CHEMISTRY 161

Chapter 4

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)

KOH(aq) + HF(aq) → ???

EXAMPLE 1

1. KOH(s) → K+(aq) + OH-(aq)

2. HF(g) → H+(aq) + F-(aq)

1.+2. K+(aq) + OH-(aq) + H+(aq) + F-(aq) →

reactants

H2O(l) + K+(aq) + F-(aq)

KOH(aq) + HF(aq) → H2O(l) + KF(aq)

Mg(OH)2(aq) + HCl(aq) → ???

EXAMPLE 2

1. Mg(OH)2(s) → Mg2+(aq) + 2 OH-(aq)

2. 2 HCl(g) → 2 H+(aq) + 2 Cl-(aq)

1.+2. Mg2+(aq) + 2 OH-(aq) + 2 H+(aq) + 2 Cl-(aq) →

reactants

2 H2O(l) + Mg2+(aq) + 2 Cl-(aq)

Mg(OH)2(aq) + 2 HCl(aq) → 2 H2O(l) + MgCl2(aq)

Mg(OH)2(aq) + 2 HCl(aq) → ???

Ba(OH)2(aq) + H2SO4(aq) → ???

EXAMPLE 3

1. Ba(OH)2(s) → Ba2+(aq) + 2 OH-(aq)

2. H2SO4(l) → 2 H+(aq) + SO42-(aq)

1.+2. Ba2+(aq) + 2 OH-(aq) + 2 H+(aq) + SO42-(aq) →

reactants

2 H2O(l) + Ba2+(aq) + SO42-(aq)

Ba(OH)2(aq) + H2SO4(aq) → 2 H2O(l) + BaSO4(s)

CHEMICAL REACTIONS

a) precipitation reactions

b) acid-base reactions (proton transfer)

c)redox reactions (electron transfer)

H → H+ + e-

(1) K → K+ + e-

(2) S + 2 e- → S-2

/ × 2

/ × 1

(1) 2 K → 2 K+ + 2 e- (2) S + 2 e- → S-2

2K(s) + S(s) → K2S(s)

REDOX REACTIONS

reactions involving transfer of electrons

(solid state reaction of potassium with sulfur)

oxidation

reduction

(1)+(2) 2K + S + 2e-→ 2 K+ + S2- +2e-

(1)+(2) 2K + S → 2 K+ + S2-

1. oxidation

KEY CONCEPTS

loss/donation of electrons (minus)

2. reduction gain/acceptance of electrons (plus)

mo r p

m o p r

3. balance electrons

4. add oxidation half reaction and reduction half reaction

(1) Ca → Ca2+ + 2e-

(2) ½ O2 + 2 e- → O2-

Ca(s) + ½ O2(g) → CaO(s)

EXAMPLE

reaction of calcium with molecular oxygen

oxidation

reduction

(1)+(2) Ca + ½ O2 + 2e-→ Ca2+ + O2- +2e-

(1)+(2) Ca + ½ O2 → Ca2+ + O2-

2 Ca(s) + O2(g) → 2 CaO(s)

/ ×2

OXIDATION NUMBER

ionic compounds ↔ molecular compounds

NaCl HF, H2

Na+Cl- ?electrons are fully transferred covalent bond

‘oxidation number’

charges an atom would have if electrons are transferred completely

HF H+ + F-

molecular compound ionic compound

F- oxidation state -1

H+ oxidation state +1

EXAMPLE 1

H2O

molecular compound ionic compound

2 H+ + O2-

H+ oxidation state +1

O2- oxidation state -2

EXAMPLE 2

H2

molecular compound ionic compound

H+ + H-

EXAMPLE 3

OXIDATION NUMBER OF FREE ELEMENTS IS ZERO

RULE 1

OXIDATION NUMBER OF FREE ELEMENTS IS ZERO

H2, O2, F2, Cl2, K, Ca

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-

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)

+1 in FO

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

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

RULE 7

charges of polyatomic molecules must be integers

(NO3-, SO4

2-)

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

NONO2

NO+

NO-NO2-

NO3-

PO43- SO4

2-

SO3

SO2

KO2

K2O

BrO-

SUMMARY

1.redox reactions

2. oxidation versus reduction

3. oxidation numbers versus charges

4. calculation of oxidation numbers

Homework

Chapter 4, p. 116-121problems