chemistry 161 chapter 4 ‘4’
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CHEMISTRY 161
Chapter 4
www.chem.hawaii.edu/Bil301/welcome.html
‘4’
1.redox reactions
2. oxidation versus reduction
3. oxidation numbers versus charges
4. calculation of oxidation numbers
REVISION
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
S(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
S(s) + O2(g) → SO2(g)
00 +4 -2
oxidation (1) S → S+4 + 4e-
reduction (2) O2 + 4 e- → 2 O-2
(1)+(2) S + O2 + 4e-→ S+4 + 2 O-2 + 4e-
(1)+(2) S + O2 → S+4 + 2 O-2
S(s) + O2(g) → SO2(g)
Mg(s) + N2(g) → Mg3N2(s)
00 +2 -3
oxidation (1) Mg → Mg+2 + 2e-
reduction (2) N2 + 6 e- → 2 N-3
(1)+(2) 3 Mg + N2 + 6e-→ 3 Mg+2 + 2 N-3 + 6e-
(1)+(2) 3 Mg + N2 → 3 Mg+2 + 2 N-3
/ × 3
3 Mg(s) + N2(g) → Mg3N2(s)
2. decomposition reactions
C → A + B
breakdown of one compound into two or more compounds
2 HgO(s) → 2 Hg(l) + O2(g)
1. oxidation numbers
2. balancing charges
HgO(s) → Hg(l) + O2(g)
0 0-2+2
oxidation (1) O-2 → ½ O2 + 2e-
reduction (2) Hg+2 + 2 e- → Hg
(1)+(2) O-2 + Hg+2 + 2e-→ Hg + 1/2 O2 + 2e- / × 2
(1)+(2) 2 O-2 + 2 Hg+2 → 2 Hg + O2
2 HgO(s) → 2 Hg(l) + O2(g)
-1 0-2+5
KClO3(s) → KCl(s) + O2(g)+1+1
oxidation (1) O-2 → ½ O2 + 2e-
reduction (2) Cl+5 + 6 e- → Cl-
/ × 3
(1)+(2) 3 O-2 + Cl+5 + 6e-→ 3/2 O2 + Cl- + 6e-
(1)+(2) 3 O-2 + Cl+5 → 3/2 O2 + Cl-
(1)+(2) ClO3- → 3/2 O2
+ Cl- (1)+(2) KClO3 → 3/2 O2
+ KCl / × 2
2 KClO3(s) → 3 O2(g) + 2 KCl(s)
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)0 0+1 -2 -2+1 +1
oxidation (1) Na → Na+ + e-
reduction (2) H+1 + e- → ½ H2
(1)+(2) Na + H+ + e-→ Na+ + ½ H2 + e- / × 2
(1)+(2) 2 Na + 2 H+ → 2 Na+ + H2
2 Na(s) + 2 H2O(l) → 2 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)0 0+1 -2 -2+3
oxidation (1) Al → Al+3 + 3 e-
reduction (2) H+1 + e- → ½ H2 / × 3
(1)+(2) Al + 3 H+ + 3 e-→ Al+3 + 3/2 H2 + 3 e- / × 2
(1)+(2) 2 Al + 6 H+ → 2 Al+3 + 3 H2
2 Al(s) + 3 H2O(l) → Al2O3(s) + 3 H2(g)
even less reactive metals form hydrogen in acids
Zn(s) + HCl(aq) → ZnCl2(aq) + H2(g)0 0+1 -1 -1+2
oxidation (1) Zn → Zn+2 + 2 e-
reduction (2) H+1 + e- → ½ H2 / × 2
(1)+(2) Zn + 2 H+ + 2 e-→ Zn+2 + H2 + 2 e-
(1)+(2) Zn + 2 H+ → Zn+2 + H2
Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g)
Li K Ba Ca Na Mg Al Zn Cr Fe Cd Co Ni Sn Pb H Cu Hg Ag Pt Au
activity series of metals
displace H from water
displace H from steam
displace H from acids
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)
Cl2(g) + 2 KBr(aq) → 2 KCl(aq) + Br2(l)0 0+1+1 -1 -1
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)+1 -1 +1 -2 0
Cl2(g) + 2 OH-(aq) → ClO-(aq) + Cl-(aq) + H2O(l)0 -2 +1 -1 -2+1-2+1
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. 121-129problems