Chapter 4: Chemical Reactions in Aqueous Solutions. 4.1 Some Electronic Properties of Aqueous Solutions

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Chapter : Chemical Reactions in Aqueous Solutions Water: the universal solvent ¾ of Earth s surface water in living organisms Some Electrical Properties of Aqueous Solutions Reactions of Acids and

1 Chapter : Chemical Reactions in Aqueous Solutions Water: the universal solvent ¾ of Earth s surface water in living organisms Some Electrical Properties of Aqueous Solutions Reactions of Acids and Bases Precipitation Reactions Reactions Involving xidation and Reduction Homework Assignment:.1A,.2A,.A,.7A 25, 31, 33, 5, 53, 55, 57, 61, 65, 67, 71, 73,.1 Some Electronic Properties of Aqueous Solutions Electric current: a flow of charged particles In metals: flow of electrons Aqueous solutions of ionic compounds: good conductors; flow of ions Electrodes: Anode: connected to the positive pole Cathode: to the negative pole Ions: Anions (-) attracted to the anode Cations (+) attracted to the cathode How electricity passes through a solution

2 A. Arrhenius s Theory of Electrolytic Dissociation Electrolyte: a solute that produces enough ions to make its aqueous solution an electrical conductor. Strong electrolytes produces a solution that conducts an electrical current very well. Weak electrolytes: conducts an electrical current quite weakly. Nonelectrolyte: its solution does not conduct a current. Nonelectrolyte Strong electrolyte Weak electrolyte The solute forms no ions; present as molecules The solute breaks apart completely into ions. The solute only partially breaks apart into ions. Nonconductor of electricity good conductor poor conductor Most molecular substances Sugars, alcohols Soluble ionic compounds A few molecular compounds NaCl, Na 2 S, NaH HCl, H 2 S Some molecular substances, esp. carboxylic acids CH 3 CH, NH 3 Electrolytes and Nonelectrolytes

3 B. Ion concentrations in Solutions of Strong Electrolytes Molar concentration of ions: [Na + ], [Cl - ] Example What are the concentrations of Na + and S ions in M Na 2 S (aq) solution? Na 2 S is a strong electrolyte. Na 2 S (s) 2 Na + (aq) + S (aq) mol Na S [Na ] = 1L soln [S mol Na 2S ] = 1L soln + 2 mol Na 1mol Na S 2 = 2 2 = 1mol S 1mol Na S M M Total ion concentration = [Na + ] + [S ] = M M = M.2 Reactions of Acids and Bases An acid produces H + ions in aqueous solution. A base produces H - ions in aqueous solution. A. Strong Acids: acids that ionize completely in solution. strong electrolytes; good conductors HCl(g) H + (aq) + Cl (aq) Representation of hydrochloric acid: HCl(aq) or H + (aq) + Cl - (aq) Weak Acids: acids that ionize only partially in solution. Most remain as intact molecules weak electrolytes CH 3 CH(aq) Acetic acid H + (aq) + CH3C - (aq) Acetate ion means a reversible reaction. Representation: CH 3 CH(aq)

4 Common strong acids HCl HBr HI HN 3 H 2 S HCl Hydrochloric acid Hydrobromic acid Hydroiodic acid Nitric acid Sulfuric acid Perchlorate acid To recognize acids from name or formula The names of acids contain acid. Acetic acid, sulfuric acid. The formula of an acid usually begins with ignizable H atoms. HN 3, H 2 S, H 3 P are acids; CH is not. Structural or condensed structural formula contains a CH group. CH 3 CH acetic acid B. Strong and Weak Bases Ionic hydroxides are strong bases NaH(s) Na + (aq) + H (aq) Most molecular substances that act as bases are weak bases. NH 3 (aq) + H 2 (l) NH + (aq) + H - (aq) The most common weak bases are ammonia and the amines. Amines are organic compounds in which one or more of the H atoms in NH 3 are replaced by a hydrocarbon alkyl group. H H H C N H or CH 3 NH 2 H Methylamine CH 3 CH 2 NH 2 Ethylamine

5 Common Strong Bases Group 1A hydroxides Group 2A hydroxides LiH Mg(H) 2 NaH Ca(H) 2 KH Sr(H) 2 RbH Ba(H) 2 CsH C. Acid-Base Reactions: Neutralization Acid + Base Salt + H 2 1. Strong acid and base HCl(aq) + NaH(aq) NaCl(aq) + H 2 (l) complete formula equation H + (aq) + Cl (aq) + Na + (aq) + H (aq) Na + (aq) + Cl (aq) + H 2 (l) ionic equation eliminate spectator ions H + (aq) + H (aq) H 2 (l) net ionic equation 2. Insoluble or only slightly soluble base Mg(H) 2 (s) + 2 HCl(aq) MgCl 2 (aq) + 2 H 2 Mg(H) 2 (s) + 2 H + (aq) + 2 Cl (aq) Mg Cl (aq) + 2 H 2 Mg(H) 2 (s) + 2 H + (aq) Mg H 2 net ionic equation Used as antacids to neutralize excess stomach acid.

6 D. More Acid-Base Reactions 1. Neutralization of ammonia by a strong acid H + (aq) + NH 3 (aq) NH + (aq) 2. Reactions of anions with acids to form gases C 3 (aq) + 2 H + H 2 (l) + C 2 (g) Baking soda, NaHC 3 reacts with some weak acid HA: HA(aq) + HC 3 (aq) A (aq) + H 2 (l) + C 2 (g) C 2 (g) causes dough to rise S + 2 H + H 2 S(g) 3. Neutralization involving weak acid + weak base CH 3 CH(aq) + NH 3 (aq) NH + (aq) + CH 3 C - (aq).3 Reactions that Form Precipitates There is a limit to how much solute will dissolve in water: maximum concentration of 5.7 M NaCl in water Insoluble in water if max. conc. of solute is less than about 0.01 M. Such as AgI(s) Precipitation results when dissolved anions and cations combine to form a compound that is insoluble (a precipitate). AgN 3 (aq) + KI(aq)

7 A. Predicting precipitation reactions AgN 3 (aq), KI(aq) 1. Swap the anions or cations to get potential products AgN 3 (aq) + KI(aq) KN 3 (?) + AgI(?) 2. A precipitation reaction will occur if either proposed product is insoluble. AgI is insoluble in water; it precipitates out of the solution. Ag +( aq) + N 3- (aq) + K + (aq) + I - (aq) AgI(s) + K + (aq) + N 3- (aq) Ag +( aq) + I - (aq) AgI(s) net ionic equation AgN 3 (aq) KI(aq) K + KN 3 (aq) Ag + + I - AgI(s) General guidelines for the Water Solubilities of Ionic Compounds Almost all nitrates, acetates, perchlorates, group 1A metal salts, and ammonium salts are soluble. Insoluble ionic compounds Pb 2+, Ag +, Hg 2 2+ chloride, bromide, iodide Ba 2+, Pb 2+, Hg 2 2+ sulfate (S ) Except (ammonium and Group 1A metal ions) carbonate, hydroxide, phosphate, sulfide e.g. Ca 2+ (aq) + C 3 (aq) CaC 3 (s)

8 B. Some Applications of Precipitation Reactions Al 3+ (aq) + 3 H - (aq) Al(H) 3 (s) Al 3+ (aq) + P 3- (aq) AlP (s) Water purification. (The gelatinous precipitates carries down suspended matter.) Removal of phosphates from wastewater. Mg 2+ (aq) + 2 H - (aq) Mg(H) 2 (s) Precipitation of magnesium ion from seawater. Ag + (aq) + Br - (aq) AgBr(s) For use in photographic film. Reactions Involving xidation and Reduction The largest category of reactions; often called redox reactions Combustion, extraction of metals from ores, manufacture of countless chemicals xidation originally used to describe reactions of a substance combining with oxygen, Reduction originally to describe the removal of oxygen. A. xidation Numbers (N): the actual charge on a monatomic ion or a hypothetic charge assigned to an atom in a molecule or in a polyatomic ion. It is a hypothetical concept, not a measured one. Useful for dealing with oxidation and reduction. Ions Ionic compound CaCl 2, made up of Ca 2+ and Cl -. Ca has an N of +2, and Cl has an N of 1. Total oxidation numbers in CaCl 2 is (-1) = 0.

In molecules, electrons are shared ; no electrons are transferred. We can assign oxidation numbers as if electrons were transferred. H 2 molecule: assign each H atom an N of +1.

9 In molecules, electrons are shared ; no electrons are transferred. We can assign oxidation numbers as if electrons were transferred. H 2 molecule: assign each H atom an N of +1. We require the total Ns be zero. N of 2 is assigned to atom. Rules for determining xidation Number (listed by priority) Sum of N is zero for a neutral species; Sum of N is equal to the charge on the ion. Group 1A elements in compounds have N = +1; 2A elements in compounds have N = +2. In compounds F has N = -1. In compounds H has N = +1. In compounds has N = -2. In binary compounds, group 7A elements have N = -1; 6A elements have N = -2; and group 5A elements have N = -3. Example.7 What are the oxidation numbers assigned to the atoms in the following compounds? (a) KCl The N of K is +1; The N of is 2; total N for four atoms is 8. For K and, the total is +1-8 = -7. The N of Cl must be +7 to give a total N of zero for all atoms. (b) Cr 2 7 The N of is 2; total N for seven is Total N for two Cr must be equal to 2 (-1) = +12. Cr 2 N for one Cr is (c) CaH 2 N of Ca is +2. (higher priority than N = +1 for H) Total N of two H atoms must be 2. N of one H atom is 1. (rather than its usual +1) H 2 Ca (d) Na 2 2 N of Na is +1. Total N of two Na atoms is +2. Total N of two atoms must be 2. N of one atom is 1. (rather than its usual 2) (e) Fe 3 N of is 2. Total N of four atoms is 8. Total N of three Fe atoms must be +8; N of Fe is +8/3. Note: the first rule (highest priority) is used last.

10 xidation Numbers xidation number of an atom in a compound is determined by rules. The oxidation number of an atom often is different in different circumstances. The oxidation numbers of atoms in elements are zero H 2, Cl 2, 2, S 8, C In some compounds the same element in different ions may have different N. NH N 3 The N in NH + has N = -3; the N in N 3- has N = +5. B. Identifying xidation-reduction Reactions xidation: process where the oxidation number of one or more elements increases Reduction: process where the oxidation number of elements decreases Al(s) + Fe 2 3 (s) 2 Fe(l) + Al 2 3 (s) Aluminum is oxidized to Al 2 3 ; Iron(III) oxide Fe 2 3 is reduced to Fe Cu(s) + 2 Ag + (aq) Cu 2+ (aq) + 2 Ag(s) Cu(s) is oxidized to Cu 2+ (aq); Ag + (aq) is reduced to Ag(s). Demonstration

11 Fundamental definition: electron transfer Cu 2+ Ag + 2e AgAg Cu xidation: Cu(s) Cu 2+ (aq) + 2e - losing 2 electrons Reduction: 2 Ag + (aq) + 2e - 2 Ag(s) gaining 2 electrons In oxidation process, electrons are lost; in reduction, electrons are gained. xidation and reduction occur simultaneously. C. Balancing xidation-reduction Reactions Mn - (aq) + Fe 2+ (aq) + H + Mn 2+ (aq) + Fe 3+ (aq) + H 2 (l) (not balanced) Mn - (aq) + Fe 2+ (aq) + 8 H + Mn 2+ (aq) + Fe 3+ (aq) + H 2 (l) (charge not balanced) 9+ charge 5+ charge By trial and error Mn - (aq) + 5 Fe 2+ (aq) + 8 H + Mn 2+ (aq) + 5 Fe 3+ (aq) + H 2 (l) 17+ charge 17+ charge (balanced) Redox equations must be balanced according to atoms and according to charge. A more systematic method of balancing redox equations exist.

12 D. xidizing and Reducing Agents The substance that is oxidized is called a reducing agent. It causes some other substances to be reduced. A reducing agent loses electrons (to an oxidizing agent) A metal, such as sodium, is good reducing agent. The substance that is reduced is called an oxidizing agent. It causes some other substances to be oxidized. A oxidizing agent gains electrons (from a reducing agent) Nonmetal, such as fluorine, is a good oxidizing agent. Both an oxidizing agent and a reducing agent are needed for a redox reaction. The number of electrons gained by the oxidizing agent must equal the number given up by the reducing agent E. xidation Numbers of Nonmetals xidizing agent Reducing agent Group 5A N N 2 + N N +2 N 2 +1 N 2 0 NH 2 H -1 N 2 H -2 NH 3-3 Group 6A S +6 S S 3 + S 2 +3 S S 2 Cl 2 +1 S 8 0 H 2 S 2-1 H 2 S -2 Group 7A Cl - +7 Cl Cl Cl 2 + Cl Cl - +1 Cl 2 0 Cl The highest N = group number. The species with highest N nonmetal is good oxidizing agent. 2. The lowest N = group number 8. Good reducing agent. 3. Intermediate N. Either an oxidizing agent or a reducing agent. But generally one role or the other is more common.

13 Why NH N 3 is an explosive Reducing agent NH N 3 xidizing agent In NH + ion, N of H is +1; total N of four H atoms is +; Total N = charge = +1. N of N = +1 (+) = -3. In N 3 - ion, N of N = +5. It has an oxidizing agent and a reducing agent They are adjacent to each other 2 NH N 3 (s) 2 N 2 (g) + 2 (g) + H 2 (g) F. Metals as Reducing Agents Metals are reducing agents; strength varies widely Activity series of metals Group 1A and 2A metals are strong reducing agents. Silver and gold are very poor reducing agents. A metal will displace from solution the ions of any metal below it in the activity series. Mg(s) + Cu 2+ (aq) Mg 2+ (aq) + Cu(s) Cu(s) + 2 Ag + (aq) Cu 2+ (aq) + 2 Ag(s) Fe(s) + Mg 2+ (aq) no reaction Any metal above hydrogen can react with an acid to produce H 2 (g). 2 Al(s) + 6 H + (aq) 2 Al 3+ (aq) + 3 H 2 (g) Ag(s) + H + (aq) no reaction K Ca Na Mg Al Cr Zn Fe Cd Ni Sn Pb H 2 Cu Ag Hg Au Strength as a reducing agent powerful strong good Fair poor Very poor

14 .5 Applications of xidation and Reduction Industry applications 1. xygen is a common oxidizing agent used in steel making. 2. xygen is used to oxidize hydrogen and acetylene in torches for welding and cutting. 3. Chlorine gas and its compounds are used as oxidizing agents in water treatment plants for killing microorganisms, and in paper and textile industries for bleaching.. Industrial reducing agents: carbon and hydrogen. Used in manufacturing iron from ore.

Chapter Four. Chapter Four. Chemical Reactions in Aqueous Solutions. Electrostatic Forces. Conduction Illustrated

Chapter Four. Chapter Four. Chemical Reactions in Aqueous Solutions. Electrostatic Forces. Conduction Illustrated 1 Electrostatic Forces 2 Chemical Reactions in Aqueous Solutions Unlike charges (+ and ) attract one another. Like charges (+ and +, or and ) repel one another. Conduction Illustrated 3 Arrhenius s Theory