Chap. AQUEOUS RXNS.1 WATER AS A SOLVENT Describe solution composition in terms of molarity Describe strong and weak electrolyte solutions, including acids and bases Use ionic equations to describe neutralization and other metathesis reactions Determine concentrations by applying solution stoichiometry to titrations Understand the key event in the redox process Water is a polar molecule Bent shape Covalent bonds Electrons not shared equally Water overcomes solute particleparticle interactions Electrostatic Dipole δ δ H O H δ IONIC SOLUTE: ELECTROLYTE Ionic solute in polar solvent Solute dissociated to ions in solution Solution conducts electricity Weak electrolytes: low concentrations of ions Solvated ion STRONG: AB n (s) A (aq) nb - (aq) WEAK: AB(s) A (aq) nb - (aq) NON-IONIC SOLUTE: NONELECTROLYTE Molecular solute in polar or nonpolar solvent Solute dispersed but not dissociated in solution Solution does not conduct electricity AB(s) AB(aq) NON-IONIC ACID SOLUTE Covalent compounds of H H X bonds polar Solvation by water leads to dissociation Solvation leads to formation of hydronium ion.2 AQUEOUS IONIC REACTIONS Many reactions of ions involve ionic compounds dissolved (dissociated) in H 2 O Not all ions participate in reactions EXAMPLE: The reaction of sodium hydroxide with hydrogen chloride (hydrochloric acid) HX(g) H (aq) X - (aq) H (ag) H 2 O(l) H 3 O (aq) NaOH(aq) HCl(g) NaCl(aq) H 2 O MOLECULAR EQUATION: Reactants and and products shown as as intact, undissociated compounds Page -1
Molecular equation does not show what happens: NaOH(s) Na (aq) OH - (aq) HCl(g) H (aq) Cl - (aq) Na (aq) OH - (aq) H (aq) Cl - (aq) Na (aq) Cl - (aq) H 2 O(l).3 Metathesis (Double Displacement) Reactions: Precipitation AX BY AY BX 2NaF CaCl 2 2NaCl CaF 2 TOTAL IONIC EQUATION All All soluble ionic ionic substances shown as as dissociated ions ions SPECTATOR IONS Do Do not not participate in in formation of of product H (aq) OH - (aq) H 2 O(l) NET IONIC EQUATION Molecular equation less less spectator ions ions Two compounds react and exchange components to to form two new compounds Aqueous media: A (aq) AX BY B (aq) Y - (aq) A (aq) Y - (aq) BX X - (aq) Reactants dissociate in solution For reaction to proceed, IONS MUST BE REMOVED FROM SOLUTION: Formation of an insoluble product (volatile gas or solid precipitate) Formation of a soluble weak/nonelectrolyte (water) Formation of insoluble product AgNO 3 soln NaNO 3 Na Ag NO 3 - NO REACTION soln AgNO 3 soln Na NO 3- AgCl(s) NaCl soln precipitate NaHCO 3 soln HCl soln Na Cl - H 2 O CO 2 (g) volatile product Table.3, p 119 SOLUBLE INSOLUBLE ANION NO - 3 C 2 H 3 O - 2 Cl - Br -, I - SO 2- SO 2- S 2- CO 2-3 PO 3- OH- - COMMENTS All All soluble soluble All All soluble soluble EXCEPT: Ag,, Hg 2 Hg 2 2, 2, Pb Pb 2 2 EXCEPT: Ag,, Hg 2 Hg 2 2, 2, Hg Hg 2 2,, Pb Pb 2 2 EXCEPT: Ca2 2,, Sr Sr 2 2,, Ba Ba 2 2,, Ag Ag,, Hg Hg 2 2,, Pb Pb 2 2 [X] < 0.01 M EXCEPT: Group Group 1A, 1A, 2A, 2A, NH EXCEPT: Group Group 1A, 1A, NH EXCEPT: Group Group 1A, 1A, NH EXCEPT: Group Group 1A, 1A, Ca2 2,, Sr Sr 2 2,, Ba Ba 2 2. Metathesis Reactions: Acid-Base Reactions Arrhenius Theory : ACID BASE Anything that produces hydrogen ions when dissolved in water HCl H Cl - Anything that produces hydroxide ions when dissolved in water NaOH Na OH - Page -2
STRONG ACIDS WEAK ACIDS STRONG BASES WEAK BASES Complete dissociation in water HX (X = Cl, Br, I), HNO 3, HClO 3, HClO, H 2 SO Partial dissociation in water HF and most other acids Dissociation in water to yield OH - Group 1A, 2A metal hydroxides: NaOH, Ca(OH) 2 Weak H acceptors (from H 2 O) NH 3, C 6 H 5 NH 2 Dissociation STRONG ACIDS AND BASES HCl(g) H 2 O(l) H 3 O (aq) Cl - (aq) NaOH(s) H 2 O(i) Na (aq) OH - (aq) WEAK ACIDS AND BASES HCN(l) H 2 O(l) H 3 O (aq) CN - (aq) NH 3 (g) H 2 O(l) NH (aq) OH - (aq) Metal hydroxides react with (NEUTRALIZE) acids to produce the salt of the acid and H 2 O: Acid-Base Titrations HBr NaOH NaBr H 2 O or H (aq) OH (aq) H 2 O(l) Weight A FORMULA WEIGHT Moles EQUATION Moles A COEFFICIENT B FORMULA WEIGHT Weight B Weak bases react with acids to produce the salt of the acid: NH 3 HCl NH Cl Not really a net metathesis reaction, by the way: SOLUTION MOLARITY Volume Volume Solution A SOLUTION MOLARITY Volume Volume Solution B NH 3 H NH Cl is a spectator ion Titrations Addition of a standard soln of V 0 known concentration to a soln of unknown concentration until stoichiometrically equivalent quantities have been reacted (equivalence or end point) STANDARD SOLN of AX V AX = V 0 - V f AX BY AY BX M AX V AX = equivalents of AX added V BY V f.5 Oxidation-Reduction (Redox) Reactions Ca(s) 2H (aq) Ca 2 (aq) H 2 (g) OXIDATION a reactant loses electron(s) Half-reaction: Ca 0 Ca 2 2e - REDUCTION a reactant gains electron(s) Half-reaction: H e - ½H 2 Electrons are transferred from one reactant to to a second reactant Oxidation is is always accompanied by by reduction Page -3
OXIDIZING AGENT Causes oxidation Is reduced Gains electrons (becomes more negative) REDUCING AGENT Causes reduction Is oxidized Loses electrons (becomes more positive) Ca 2H Ca 2 H 2 2e Examples OXIDATION (LOSE ELECTRONS, become MORE POSITIVE): E 0 E n ne Zn Zn2 2e E m E (mn) ne Fe 2 Fe 3 e E n E 0 ne N 3 ½N 2 3e REDUCTION (GAIN ELECTRONS, become MORE NEGATIVE): E 0 ne E n ½O 2 2e O2 E m ne E (m n) Fe 3 e Fe 2 E n ne E 0 Ag e Ag Oxidation Numbers (States): Monitoring Electron Transfer The charge that an atom (X) in an XY bond would have if its shared electrons were held completely by the atom that attracts the electrons more strongly The oxidation number (O.N.) of the oxidizing agent DECREASES in a redox reaction The O.N. of the reducing agent INCREASES in a redox reaction Oxidation Number Rules 1. For any atom in its elemental form, O.N. = 0 2. For any monatomic ion, O.N. = ionic charge (K, O.N. = 1; S 2, O.N. = 2) 3. The O.N. for hydrogen is 1, with the sole exception of metal hydrides (MH n ), where O.N. = 1. The O.N. for oxygen is 2, with the sole exception of peroxides (O 2 2 ), where O.N. = 1 5. The O.N. for other nonmetals is <0, with the exception of oxyanions, where O.N. > 0 6. The sum of all O.N. in a neutral compound is 0, otherwise ΣO.N. = ion charge.6 REDOX Reaction Types COMBINATION REACTIONS: Two or more reactants form a single product Decomposition A B C 2H 2 O 2 2H 2 O O 2 DECOMPOSITION REACTIONS: A single reactant forms two or more products DISPLACEMENT (SINGLE REPLACEMENT) REACTIONS: Reactant atoms or ions exchange places O: -1 0 O: -1-2 Page -
Combination Displacement (Single) A B C A BX AX B 2Mg O 2 2MgO H 2 CuO Cu H 2 O O: 0-2 Mg: 0 2 H: 0 1 Cu: 2 0 Can we we predict which elements will displace another element? Activity Series Table.1, p 130 Metals have different strengths as reducing agents Nonmetals, particularly halogens, have different strengths as oxidizing agents Redox reactants can be organized by relative strength in ACTIVITY SERIES, allowing prediction of a reaction s occurrence In general: A more active metal reduces a less active metal A more active halogen oxidizes a less active halogen Redox activity is also reflected in the O.N.s of elements in covalent compounds METAL OXIDATION REACTION Lithium Li Li e - Potassium K K e - Aluminum Al Al 3 3e - Zinc Zn Zn 2 2e - Lead Pb Pb 2 2e - Hydrogen ½H 2 H e - Copper Cu Cu 2 2e - Gold Au Au 3 3e - More easily oxidized More easily reduced HALOGEN Fluorine Chlorine Bromine Iodine REDUCTION REACTION ½F 2 e - F ½Cl 2 e Cl ½Br 2 e Br ½I 2 e I More easily oxidized More easily reduced Implication for O.N.: Br Cl 1 1 Page -5