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CHEMISTRY 161 Chapter 4
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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)
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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)
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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)
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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)
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H → H+ + e- CHEMICAL REACTIONS a) precipitation reactions
b) acid-base reactions (proton transfer) c)redox reactions (electron transfer) H → H+ + e-
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(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)
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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
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(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)
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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
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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
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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
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EXAMPLE 3 H2 H+ + H- ionic compound molecular compound
OXIDATION NUMBER OF FREE ELEMENTS IS ZERO
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RULE 1 OXIDATION NUMBER OF FREE ELEMENTS IS ZERO H2, O2, F2, Cl2, K, Ca
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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-
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RULE 3 oxidation number of hydrogen +1 in most compounds
(H2O, HF, HCl, NH3) -1 binary compounds with metals (hydrides) (LiH, NaH, CaH2, AlH3)
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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
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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-)
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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
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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-)
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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
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K2O NO+ SO42- PO43- KO2 SO3 NO3- NO NO2 BrO- SO2 NO2- NO-
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SUMMARY redox reactions 2. oxidation versus reduction
3. oxidation numbers versus charges 4. calculation of oxidation numbers
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Homework Chapter 4, p problems
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