1. CHEMISTRY 161
Chapter 4

2. 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)

3. EXAMPLE 1 KOH(aq) + HF(aq) → ??? KOH(aq) + HF(aq) → H2O(l) + KF(aq)
1. KOH(s) → K+(aq) + OH-(aq)
reactants
2. HF(g) → H+(aq) + F-(aq)
1.+2. K+(aq) + OH-(aq) + H+(aq) + F-(aq) →
H2O(l) +
K+(aq) + F-(aq)
KOH(aq) + HF(aq) → H2O(l) + KF(aq)

4. EXAMPLE 2 Mg(OH)2(aq) + HCl(aq) → ??? Mg(OH)2(aq) + 2 HCl(aq) → ???
1. Mg(OH)2(s) → Mg2+(aq) + 2 OH-(aq)
reactants
2. 2 HCl(g) → 2 H+(aq) + 2 Cl-(aq)
1.+2. Mg2+(aq) + 2 OH-(aq) + 2 H+(aq) + 2 Cl-(aq) →
2 H2O(l) +
Mg2+(aq) + 2 Cl-(aq)
Mg(OH)2(aq) + 2 HCl(aq) → 2 H2O(l) + MgCl2(aq)

5. EXAMPLE 3 Ba(OH)2(aq) + H2SO4(aq) → ??? reactants
1. Ba(OH)2(s) → Ba2+(aq) + 2 OH-(aq)
reactants
2. H2SO4(l) → 2 H+(aq) + SO42-(aq)
1.+2. Ba2+(aq) + 2 OH-(aq) + 2 H+(aq) + SO42-(aq) →
2 H2O(l) +
Ba2+(aq) + SO42-(aq)
Ba(OH)2(aq) + H2SO4(aq) → 2 H2O(l) + BaSO4(s)

6. H → H+ + e- CHEMICAL REACTIONS a) precipitation reactions
b) acid-base reactions (proton transfer)
c)redox reactions (electron transfer)
H → H+ + e-

7. (1)+(2) 2K + S + 2e-→ 2 K+ + S2- +2e- (1)+(2) 2K + S → 2 K+ + S2-
REDOX REACTIONS
reactions involving transfer of electrons
(solid state reaction of potassium with sulfur)
(1) K → K+ + e-
/ × 2
oxidation
(2) S + 2 e- → S-2
/ × 1
reduction
(1) 2 K → 2 K+ + 2 e-
(2) S + 2 e- → S-2
(1)+(2) 2K + S + 2e-→ 2 K+ + S2- +2e-
(1)+(2) 2K + S → 2 K+ + S2-
2K(s) + S(s) → K2S(s)

8. KEY CONCEPTS 1. oxidation 2. reduction loss/donation of electrons
(minus)
2. reduction
gain/acceptance of electrons
(plus) o m r p
m o p r
3. balance electrons
4. add oxidation half reaction and reduction half reaction

9. (1)+(2) Ca + ½ O2 + 2e-→ Ca2+ + O2- +2e-
EXAMPLE
reaction of calcium with molecular oxygen
(1) Ca → Ca2+ + 2e-
oxidation
(2) ½ O2 + 2 e- → O2-
reduction
(1)+(2) Ca + ½ O2 + 2e-→ Ca2+ + O2- +2e-
(1)+(2) Ca + ½ O2 → Ca2+ + O2-
Ca(s) + ½ O2(g) → CaO(s)
/ ×2
2 Ca(s) + O2(g) → 2 CaO(s)

10. NaCl HF, H2 ? Na+Cl- OXIDATION NUMBER
ionic compounds ↔ molecular compounds
NaCl
HF, H2 ?
Na+Cl-
covalent bond
electrons are fully transferred
‘oxidation number’
charges an atom would have if electrons are transferred completely

11. EXAMPLE 1 H+ + F- HF H+ oxidation state +1 F- oxidation state -1
molecular compound
ionic compound
H+ oxidation state +1
F oxidation state -1

12. EXAMPLE 2 H2O 2 H+ + O2- H+ oxidation state +1 O2- oxidation state -2
molecular compound
ionic compound
H+ oxidation state +1
O oxidation state -2

13. EXAMPLE 3 H2 H+ + H- ionic compound molecular compound
OXIDATION NUMBER OF FREE ELEMENTS IS ZERO

14. RULE 1
OXIDATION NUMBER OF FREE ELEMENTS IS ZERO
H2, O2, F2, Cl2, K, Ca

15. 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 F-

16. RULE 3 oxidation number of hydrogen +1 in most compounds
(H2O, HF, HCl, NH3)
-1 binary compounds with metals (hydrides)
(LiH, NaH, CaH2, AlH3)

17. 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)
+1 in FO

18. 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-)

19. RULE 6 neutral molecule: sum of oxidation numbers must be zero
NH3 H: +1 N: -3
3 × (+1) + 1 × (-3) = 0
polyatomic ion: sum of oxidation numbers equals charge of ion
NH4+ H: +1 N: -3
4 × (+1) + 1 × (-3) = +1

20. RULE 7 -1/2 in superoxide ion (O2-)
charges of polyatomic molecules must be integers
(NO3-, SO42-)
oxidation numbers do not have to be integers
-1/2 in superoxide ion (O2-)

22. MENUE oxidation states of group I – III metals
oxidation state of hydrogen (+1, -1)
oxidation states of oxygen (-2, -1, -1/2, +1)
oxidation state of halogens
remaining atoms

23. K2O
NO+
SO42-
PO43-
KO2
SO3
NO3- NO
NO2
BrO-
SO2
NO2-
NO-

24. SUMMARY redox reactions 2. oxidation versus reduction
3. oxidation numbers versus charges
4. calculation of oxidation numbers

25. Homework
Chapter 4, p
problems