The Major Classes of Chemical Reactions

Transcription

1 Chapter 4 The Major Classes of Chemical Reactions 5.1 Properties of compounds in Aqueous Solution 5.2 Precipitation Reaction 5.3 Acids and Bases 5.4 Reactions of Acid and Bases 5.5 Gas-forming Reactions 5.7 Oxidation-Reduction Reactions 5.8 Measuring Concentrations of Compounds in Solution 5.9 ph- a Concentration Scale for Acids and Bases 5.10 Stoichiometryof Reactions in Aqueous Solution Electron distribution in molecules of H 2 and H 2 O. In H 2, the nuclei are identical, they attract electrons equally. The central region of higher electron density is balanced by the two outer regions of less electron density. In H 2 O, the O nucleus attracts shared electrons more strongly than the H nucleus, creating an even charge distribution in each bond, the partially negative O is designated δ - and H δ +. Ball-and-stick model: a polar arrow points to the negative end of each O-H bond. The two polar O-H bonds and the bent molecule shape give rise to the polar nature of H 2 O Water separates ions in a process reducing the electrostatic force of attraction between them. Water interacts molecules strongly with the reactants and affects their bonds. 1

2 The dissolution of an ionic compound. The negative end of water molecules are attracted to cations andpositive end to anions. The ionic tug of war occurs as the ions become partially solvated. The attraction of ion and water overweighs the attraction of theoppositely charged ions. The ions become solvated as they separate from each other and move randomly throughout the solution. The electrical conductivity of ionic solutions. Weak electrolyte conduct electricity poorly. The compound is partially dissociated. Acetic acid, CH 3 COOH. Strong electrolyte conduct electricity well. The compound is 100% dissociated. Sodium chloride, NaCl. Soluble in water electrostatic attraction of water and ion dominates Insoluble in water attraction of ion among ions in compound favors. Noneletrolyte substances dissolve in water, but do not conduct electricity. Eletrolyte substance dissolve in water and dissociate, so conduct electriccurrent. 2

3 Solubility Rules For Ionic Compounds in Water Soluble Ionic Compounds 1. All common compounds of Group 1A(1) ions (Li +, Na +, K +, etc.) and ammonium ion (NH 4+ ) are soluble. 2. All common nitrates (NO 3- ), acetates (CH 3 COO - or C 2 H 3 O 2- ) and most perchlorates(clo 4- ) are soluble. 3. All common chlorides (Cl - ), bromides (Br - ) and iodides (I - ) are soluble, except those of Ag +, Pb 2+, Cu +, and Hg Insoluble Ionic Compounds 1. All common metal hydroxides are insoluble, except those of Group 1A(1) and the larger members of Group 2A(2)(beginning with Ca 2+ ). 2. All common carbonates (CO 3 2- ) and phosphates (PO 4 3- ) SO 3 2-, SO 4 2-, S 2- are insoluble, except those of Group 1A(1) and NH All common sulfides are insoluble except those of Group 1A(1), Group 2A(2) and NH 4+. Writing Equations for Aqueous Ionic Reactions The molecular equation shows all of the reactants and products as intact, undissociated compounds. The total ionic equation shows all of the soluble ionic substances dissociated into ions. The net ionic equation eliminates the spectator ions and shows the actual chemical change taking place. 3

4 A precipitation reaction and its equation. When water solutions of two different ionic compounds are mixed, an insoluble solid separated out of the solution. Predicting Whether a Precipitate Will Form 1. Note the ions present in the reactants. 2. Consider the possible cation-anion combinations. 3. Decide whether any of the ion combinations is insoluble. solubility rules for ionic compounds in water 4

5 Sample Problem Predicting Whether a Precipitation Reaction Occurs; Writing Ionic Equations Predict whether a reaction occurs when each of the following pairs of solutions are mixed. If a reaction does occur, write balanced molecular, total ionic, and net ionic equations, and identify the spectator ions. write ions combine anions & cations check for insolubility Table (a) sodium sulfate(aq) + strontium nitrate(aq) (b) ammonium perchlorate(aq) + sodium bromide(aq) (a) Na 2 SO 4 (aq) + Sr(NO 3 ) 2 (aq) 2NaNO 3 (aq) + SrSO 4 (s) 2Na + (aq) +SO 4 2- (aq)+ Sr 2+ (aq)+2no 3 - (aq) 2Na + (aq) +2NO 3 - (aq)+ SrSO 4 (s) SO 4 2- (aq)+ Sr 2+ (aq) (b) NH 4 ClO 4 (aq) + NaBr(aq) SrSO 4 (s) NH 4 Br(aq) + NaClO 4 (aq) eliminate spectator ions for net ionic equation All reactants and products are soluble so no reaction occurs. Table Selected Acids and Bases Acids Strong Bases Strong hydrochloric acid, HCl hydrobromicacid, HBr hydroiodicacid, HI nitric acid, HNO 3 sulfuric acid, H 2 SO 4 sodium hydroxide, NaOH potassium hydroxide, KOH calcium hydroxide, Ca(OH) 2 strontium hydroxide, Sr(OH) 2 barium hydroxide, Ba(OH) 2 perchloricacid, HClO 4 Weak hydrofluoric acid, HF Weak ammonia, NH 3 phosphoric acid, H 3 PO 4 acetic acid, CH 3 COOH (or HC 2 H 3 O 2 ) 5

6 Sample Problem Writing Ionic Equations for Acid-Base Reactions Write balanced molecular, total ionic, and net ionic equations for each of the following acid-base reactions and identify the spectator ions. (a) strontium hydroxide(aq) + perchloricacid(aq) (b) barium hydroxide(aq) + sulfuric acid(aq) reactants are strong acids and bases and therefore completely ionized in water products are water spectator ions (a) Sr(OH) 2 (aq)+2hclo 4 (aq) 2H 2 O(l)+Sr(ClO 4 ) 2 (aq) Sr 2+ (aq) + 2OH - (aq)+ 2H + (aq)+ 2ClO - 4 (aq) 2H 2 O(l)+Sr 2+ (aq)+2clo - 4 (aq) 2OH - (aq)+ 2H + (aq) (b) Ba(OH) 2 (aq) + H 2 SO 4 (aq) 2H 2 O (l) 2H 2 O(l) + BaSO 4 (aq) Ba 2+ (aq) + 2OH - (aq)+ 2H + (aq)+ SO 4 2- (aq) 2H 2 O(l)+Ba 2+ (aq)+so 4 2- (aq) 2OH - (aq)+ 2H + (aq) 2H 2 O (l) An acid-base reaction that forms a gaseous product. Molecular equation NaHCO 3 (aq) + CH 3 COOH(aq) CH 3 COONa(aq) + CO 2 (g) + H 2 O(l) Total ionic equation Na + (aq) + HCO 3- (aq) + CH 3 COOH(aq) CH 3 COO - (aq) + Na + (aq) + CO 2 (g) + H 2 O(l) Net ionic equation HCO 3- (aq) + CH 3 COOH(aq) CH 3 COO - (aq) + CO 2 (g) + H 2 O(l) 6

7 Rules for Assigning an Oxidation Number (O.N.) General rules 1. For an atom in its elemental form (Na, O 2, Cl 2, etc.): O.N. = 0 2. For a monoatomicion: O.N. = ion charge 3. The sum of O.N. values for the atoms in a compound equals zero. The sum of O.N. values for the atoms in a polyatomic ion equals the ion s charge. Rules for specific atoms or periodic table groups 1. For Group 1A(1): O.N. = +1 in all compounds 2. For Group 2A(2): O.N. = +2 in all compounds 3. For hydrogen: O.N. = +1 in combination with nonmetals 4. For fluorine: O.N. = -1 in combination with metals and boron 5. For oxygen: O.N. = -1 in peroxides O.N. = -2 in all other compounds(except with F) 6. For Group 7A(17): O.N. = -1 in combination with metals, nonmetals (except O), and other halogens lower in the group Sample Problem Determining the Oxidation Number of an Element Determine the oxidation number (O.N.) of each element in these compounds: (a) zinc chloride (b) sulfur trioxide (c) nitric acid The O.N.s of the ions in a polyatomic ion add up to the charge of the ion and the O.N.s of the ions in the compound add up to zero. (a) ZnCl 2. The O.N. for zinc is +2 and that for chloride is -1. (b) SO 3. Each oxygen is an oxide with an O.N. of -2. Therefore the O.N. of sulfur must be +6. (c) HNO 3. H has an O.N. of +1 and each oxygen is -2. Therefore the N must have an O.N. of +5. 7

8 A summary of terminology for oxidation-reduction (redox) reactions. e - X transfer or shift of electrons Y X loses electron(s) X is oxidized Y gains electron(s) Y is reduced X is the reducing agent X increases its oxidation number Y is the oxidizing agent Y decreases its oxidation number Sample Problem Recognizing Oxidizing and Reducing Agents Identify the oxidizing agent and reducing agent in each of the following: (a) 2Al(s) + 3H 2 SO 4 (aq) (b) PbO(s) + CO(g) (c) 2H 2 (g) + O 2 (g) Al 2 (SO 4 ) 3 (aq) + 3H 2 (g) Pb(s) + CO 2 (g) 2H 2 O(g) Assign an O.N. for each atom and see which atom gained and which atom lost electrons in going from reactants to products. An increase in O.N. means the species was oxidized (and is the reducing agent) and a decrease in O.N. means the species was reduced (is the oxidizing agent) (a) 2Al(s) + 3H 2 SO 4 (aq) Al 2 (SO 4 ) 3 (aq) + 3H 2 (g) The O.N. of Al increases; it is oxidized; it is the reducing agent. The O.N. of H decreases; it is reduced; H 2 SO 4 is the oxidizing agent. 8

9 Sample Problem Recognizing Oxidizing and Reducing Agents (b) PbO(s) + CO(g) Pb(s) + CO 2 (g) The O.N. of C increases; it is oxidized; CO is the reducing agent. The O.N. of Pb decreases; it is reduced; PbO is the oxidizing agent (c) 2H 2 (g) + O 2 (g) 2H 2 O(g) The O.N. of H increases; it is oxidized; it is the reducing agent. The O.N. of O decreases; it is reduced; it is the oxidizing agent. The ph values of some aqueous solutions ph = -log [H 3 O + ] poh = - log [OH - ] ph + poh = 14 9

10 Calculating [H 3 O + ], ph, [OH - ], and poh In an art restoration project, a conservator prepares copper-plate etching solutions by diluting concentrated (12 M) HNO 3 to 2.0 M, 0.30 M, and M HNO 3. Calculate [H 3 O + ], ph, [OH - ], and poh of the three solutions at 25 0 C. HNO 3 is a strong acid so [H 3 O + ] = [HNO 3 ]. Use K w to find the [OH - ] and then convert to ph and poh. For 2.0M HNO 3, [H 3 O + ] = 2.0M and -log [H 3 O + ] = = ph [OH - ] = K w / [H 3 O + ] = /2.0 = M; poh = Students to do : 0.30 M and M For 0.3M HNO 3, [H 3 O + ] = 0.30M and -log [H 3 O + ] = 0.52 = ph [OH - ] = K w / [H 3 O + ] = /0.30 = M; poh = For M HNO 3, [H 3 O + ] = M and -log [H 3 O + ] = 2.20 = ph [OH - ] = K w / [H 3 O + ] = /6.3x10-3 = M; poh = Determining the Molarityof H + Ions in Aqueous Solutions of Acids Nitric acid is a major chemical in the fertilizer and explosivesindustries. In aqueous solution, each molecule dissociates and the H becomes a solvated H + ion. What is the molarityof H + (aq) in 1.4M nitric acid? Use the formula to find the molarityof H +. Molarityis the concentration of solute in solution. Molarity(M) = moles of solute / liters of solution. One mole of H + (aq) is released per mole of nitric acid (HNO 3 ) HNO 3 (l) H 2 O H + (aq) + NO 3 - (aq) 1.4M HNO 3 (aq) should have 1.4M H + (aq). 10

11 Determining Moles of Ions in Aqueous Ionic Solutions How many moles of each ion are in the following solutions? (a) 5.0 mol of ammonium sulfate dissolved in water (b) 78.5 g of cesium bromide dissolved in water (c) 7.42x10 22 formula units of copper(ii) nitrate dissolved in water (d) 35 ml of 0.84 M zinc chloride We have to relate the information given and the number of moles of ions present when the substance dissolves in water. (a) (NH 4 ) 2 SO 4 (s) H 2 O 5.0 mol (NH 4 ) 2 SO 4 2 mol NH 4 + 2NH 4 + (aq) + SO 4 2- (aq) 1 mol (NH 4 ) 2 SO 4 = 10. mol NH mol SO 4 2- Sample Problem Finding the Concentration of Acid from an Acid-Base Titration volume(l) of base mol of base mol of acid M of acid You perform an acid-base titration to standardize an HClsolution by placing ml of HClin a flask with a few drops of indicator solution. You put M NaOHinto the buret, and the initial reading is 0.55 ml. At the end point, the buretreading is ml. What is the concentration of the HClsolution? multiply by M of base molar ratio divide by L of acid NaOH(aq) + HCl(aq) ( ) ml x 1L 10 3 ml NaCl(aq) + H 2 O(l) = L L X M = 5.078x10-3 mol NaOH Molar ratio is 1: x10-3 mol HCl L = M HCl 11