1A03/1E03 Types of Reactions (2)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 AgNO 3 (aq)  Cu(NO 3 ) 2 (aq) + 2 Ag(s) Transfer of electrons: Cu 0 gives 2 electrons to 2 Ag + to form Cu 2+ and 2 Ag 0 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(NO 3 ) 2 O.N. = +2 Ag in AgNO 3 O.N. = +1 Ag in Ag(s)O.N. = 0 (see Fig in Kotz)

1A03/1E03 Types of Reactions (2)3 RULES for OXIDATION NUMBERSO.N. (see Kotz, p. 191) Atoms in pure ELEMENT (Cu, I 2, S 8 )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 (CaH 2 ) where O.N. = -1 oxygen in most compounds (MgO, Na 2 O)-2 –EXCEPT in peroxides (H 2 O 2 ) 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 in H 2 SO 4 ? 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 in H 2 SO 4 is : What is O.N. of N in HNO 3 ? 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 in HNO 3 is : this shows the nitrate group: “ NO 3 “ has a net O.N. of (and thus a charge) of THUS the O.N. of Ag in AgNO 3 is

1A03/1E03 Types of Reactions (2)5 Another example of an OXIDATION-REDUCTION REACTION Fe 2 O 3 (s) + 3 CO (g)  2 Fe (s) + 3 CO 2 (g) Species Fe in Fe 2 O 3 C in COFe C in CO 2 O.N Thus: Fe in Fe 2 O 3 GAINS 3 electrons- Fe 2 O 3 is the OXIDIZING AGENT C in CO LOSES 2 electrons- CO is the REDUCING AGENT The iron in Fe 2 O 3 was REDUCED to Fe The carbon in CO was OXIDIZED to CO 2 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 ) 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) + H 2 O 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 HCO 3 - (aq)  H + (aq) + CO 3 2- (aq) BASE - substance that accepts H + from another species NH 3 + H 2 O  NH 4 + (aq) + OH - (aq) TYPICAL PROPERTIES of ACIDS and BASES ACIDS sour tase corrosive reacts with bases turns natural dyes red generates CO 2 from limestone generates H 2 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 + HBr (aq) + NH 3 (aq)  NH 4 + (aq) + Br - (aq) ACID BASE conjugate ACID conjugate BASE of NH 3 of HBr - H + + H + ALL ACID-BASE REACTIONS involve TWO CONJUGATE ACID-BASE PAIRS Acid(1)+ Base(2)  Conj-Acid(2)+Conj-Base(1) HCl +NH 3  NH 4 + +Cl - H 2 O+NH 3  NH 4 + +OH - CH 3 CO 2 H+H 2 O  H 3 O + +CH 3 CO 2 -

1A03/1E03 Types of Reactions (2)9 RELATIVE STRENGTHS of ACIDS and BASES STRONG ACIDS - react completely with water to form H 3 O + (aq) HCl (aq) + H 2 O  H 3 O + (aq) + Cl - (aq) STRONG BASES - react completely with water to form OH - (aq) Li 2 O + H 2 O  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) CH 3 CO 2 H + H 2 O  CH 3 CO 2 - (aq) + H 3 O + (aq) WEAK BASE: NH 3 (g) + H 2 O  NH 4 + (aq) + OH - (aq)

1A03/1E03 Types of Reactions (2)10 Relative Strengths of acids/bases depend on the solvent : In water:H 3 O + 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, H 2 SO 4, H 3 PO 4, HNO 3, HClO 4 WEAK ACIDS:CH 3 CO 2 H, “H 2 CO 3 ” (= CO 2 ), HS, HCN, HF WEAK BASES:NH 3, Na 2 CO 3 STRONG BASES:NaOH, Li 2 O, 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. CaCO 3 (s) + 3 HCl (aq)  CaCl 2 (aq) + H 2 O (l) + CO 2 (g)  Zn (s) + 2 HCl (aq)  ZnCl 2 (aq) + H 2 (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.CaCO 3 (s) + 3 HCl (aq)  CaCl 2 (aq) + H 2 O (l) + CO 2 (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 H 2 CO 3 called CARBONIC ACID, which immediately dissociates to H 2 O and CO 2 2 H + + CO 3 2-  “H 2 CO 3 “  H 2 O + CO 2  B.Zn (s) + 2 HCl (aq)  ZnCl 2 (aq) + H 2 (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: F 2 (gas), Cl 2 (gas), Br 2 (liquid), I 2 (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 strengthF > Cl > Br > I Compounds most stable form: HALIDES (A + X - ) hydrogen halides: ACIDS: HCl (strong), others (weak)

1A03/1E03 Types of Reactions (2)14 prepared by electrochemical reaction (chlor-alkali process) 2 NaCl + H 2 O > Cl NaOH 8th most important industrial chemical by mass (10 10 kg/yr) IMPORTANT USES disinfectant of water (as hpyochlorous acid (HOCl(aq)) Cl H 2 O  H 3 O + + Cl - + HOCl (aq) bleach for textiles, paper, etc (as hypochlorite (OCl-) Cl OH -  H 2 O + Cl - + OCl - (aq) organochlorine compounds: plastics, drugs ELECTROLYSIS CHLORINE

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 Solubilityguidelines: Kotz, p. 168 Oxidation Numberrules: 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 Cl 2 (g)  2 FeCl 3 (s) (black) sugar and sulfuric acid: 2C 6 H 12 O 6 (s) + 12 H 2 SO 4 (aq)  6C (s) + 6 CO 2 (g)  + 24 H 2 O (l) + 12 SO 2 (g) Acid-Base acid and limestone: 2 H + (aq) + CaCO 3 (s)  Ca 2+ (aq) + CO 2 (g)  + H 2 O(l) acid and metal: 2 H + (aq) + Zn (s)  Zn 2+ (aq) + H 2 (g)