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CHEMISTRY - science of properties and transformations of matter

CHEMISTRY - science of properties and transformations of matter. CHEMICAL REACTIONS - central to CHEMSITRY. CLASSIFICATION of REACTIONS (Kotz, Ch. 4) Precipitation Oxidation-reduction Acid-base Gas forming. OXIDATION-REDUCTION REACTIONS.

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CHEMISTRY - science of properties and transformations of matter

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  1. CHEMISTRY - science of properties and transformations of matter CHEMICAL REACTIONS - central to CHEMSITRY CLASSIFICATION of REACTIONS (Kotz, Ch. 4) Precipitation Oxidation-reduction Acid-base Gas forming 1A03/1E03 Types of Reactions (2)

  2. OXIDATION-REDUCTION REACTIONS Cu (s) + 2 AgNO3 (aq) Cu(NO3)2 (aq) + 2 Ag(s) (see Fig. 4.17 in Kotz) Transfer of electrons: Cu0 gives 2 electrons to 2 Ag+ to form Cu2+ and 2 Ag0 species that provides electrons …….. called the REDUCING AGENT species that accepts electrons …….. called the OXIDIZING AGENT species that loses electrons is said to have been OXIDIZED species that gains electrons is said to have been REDUCED Chemists use the concept of OXIDATION NUMBER to recognize OXIDATION-REDUCTION reactions Here Cu in Cu (s) O.N. = 0 Cu in Cu(NO3)2 O.N. = +2 Ag in AgNO3 O.N. = +1 Ag in Ag(s) O.N. = 0 1A03/1E03 Types of Reactions (2)

  3. RULES for OXIDATION NUMBERS O.N.(see Kotz, p. 191) • Atoms in pure ELEMENT (Cu, I2, S8) 0 • Ions of single atom (Al#+) charge on ion • Fluorine in compounds (NaF) -1 • other halogens in compounds (KCl, CsI) -1 • EXCEPT when combined with F or O where O.N. = +1 • hydrogen in most compounds (HCl, HI) +1 • EXCEPT metal hydrides (CaH2) where O.N. = -1 • oxygen in most compounds (MgO, Na2O) -2 • EXCEPT in peroxides (H2O2) where O.N. = -1 • the O.N. of all other elements in a compound are determined by requiring that SUM of O.N. of all elements = CHARGE on the compound 1A03/1E03 Types of Reactions (2)

  4. EXAMPLES: What is O.N. of S inH2SO4 ? • H has an O.N. of • O has an O.N. of • 2 * O.N. (H) + 4 * O.N. (O) = - overall charge of molecule is 0 therefore O.N. of S inH2SO4 is : What is O.N. of N inHNO3 ? • H has an O.N. of • O has an O.N. of • O.N. (H) + 3 * O.N. (O) = - overall charge of molecule is 0, therefore O.N. of N inHNO3 is : this shows the nitrate group: “ NO3 “ has a net O.N. of (and thus a charge) of THUS the O.N. of Ag in AgNO3 is 1A03/1E03 Types of Reactions (2)

  5. Another example of an OXIDATION-REDUCTION REACTION Fe2O3 (s) + 3 CO (g)  2 Fe (s) + 3 CO2 (g) Species Fe in Fe2O3 C in CO Fe C in CO2 O.N. +3 +2 0 +4 Thus: Fe in Fe2O3 GAINS 3electrons - Fe2O3 is the OXIDIZING AGENT C in CO LOSES 2 electrons - CO is the REDUCING AGENT The iron in Fe2O3 was REDUCED to Fe The carbon in CO was OXIDIZED to CO2 • NOTES: • oxidation-reduction reactions DO NOT require ions; can involve gases • OXIDATION NUMBERS do not represent actual charge • - rather they are a convenient book-keeping device which help to • identify and classify reactions 1A03/1E03 Types of Reactions (2)

  6. ACID-BASE REACTIONS(see alsoKotz Ch 17 - sections 1 to 5 (pp. 794-804) TYPES OF ACIDS/BASES • Arrhenius: • ACID - substance that donates H+ in water HCl  H+(aq) + Cl- (aq) • BASE - substance that donates OH- in water NaOH  Na+(aq) + OH- (aq) • SALT - ionic product of an ACID - BASE reaction, composed of a +ve CATION from the base and a -ve ANION from the acid HCl (aq) + NaOH (aq)  NaCl (aq) + H2O ACID + BASE  SALT + water 1A03/1E03 Types of Reactions (2)

  7. Brnsted - Lowry: ( sect 17.3, p. 707) • ACID - substance that donates H+ to another species • HCO3- (aq)  H+(aq) + CO32- (aq) • BASE - substance that accepts H+ from another species • NH3 + H2O  NH4+(aq) + OH- (aq) TYPICAL PROPERTIES of ACIDS and BASES • ACIDS • sour tase • corrosive • reacts with bases • turns natural dyes red • generates CO2 from limestone • generates H2 with metals • BASES • soapy feel • restores natural dyes to blue • reacts with acids 1A03/1E03 Types of Reactions (2)

  8. CONJUGATE ACID-BASE PAIRS A pair of compounds that differ in composition by one H+ - H+ + H+ HBr (aq) + NH3 (aq)  NH4+ (aq) + Br- (aq) ACID BASE conjugate ACID conjugate BASE of NH3of HBr ALL ACID-BASE REACTIONS involve TWO CONJUGATE ACID-BASE PAIRS Acid(1) + Base(2) Conj-Acid(2) + Conj-Base(1) HCl + NH3 NH4+ + Cl- H2O + NH3 NH4+ + OH- CH3CO2H + H2O  H3O+ + CH3CO2- 1A03/1E03 Types of Reactions (2)

  9. RELATIVE STRENGTHS of ACIDS and BASES STRONG ACIDS - react completely with water to form H3O+ (aq) HCl (aq) + H2O  H3O+ (aq) + Cl- (aq) STRONG BASES - react completely with water to form OH- (aq) Li2O + H2O  2 Li+ (aq) + OH- (aq) Weak ACIDS/ weak BASES only react partially with water - an EQUILIBRIUM is formed : the conjugate ACID and the conjugate BASE are both present at the same time WEAK ACID: (acetic acid a.k.a. vinegar) CH3CO2H + H2O CH3CO2- (aq) + H3O+ (aq) WEAK BASE: NH3 (g) + H2O  NH4+ (aq) + OH- (aq) 1A03/1E03 Types of Reactions (2)

  10. Relative Strengths of acids/bases depend on the solvent : In water: H3O+ is the STRONGEST ACID OH- is the STRONGEST BASE Once an acid or base is fully reacted (dissociated) to form these species, one can no longer distinguish relative strengths. This is called the LEVELLING ACTION of water (The relative strengths of two strong acids can only be determined in non-aqueous solvents) STRONG ACIDS: HCl, H2SO4, H3PO4, HNO3, HClO4 WEAK ACIDS: CH3CO2H, “H2CO3” (= CO2), HS, HCN, HF WEAK BASES: NH3, Na2CO3 STRONG BASES: NaOH, Li2O, NaH, Ca(OH)2 Kotz, Table 17.3 In Br nsted acid-base reactions - H+ transfer occurs from STRONGER to WEAKER congugate acid-base pair 1A03/1E03 Types of Reactions (2)

  11. GAS FORMING REACTIONS - some reactions generate gases. When carried out in an OPEN system, the escape of the gas acts as a DRIVING FORCE for the chemical reaction - loss of gas product pulls reaction to completion just as formation of a precipitate e.g. CaCO3 (s) + 3 HCl (aq)  CaCl2 (aq) + H2O (l) + CO2 (g) Zn (s) + 2 HCl (aq)  ZnCl2 (aq) + H2 (g) NB. Each of these reactions can also be classified as one of the other 3 types of reactions - WHICH ONES ? 1A03/1E03 Types of Reactions (2)

  12. GAS FORMING REACTIONS - alternative classification A. CaCO3 (s) + 3 HCl (aq)  CaCl2 (aq) + H2O (l) + CO2 (g) There is no precipitate and no change in O.N. (Ca stays as +2, Cl as -1) It is an ACID-BASE REACTION involving an intermediate H2CO3 called CARBONIC ACID, which immediately dissociates to H2O and CO2 2 H+ + CO32- “H2CO3 “  H2O + CO2  B. Zn (s) + 2 HCl (aq)  ZnCl2 (aq) + H2 (g) There is no precipitate. However there is an obvious change in O.N. It is also an example of an OXIDATION-REDUCTION REACTION Zn goes from O.N. = 0 in the metal to O.N. = +2 in the chloride salt H goes from O.N. = +1 in HCl (aq) to O.N. = 0 in the elemental gas 1A03/1E03 Types of Reactions (2)

  13. CHEMISTRY of HALOGENS • The Elements • Group 7A - all non-metals • diatomc molecules: F2 (gas), Cl2 (gas), Br2 (liquid), I2 (solid) • most stable oxidation state: -1 • Undergo OXIDATION-REDUCTION reactions readily • with metals (all elements on right and middle of P.T.) • Trends: • SIZE: I > Br > Cl > F • Electron affinity: F > Cl > Br > I • Oxidizing strength F > Cl > Br > I Compounds most stable form: HALIDES (A+X-) hydrogen halides: ACIDS: HCl (strong), others (weak) 1A03/1E03 Types of Reactions (2)

  14. CHLORINE • prepared by electrochemical reaction (chlor-alkali process) • 2 NaCl + H2O -------> Cl2 + 2 NaOH • 8th most important industrial chemical by mass (1010 kg/yr) • IMPORTANT USES • disinfectant of water (as hpyochlorous acid (HOCl(aq)) • Cl2 + 2 H2O  H3O+ + Cl- + HOCl (aq) • bleach for textiles, paper, etc (as hypochlorite (OCl-) • Cl2 + 2 OH- H2O + Cl- + OCl- (aq) • organochlorine compounds: plastics, drugs ELECTROLYSIS 1A03/1E03 Types of Reactions (2)

  15. CHLORINE: BANE or BOON ??? Kotz, pp 88,89 Consumer friend . . . . . . . or environmental foe ?? 1A03/1E03 Types of Reactions (2)

  16. SUMMARY: Key Concepts from “Types of Reactions” (Kotz Ch. 4) • Classes of reactions: precipitation • oxidation-reduction • acid-base • gas-forming • Ionic versus non-ionic compounds • Solubility guidelines: Kotz, p. 168 • Oxidation Number rules: Kotz, p. 191 • Acid/base types: Arrhenius, Br nsted- Lowry • Typical chemistry of halogens • Some reactions PROCEED TO COMPLETION • e.g. precipitation, gas forming, strong acid/strong base • Some reactions establish an EQUILIBRIUM of reactants and products • e.g. oxidation-reduction, weak acids, weak bases LEARN TO BALANCE CHEMICAL EQUATIONS - both MASS and CHARGE balance are important 1A03/1E03 Types of Reactions (2)

  17. CHEMISTRY of DEMONSTRATIONS Oxidation-Reduction iron and chlorine: 2 Fe (s) + 3 Cl2 (g)  2 FeCl3 (s) (black) sugar and sulfuric acid: 2C6H12O6 (s) + 12 H2SO4 (aq)  6C (s) + 6 CO2 (g)  + 24 H2O (l) + 12 SO2 (g) Acid-Base acid and limestone: 2 H+ (aq)+ CaCO3 (s)  Ca2+ (aq) + CO2 (g)  + H2O(l) acid and metal: 2 H+ (aq) + Zn (s)  Zn2+ (aq) + H2 (g) 1A03/1E03 Types of Reactions (2)

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