- (aq)  CNO- (aq) + MnO2 (S) (basic solution)
Reduction-Half:
Oxidation-Half:
________________________________________________________________________
Balancing Redox Equations via Half-Reactions
Directions: Consider the incomplete half-reactions below.
(a) Use oxidation #s to identify the reactions below as oxidation or reduction
(b) Place the correct # of electrons on the appropriate side of the reaction to complete
the half-reaction
1.
Sr (s)  Sr2+(aq)
2.
Cr2+(aq)  Cr3+(aq)
3.
MnO4– (s) + 8 H+ (aq)  2 Mn+ (aq) + 4 H2O (l)
4.
10 H+ (aq) + NO3– (aq)  NH4+ (aq) + 3 H2O (l)
Directions: Complete and balance the following equations using the half-reaction method in
ACIDIC solutions.
1. Cr2O72- (aq) + Cl- (aq)  Cr3+ (aq) + Cl2 (g)
2. Cu (s) + NO3- (aq)  Cu2+ (aq) + NO2 (g)
3. Mn2+ (aq) + NaBiO3 (s)  Bi3+ (aq) + MnO4- (aq) + Na+ (aq)
4. I2 (s) + OCl- (aq)  IO3- (aq) +Cl- (aq)
5. As2O3 (s) + NO3- (aq)  H3AsO4 (aq) + N2O3 (aq)
6. Cr2O72- (aq) + CH3OH (aq)  HCO2H (aq) + Cr3+ (aq)