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)

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)

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)

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)

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

(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)

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

(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)

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

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

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

EXAMPLE 3 H2 H+ + H- ionic compound molecular compound 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 -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-)

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

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

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

Homework Chapter 4, p. 116-121 problems