Chapter 4 Reactions in Aqueous Solutions

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1 Chapter 4 Reactions in Aqueous Solutions Ionic compounds dissociate in water yielding electrolyte solutions. H 2 O NaCl(s) Na + (aq) + Cl - (aq) The ions are hydrated by water. The Electrical Conductivity of Ionic Solutions Solubility Rules for Ionic Compounds 1

Strong & Weak Electrolytes Strong Electrolytes are compounds that completely (100%) dissociate or ionize in water All soluble ionic compounds are strong electrolytes.

H 2 O C 12 H 22 O 11 (s) C 12 H 22 O 11 (aq) Since no ions are present, these solutions do not conduct electricity.

2 Strong & Weak Electrolytes Strong Electrolytes are compounds that completely (100%) dissociate or ionize in water All soluble ionic compounds are strong electrolytes. Weak electrolytes only partly ionize in solution. Nonelectrolytes Nonelectrolyte solutions result from dissolving molecular compounds. H 2 O C 12 H 22 O 11 (s) C 12 H 22 O 11 (aq) Since no ions are present, these solutions do not conduct electricity. Arrhenius Acid Base Theory Acid: A substance that produces hydronium ion, H 3 O + (or hydrogen ion, H + ) in water HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl - (aq) HCl(aq) H + (aq) + Cl - (aq) Base: A substance that produces OH - in water NaOH(aq) Na + (aq) + OH - (aq) Acids and Bases neutralize by combining H + and OH -. HCl(aq) + NaOH(aq) H 2 O(l) + NaCl(aq) 2

3 Acids and Bases Acid Properties sour taste (a bad test!) turn blue litmus red corrode many metals some common examples battery acid, H 2 SO 4 muriatic acid, HCl(aq) acetic acid (in vinegar), HC 2 H 3 O 2 ascorbic acid (vitamin C), H 2 C 6 H 6 O 6 citric acid (in citrus fruits), H 3 C 6 H 5 O 7 acetylsalicylic acid (aspirin), HC 9 H 7 O 4 In the formula for an acid, generally H is written first. Acids and Bases Base Properties bitter taste (again, a bad test!) turn red litmus blue feel slick to touch react with many dissolved metals to form a precipitate. some common examples sodium hydroxide, NaOH milk of magnesia, Mg(OH) 2 ammonium hydroxide, NH 4 OH In the formula for a base, generally OH - follows a cation. Strong Acids Strong acids are those that are strong electrolytes. They ionize 100% in water There are seven: HCl(aq) HBr(aq) HI(aq) HNO 3 (aq) H 2 SO 4 (aq) HClO 4 (aq) HClO 3 (aq) hydrochloric acid hydrobromic acid hydroiodic acid nitric acid sulfuric acid perchloric acid chloric acid 3

4 Strong Bases Soluble metal hydroxides are strong electrolytes, and thus strong bases. Note that all soluble ionic compounds are considered strong electrolytes, but only the soluble metal hydroxides are considered strong bases. LiOH NaOH KOH RbOH CsOH Ca(OH) 2 Sr(OH) 2 Ba(OH) 2 Weak Acids and Bases Weak acids and bases are called weak because they don t ionize or dissociate completely in water, i.e. they are weak electrolytes Acetic acid (HC 2 H 3 O 2 or HAc) is a weak acid HAc(aq) H + (aq) + Ac - (aq) (< 1%) Hydrofluoric acid is a weak acid HF(aq) H + (aq) + F - (aq) (< 1%) The forward reaction competes with the reverse reaction and establish a dynamic equilibrium. Any acid that is not a strong acid is a weak acid. (Dynamic) Chemical Equilibrium Why not an equilibrium for HCl(aq)? HCl(aq) H + (aq) + Cl - (aq) (100%) The symbol is not used for strong electrolytes KCl(aq) NaOH(aq) K + (aq) + Cl - (aq) Na + (aq) + OH - (aq) 4

5 Polyprotic Acids Acids that have more than one proton donate H 3 PO 4 (aq) H + (aq) + H 2 PO 4- (aq) H 2 PO 4- (aq) H + (aq) + HPO 4 2- (aq) HPO 4 2- (aq) H + (aq) + PO 4 3- (aq) Acidic Anhydrides Nonmetal oxides that react with water to make Arrhenius acids CO 2 + H 2 O H 2 CO 3 (aq) H 2 CO 3 (aq) 2H + (aq) + CO 2-3 (aq) N 2 O 5 + H 2 O 2HNO 3 (aq) HNO 3 (aq) H + (aq) + NO 3- (aq) SO 3, P 4 O 10, others. Basic Anhydrides Metal oxides that react with water to make Arrhenius bases Na 2 O + H 2 O 2NaOH(aq) NaOH(aq) Na + (aq) CaO + H 2 O Ca(OH) 2 (aq) Ca 2+ (aq) + OH - (aq) Ca(OH) 2 (aq) + 2OH - (aq) MgO, SrO, K 2 O, others. 5

6 Molecular Base NH 3 (g) + H 2 O NH 4 OH(aq) The base reacts with water to accept a H +. NH 4 OH(aq) NH 4+ (aq) + OH - (aq) Notice NH 3 also fits the definition of an Arrhenius base. Electrolyte Summary Ionic Compounds Strong Electrolytes Soluble Weak Electrolytes None Nonelectrolytes None Molecular Compounds Strong Acids Weak Acids & Weak Bases All other compounds Not all ionic compounds are significantly (> 0.01 mol/l) soluble in water, but if an ionic compound dissolves to any extent, it is completely (100%) dissociated. Exchange Reactions involve exchanging ions in solution. (Also called Double Replacement or Metathesis Reactions.) NaCl(aq) + AgNO 3 (aq) AgCl(s) + NaNO 3 (aq) 6

7 3 Types of Chemical Equations for Ionization/Dissociation Molecular Equation - write complete formulas, e.g., the acid-base neutralization rxn. HCl(aq) + NaOH(aq) H 2 O + NaCl(aq) Ionic Equation - write soluble strong electrolytes as their ions Net Ionic Equation - eliminate spectator ions (ions which are the same on both sides of the equation.) For ionic and net ionic eqns: write insoluble salts, weak electrolytes and molecular compounds in molecular form. M.E. HAc(aq) + NaOH(aq) I.E. N.I.E. For ionic and net ionic eqns: write insoluble salts, weak electrolytes and molecular compounds in molecular form. M.E. Na 2 SO 4 (aq) + Pb(NO 3 ) 2 (aq) I.E. N.I.E. 7

8 Evidence for Metathesis Reactions If ions exchange partners giving these products then a reaction would occur. a precipitate, AgCl(s) Solubility rules (used to predict rxns) or a weak electrolyte Any weak acid HAc (odor) or molecular compound, H 2 O (acid/base neutralization, often heat) H 2 S (odiferous gas) CO 2 (gas, bubbles) NH 3 (gas, characteristic odor) Gaseous Products Some gases are direct products of the metathesis reactions with acids. Sulfides, (reacting with acids) 2H + + S 2- H 2 S(g) Cyanides, (reacting with acids) H + + CN - HCN(g) Three common gas products result from secondary decomposition of unstable products. Carbonates, (reacting with acids) 2H + + CO 3 2- [H 2 CO 3 (aq)] CO 2 (g) + H 2 O(l) Sulfites, (reacting with acids) 2H + + SO 3 2- [H 2 SO 3 (aq)] SO 2 (g) + H 2 O(l) Ammonium salts, (reacting with bases) 2NH 4+ + OH - [NH 4 OH(aq)] NH 3 (g) + H 2 O(l) Predicting: When will a Metathesis reaction occur? 2 AgNO 3 (aq) + Na 2 CO 3 (aq) 3Na 2 SO 4 (aq) + 2K 3 PO 4 (aq) Na 2 S (aq) + H 2 SO 4 (aq) 8

9 Oxidation-Reduction Reactions [Redox Reactions] 2 Na(s) + Cl 2 (g) 2 NaCl(s) oxidation - loss of e - (increase oxidation #) Na Na + + e - reduction - gaining of e - (decrease oxidation #) Cl 2 + 2e - 2Cl - Oxidation Numbers: 1. The O.N. of all free elements are zero. (including H 2, Cl 2, etc.) 2. The O.N. of monatomic ions is the charge (e.g. Na +, Cl - ) 3. The sum of O.N. s in molecules or polyatomic ions is the charge on entire specie (NaCl is 0, SO 2-4 is 2) 4. Fluorine is F - in compounds 5. Hydrogen is H + in compounds 6. Oxygen is O 2- in compounds (except peroxide, O 2-2, each O is 1) Oxidation Numbers Rely on one element in a formula to determine the oxidation number (ON) of another. FeCl 3 ClO 4 - Cr 2 O Redox Reactions In a reaction, if something is oxidized then something else must be reduced. Reducing Agent: the substance which is oxidized. Oxidizing Agent: the substance which is reduced. Is this a redox reaction? Fe + 3O 2 2Fe 2 O 3 9

10 Simple Redox Reactions Simple redox reactions may be classified as: Combination Reactions A + B C Decomposition Reactions C A + B Hydrogen Displacement (e.g. by a metal) M + HX MX + H 2 Metal Displacement (e.g. 2 metals N and M) N + MX NX + M Halogen Displacement (e.g. 2 halogens X and Y) X 2 + MY MX + Y 2 Hydrogen Displacement: Acid reactions with metals Metals are oxidized by acids (if they react). Something must be reduced For example: Mg(s) + H 2 SO 4 MgSO 4 (aq) + H 2 (g) Hydrogen Displacement: Acid reactions with metals Mg + H 2 SO 4 MgSO 4 + H 2 H + is replaced by Mg The reaction proceeds in the direction shown because Mg is more active than H 2. HCl(aq) behaves similarly: Co(s) + 2 HCl(aq) CoCl 2 (aq) + H 2 (g) Co displaces H + Cobalt is more active than hydrogen Co is the reducing agent. H + is the oxidizing agent in Hydrogen Displacement reactions. 10

11 Hydrogen Displacement: Water reactions with metals Some metals are so active that they can readily replace hydrogen from cold water. 2 Na(s) + 2 H 2 O(l) 2 NaOH(aq) + H 2 (g) Some less active metals react with steam. Ti, Cr, Al Still less active metals won t displace hydrogen from water but will from acids. Ni, Pb Some metals have such low activities that they resist oxidation by water and acids. Au, Hg Metal Displacement: A more active metal may replace a less active metal. Generally, a metal may replace another metal ion Zn + CuSO 4 ZnSO 4 + Cu Zn replaced Cu (Zn is more active) If a reaction will not proceed in the direction written, it will in the opposite direction Pb + Mg(NO 3 ) 2 Pb(NO 3 ) 2 + Mg Mg would replace Pb (Mg is more active) Mg + Pb(NO 3 ) 2 Mg(NO 3 ) 2 + Pb Results like these are summarized in an activity series. Metal Activity Series The position of hydrogen in the table shows which metals will reduce H +. (Which metals will react with ordinary acids?) More active elements are more easily oxidized Thus are better reducing agents. Gold Mercury Silver Copper Hydrogen Lead Tin Cobalt Cadmium Iron Chromium Zinc Manganese Aluminum Magnesium Sodium Calcium Strontium Barium Potassium Rubidium Cesium Au 3+ Hg 2+ Ag + Cu 2+ H + Pb 2+ Sn 2+ Co 2+ Cd 2+ Fe 2+ Cr 3+ Zn 2+ Mn 2+ Al 3+ Mg 2+ Na + Ca 2+ Sr 2+ Ba 2+ K + Rb + Cs + Will Zn replace Cu 2+? Will Mg replace Pb 2+? Will Mn be oxidized by an acid? Will Au be oxidized by an acid? 11

(easily lose electrons) Very reactive nonmetals are strong oxidizers. (easily gain electrons.

12 Activities and the Periodic Table Electronegativity - The ability of an atom to attract electrons in a chemical bond. increases left to right increases up Very reactive metals are easily oxidized. (easily lose electrons) Very reactive nonmetals are strong oxidizers. (easily gain electrons. F Cl Br I M = moles Volume (L) Molarity moles solute/liter solution capital M for molarity capital L for liter mol for moles 0.35 M HCl means 0.35 mol HCl 1L soln Molarity What is the molar concentration of each ion in 0.20 M (NH 4 ) 2 SO 4? 12

Molarity What is the molarity of NaOH if 4.0 g of NaOH is dissolved to 500.0 ml of water? Molarity How many ml of 0.

13 Molarity What is the molarity of NaOH if 4.0 g of NaOH is dissolved to ml of water? Molarity How many ml of 0.35 M HCl should be measured out to give moles of HCl? Molarity How many grams of NaOH should be weighed out to prepare 250 ml of 0.10 M NaOH solution? 13

Dilutions Preparing a less conc solution from a higher conc M 1 V 1 = M 2 V 2 Molarity of stock soln Volume of stock soln Molarity of dilute soln Volume of dilute soln How many ml of the 0.

14 Dilutions Preparing a less conc solution from a higher conc M 1 V 1 = M 2 V 2 Molarity of stock soln Volume of stock soln Molarity of dilute soln Volume of dilute soln How many ml of the M NaOH solution would be required to prepare ml of a M NaOH solution? Stoichiometry grams A MM moles A moles B Stoic. Eq. MM grams B M x V = moles M x V = moles Molarity A and Volume A Molarity B and Volume B Acid-Base Titration: A quantitative stoichiometric method of determining acid or base concentrations (or amounts). A sample of unknown concentration is put in a flask. e.g., ml an acid solution The volume must be known. A buret is used to deliver a precise volume of a standard solution. The standard solution has a known concentration. e.g., an M base solution An indicator is used to detect the endpoint of a reaction. e.g., all acid is used up At the equivalence point, the amount of base added equals the amount of acid which was present to start. 14

Titration (acid-base) Phenolphthalein indicator is colorless in acidic solution and pink in basic solution so it can tell us when the equivalence point is passed.

2 drops of phenolphthalein indicator solution is added to the acid (colorless in acid) The solution is titrated with a standard 0.

15 Titration (acid-base) Phenolphthalein indicator is colorless in acidic solution and pink in basic solution so it can tell us when the equivalence point is passed. Titration (acid-base) ml of H 2 SO 4 solution (conc. unk.) is placed in an Erlenmeyer flask. 2 drops of phenolphthalein indicator solution is added to the acid (colorless in acid) The solution is titrated with a standard M NaOH until the endpoint is reached (colorless to pink) ml of NaOH was required. What is the molarity of unknown H 2 SO 4 acid? 2NaOH + H2 SO4 Na 2SO4 + 2H 2O Titration (acid-base) 2NaOH + H2 SO4 Na 2SO4 + 2H 2O ml mol/l ml? mol/l M NaOH x V NaOH mol NaOH mol H 2 SO 4 mol/l 15

Solubility Rules for Ionic Compounds Arrhenius Acid Base Theory

Solubility Rules for Ionic Compounds Arrhenius Acid Base Theory Chapter 4 Reactions in Aqueous Solutions Ionic compounds dissociate in water yielding electrolyte solutions. H 2 O NaCl(s) Na + (aq) + Cl - (aq) The ions are hydrated by water. Strong & Weak Electrolytes