CHEMISTRY 161
Oxidation-Reduction
Reactions
Chapter 6
REVISION
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
H+(aq) + OH-(aq) → H2O(l)
(Arrhenius)
HCl(aq) + NH3(aq) → NH4Cl(aq)
H+(aq) + NH3(aq) → NH4+(aq)
(Bronsted)
CHEMICAL REACTIONS
a) precipitation reactions
b) acid-base reactions (proton transfer)
c) oxidation-reduction reactions
(electron transfer)
(redox reactions)
KEY CONCEPTS
1. oxidation
loss of electrons
2. reduction
acceptance of electrons
NUMBER OF ELECTRONS MUST BE CONSERVED
EXAMPLE
Na+Cl-
1. oxidation
Na  Na+ + e
2. reduction
Cl2 + 2 e  2 Cl-
!!!balance electrons!!!
CaO, Al2O3
substance that lost the electrons
reduction agent
substance that gained the electrons
oxidizing agent
oxidizing agent is reduced
reducing agent is oxidized
2 Na + Cl2  2 Na+Cl-
EXAMPLE 1
solid state reaction of potassium with sulfur
to form potassium sulfide
EXAMPLE 2
solid state reaction of iron with oxygen
to form iron(III)oxide
OXIDATION NUMBER
ionic compounds ↔ molecular compounds
NaCl
HF, H2
Na+Cl-
?
electrons are fully transferred
covalent bond
charges an atom would have if electrons are
transferred completely
EXAMPLE 1
H+ + F-
HF
molecular compound
ionic compound
H+
oxidation state +1
F-
oxidation state -1
EXAMPLE 2
H2O
2 H+ + O2-
molecular compound
ionic compound
H+
oxidation state +1
O2-
oxidation state -2
EXAMPLE 3
H2
molecular compound
H+ + Hionic compound
OXIDATION NUMBER OF FREE ELEMENTS IS ZERO
RULE 1
OXIDATION NUMBER OF FREE ELEMENTS IS ZERO
H2, O2, F2, Cl2, K, Ca, P4, S8
RULE 2
monoatomic ions
oxidation number equals the charge of the ion
group I
M+
group II
M2+
group III
M3+ (Tl: also +1)
group VII (w/ metal)
X-
RULE 3
oxidation number of hydrogen
+1 in most compounds
(H2O, HF, HCl, NH3)
-1 binary compounds with metals (hydrides)
(LiH, NaH, CaH2, AlH3)
RULE 4
oxidation number of oxygen
-2 in most compounds
(H2O, MgO, Al2O3)
-1 in peroxide ion (O22-) (H2O2, K2O2, CaO2)
-1/2 in superoxide ion (O2-) (LiO2)
RULE 5
oxidation numbers of halogens
F: -1 (KF)
Cl, Br, I: -1 (halides) (NaCl, KBr)
Cl, Br, I: positive oxidation numbers if combined
with oxygen (ClO4-)
RULE 6
charges of polyatomic molecules must be integers
(NO3-, SO42-)
oxidation numbers do not have to be integers
-1/2 in superoxide ion (O2-)
MENUE
1.oxidation states of group I – III metals
2.oxidation state of hydrogen (+1, -1)
3. oxidation states of oxygen (-2, -1, -1/2, +1)
4.oxidation state of halogens
5.remaining atoms
oxidizing agents
?????
OCl-
Cl-
EXP1
reducing agent
2 Na + 2 H2O  H2 + 2 NaOH
EXP2
K2O
PO43-
NO+
SO42-
KO2
SO3
NO3NO2
NO
KClO4
BrO-
SO2
NO2-
NO-
REVISION
1.redox reactions
2. oxidation versus reduction
3. oxidation numbers versus charges
4. calculation of oxidation numbers
TYPES OF REDOX REACTIONS
1.combination reactions
A+B→C
2. decomposition reactions
C→A+B
3. displacement reactions
A + BC → AC + B
4. disproportionation reactions
1.combination reactions
A+B→C
two or more compounds combine to form a single product
S8(s) + O2(g) → SO2(g)
1. oxidation numbers
2. balancing charges
MENUE
1.oxidation states of group I – III metals
2.oxidation state of hydrogen (+1, -1)
3. oxidation states of oxygen (-2, -1, -1/2, +1)
4.oxidation state of halogens
5.remaining atoms
2. decomposition reactions
C→A+B
breakdown of one compound into two or more compounds
HgO(s) → Hg(l) + O2(g)
KClO3(s) → KCl(s) + O2(g)
1. oxidation numbers
2. balancing charges
3. displacement reactions
A + BC → AC + B
an ion or atom in a compound is replaced by an ion or atom
of another element
3.1. Hydrogen displacement
3.2. Metal displacement
3.3. Halogen displacement
3.1. Hydrogen displacement
group I and some group II metals (Ca, Sr, Ba)
react with water to form hydrogen
Na(s) + H2O(l) → NaOH + H2(g)
less reactive metals form hydrogen and the oxide in
water (group III, transition metals)
Al(s) + H2O(l) → Al2O3(s) + H2(g)
3.1. Hydrogen displacement
even less reactive metals form hydrogen in acids
Zn(s) + HCl(aq) → ZnCl2(aq) + H2(g)
EXP3
activity series of metals
Li K Ba Ca Na Mg Al Zn Cr Fe Cd Co Ni Sn Pb H Cu Hg Ag Pt Au
displace H from water
displace H from steam
displace H from acids
Li K Ba Ca Na Mg Al Zn Cr Fe Cd Co Ni Sn Pb H Cu Hg Ag Pt Au
does not like so much to donate electrons
likes to donate electrons
EXP4
3.2. Metal displacement
V2O5(s) + 5 Ca(s) → 2 V(s) + 5 CaO(s)
TiCl4(g) + 2 Mg (l) → Ti(s) + 2 MgCl2(l)
3.3. Halogen displacement
F2 > Cl2 > Br2 > I2
reactivity (‘likes’ electrons)
0
+1 -1
+1 -1
0
Cl2(g) + 2 KBr(aq) → 2 KCl(aq) + Br2(l)
Br2(g) + 2 KI(aq) → 2 KBr(aq) + I2(s)
4. disproportionation reactions
an element in one oxidation state is oxidized and reduced
at the same time
H2O2(aq) → 2 H2O(l) + O2(g)
Cl2(g) + 2 OH-(aq) → ClO-(aq) + Cl-(aq) + H2O(l)
SUMMARY
1.combination reactions
A+B→C
2. decomposition reactions
C→A+B
3. displacement reactions
A + BC → AC + B
4. disproportionation reactions
Homework
Chapter 4, p. 121-129
problems