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CH 1 301 Chapter 4 & 10 Acids and Bases Chapter 4 Revisited: 4-2 (part 2) Strong and Weak Acids Acids are defined as substances that produce H+ in aqueous solution. The strong acids completely ionized (or almost completely ionize) when in dilute aqueous solution. The H+ ions produced in aqueous solutions produce acidic solutions. By now, you should be thoroughly familiar with the seven strong acids: Three are derived from the halogens (Group 7) HCl HBr HI Four are derived from the polyatomic ions (oxoacids): H2SO4 HNO3 HClO3 HClO4 Note: An acid cannot be an acid unless it is in solution--that is, it must be able to produce H+ and that can happen only in solution. Often, and correctly so, acids are named with the symbol (aq) because acids are more commonly dissolved in water. E.g. HClO3 (aq) chloric acid (a strong acid) When such an acid is added to water, hydrogen ions are produced: HClO3 (aq) * H 2O H + (aq) + ClO3- (aq) *(~100%) This type of notation indicates that the strong acid, chloric acid, is completely ionized in aqueous solution. Weak acids, on the other hand, only partially ionize in aqueous solution. Generally the degree to which these acids ionize is very small such as acetic acid that only ionizes about 0.5%. When a weak acid is added to water, the numbers of hydrogen ions that are produced are very small. The H+ produced combine with the anion to reform the weak acid molecule. This is called a reversible reaction. 2 CH 3COOH (aq) H 2O H + (aq) + CH 3COO- (aq) (reversible ; favorsthe formationof thereactant ) The double arrows in the reversible reaction indicates that when the reactant ionizes to form H+ and CH3COO- ions, the ions tend to recombine to form the original weak acid molecule. The bottom arrow, which is longer than the top arrow, indicates that the nonionized form of the weak acid is favored--that is, if you could look into a dilute acetic acid solution, you would see mostly CH3COOH molecules and very few H+ and CH3COO- ions. There are many weak acids and only seven strong acids. To determine if an acid is strong or weak, you should realize if it is not one of the seven strong acids, it must be a weak acid. There are several common weak acids that you should recognize immediately. (TABLE 4-6 and a few additional weak acids.) HF CH3COOH H2CO3 HCN H3PO4 HNO2 (COOH)2 HClO HClO2 H2SO3 You should be able to name and/or write the structural formulas to the above ten weak acids. Naming Acids: (Binary Acids, page 163-164) Binary acids, (acids of group 6 and 7), are composed of hydrogen and one other nonmetal element (except oxygen). Binary acids are named by using the prefix "hydro-" and changing the suffix from "-ide" to "-ic". For example: Strong binary acids: HCl (aq) HBr (aq) HI (aq) hydrochloric acid hydrobromic acid hydroiodic acid 3 Weak binary acids: HF (aq) H2S (aq) HCN (aq) hydrofluoric acid hydrosulfuric acid hydrosulfuric acid Note: Without the (aq), these compounds are named as covalent compounds. One observation you should make, however, is that H2S is named as hydrogen sulfide rather than dihydrogen sulfide--this is the common name, but the latter is technically correct. Naming Acids: (Ternary Acids, page 165-167) Ternary acids are also called oxoacids. Ternary acids are composed of hydrogen, oxygen, and one other nonmetal. The numbers of oxygens in these acids vary. (Polyatomic ions with hydrogens). An important concept to learn, concerning the naming of oxoacids, is oxidation states. Oxidation states of the central atoms (the atoms that are not hydrogen or oxygen) are assigned as follows: 1. 2. 3. The oxidation state of hydrogen is +1. The oxidation state of oxygen is -2. The oxidation states of the central atoms in the acids are determined by balancing the charges of the hydrogen and oxygen so that the overall charge on the molecule equals zero. For example: H2SO4 (sulfuric acid): +1 -2 H2SO4 each hydrogen has a +1 oxidation state each oxygen has a -2 oxidation state The oxidation state of the sulfur atom is determined by balancing the overall charges of the hydrogen and oxygen so that the molecule has a net charge of zero. Let X = oxidation state of the sulfur atom. 2 (+1) + X + 4 (-2) = 0 (+ 2) + X + (-8) = 0 X = +6 The oxidation state of the sulfur atom is + 6. 4 Compare the oxidation state of sulfuric acid to the weak oxoacid H2SO3. H2SO3 (sulfurous acid): Using the same process: +1 -2 H2SO3 each hydrogen has a +1 oxidation state each oxygen has a -2 oxidation state Let X = oxidation state of the sulfur atom. 2 (+1) + X + 3 (-2) = 0 (+ 2) + X + (-6) = 0 X = +4 The oxidation state of the sulfur atom is + 4. The weaker acid, sulfurous acid, has the lower oxidation state. The acid with the lower oxidation state is given the "-ous" ending. It should also be noted that the oxoacids are NOT given the "hydro-" prefix. To name an oxoacid, simply the give the acid the name of the central atom with the either: 1. 2. The -ous ending (the lower ox. state) The -ic ending (the higher ox. state) HNO3 HNO2 nitric acid (+5) nitrous acid (+3) There are oxoacids with more than two possible structures (with only a difference in the numbers of oxygens): These acids are derived from the halogens (except fluorine). The naming of such oxoacids is as follows: (pages 166-167). ClO ClO2- ClO3- ClO4- - Ion Name hypochlorite ion chlorite ion chlorate ion perchlorate ion Acid HClO (aq) HClO2 (aq) HClO3 (aq) HClO4 (aq) Name hypochlorous acid chlorous acid chloric acid perchloric acid A more complete list of weak acids can be seen in Appendix F. 5 The Strong Soluble Bases: (Table 4-7) Strong soluble bases are ionic metal hydroxides. Strong soluble bases dissolve in water to produce hydroxide ions (OH-). The hydroxide ions produced in water creates a basic solution. Group IA metals combine with the hydroxide ion form strong soluble bases. Group IIA metals, Ca2+, Sr2+, and Ba2+, combine with the hydroxide ion to form strong soluble bases. Ba(OH )2 H 2O Ba2+ (aq) + 2 OH - (aq) (~ 100 %) The remaining metal hydroxides are not very soluble in water and, therefore; do not form solutions that are very basic. Note: There is a difference between insoluble metal hydroxides and weak bases. Insoluble metal hydroxides do not dissolve in water whereas; weak bases dissolve in water but do not ionize very well. The Weak Soluble Bases: (Table 4-9) The weak soluble bases are ammonia, NH3, and the derivatives of ammonia--called amines. When CH3 (methyl) groups are used to replace the hydrogens of the ammonia molecule, the molecule becomes an amine. NH3 CH3NH2 ammonia methyl amine (one hydrogen was replaced with a methyl group.) dimethyl amine (two hydrogens were replaced with methyl groups.) These two amines will be the only two ammonia derivatives that we will use as weak bases. A more complete list of such weak bases can be seen in Appendix G. Weak soluble bases are reversible and do not produce strong basic solutions. (CH3)2 NH2 NH 3 (aq) H 2O NH 4+ (aq) + OH - (aq) (reversible; reactant is favored ) H 2O CH 3 NH 2 (aq) CH 3 NH 3+ (aq) + OH - (aq) (reversible) 6 Chapter 10: Reactions in Aqueous Solutions I: Acids, Bases, and Salts 10-1 Properties of Acids and Bases: Acidic solutions Taste Consistency Color Reactions Sour same as water turn litmus red Basic solutions Bitter slippery turn litmus blue react with some metals to produce H2 gas (not aqueous HNO3) neutralize metal oxides and metal hydroxides to produce salts and water strong acids are strong electrolytes weak acids are weak electrolytes neutralize acids to form salts and water Electrolyte strong soluble bases are strong electrolytes weak bases and insoluble bases are weak electrolytes The Arrhenius Theory: Acids produce H+ ions Bases produce OH- ions H2O (l) neutralization H+ (aq) + OH- (aq) But: We know that H+ ions do not exist in aqueous solutions. Rather, the H+ ions form coordinate covalent bonds with water. We also know that ammonia does not have an --OH group (ammonia is a weak base)> .. .. + H + : O--H H--O--H+ H H3O+ is called the hydronium ion. To account for this observation, Br nsted and Lowry suggested a different definition of acids and bases: H An acid is a proton donor A base is a proton acceptor 7 During neutralization, an acid donates a proton (H ) to a base. Br nsted and Lowry neutralization reactions are described in terms of the conjugate acid and conjugate base. Strong acid and water: .. HBr + : O--H strong acid H strong base .. H--O--H+ + H weak conjugate acid .. :Br: - .. weak conjugate base + Weak acid and water: .. .. HF + :O--H H--O--H+ + weak acid H weak base F- H strong strong conjugate conjugate acid base (the H+donor) (the H+ acceptor) The weak acid, HF, produces a strong conjugate base--a very good proton acceptor F-. The base, H2O, produces a strong conjugate acid--a very good proton donor, H3O+. Weak base and water: NH3 + weak base .. : O--H H weak acid .. : O--H- + .. NH4+ conjugate acid (the H+ donor) conjugate base (the H+ acceptor) The weak base, NH3, produces a strong conjugate acid. The acid, H2O, produces a strong conjugate base. Water is can amphiprotic--it either donate or accept protons. Water is amphoteric--it can either act as an acid or as a base. Water will autoionize--it will undergo an acid-base reaction with itself. H2O (l) + H2O (l) H3O+ (aq) + OH- (aq) 8 Strong acid and strong base: HCl (aq) + NaOH (aq) strong acid strong base H2O (l) + NaCl (aq) weak conjugate weak conjugate acid base (Cl-) 10- 7 Strengths of Acids Binary acids: The strength of a binary acid is dependent upon two factors: 1. The ease of breaking the H--X bond. (X represents the nonmetal in the acid. Usually, X represents a halogen. An H--X bond is generally called a hydrogen halide bond.) 2. Of the halogens, the H--F requires the most energy to break. We refer to the bond strength between two atoms as the bond energy (energy required to break the bond). The stability of the ions produced in the solution. The fluoride ion is very small and highly charged. When released into aqueous solutions, F- causes the water molecules to become more ordered which is thermodynamically (entropy) unfavorable. Generally, the weak binary acids have high bond energies and usually have smaller nonmetal ions. Strong binary acids (HCl, HBr, and HI) completely ionize in water. Smaller bond energies Larger nonmetal ions However, it is difficult to determine which of the strong binary acids are stronger in aqueous solution. Think about it, if all three completely ionize, which ionizes to the greater extent? Therefore, we say that the H3O+ ion is the strongest acid that can exist in water, and that OH- ion is the strongest base that can exist in water. This is called the leveling effect of water. HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq) 9 The hydronium ion is a weaker acid than HCl, and therefore; the hydronium ion exists in water. NaOH (aq) + H2O (l) OH- (aq) + H2O (l) Na+ (aq) + OH- (aq) OH- (aq) + H2O (l) The reaction is complete. The hydroxide ions may react with the hydrogens of water molecules, but the outcome will only be the production of hydroxide ions. Therefore, the hydroxide ion exists in water from this reaction. Ternary acids: (Oxoacids) The strength of a ternary acid is determined by the oxidation state if the acid has the same central atom and different numbers of oxygen atoms. The central atom with the largest oxidation number is the strongest acid. HNO3 HNO2 the nitrogen has a +5 ox. state the nitrogen has a +3 ox. state HNO3 > HNO2 You should try this method of assigning ox. states to determine the acid strength of all oxoacids that have the same central atom. The strength of a ternary acid is determined by the electronegativity of the central atom, if the acids compared have a central atom with the same ox. state. The central atom with the largest electronegativity is the strongest acid. HClO4 Cl is the most electronegative of the halogens that form strong oxoacids. (Remember, fluorine does not form oxoacids.) HClO4 > HBrO4 > HIO4 Note: that the bond energies associated with breaking the O--H bonds in oxoacids is less than the bond energies required to break the bonds between the central atoms and the oxygen bonds. You should look at the structures of the oxoacids shown on page 378. 10 Acid Base Reactions in Aqueous Solutions Neutralization (formation of water is the driving force) Three different chemical equations can be written for the reactions that occur in aqueous solutions. Since neutralization occurs in aqueous solutions, the three different equations will be presented for the different possible acid--base reactions. The molecular (formula) chemical equation--the normal chemical equation for a reaction. The complete ionic equations--the ions in solution are presented on both sides of the chemical equation. The net ionic equation--the actual reaction (the driving force) of the reaction is presented. Strong acid and Strong base neutralization reaction: Molecular equation: 2 HNO3 (aq) + Ca(OH)2 (aq) 2 H2O (l) + Ca(NO3)2 (aq) means the compound is dissolved in water. Since we know that strong acids and strong soluble bases completely ionize in water, we write each ion. Complete ionic: Show all (aq) compounds that ionize as ions. All compounds with (l), (s), or (g) are not shown as ions. HNO3 sol'n Ca(OH)2 sol'n H+ NO3- H+ + H NO3- - NO3 H+ NO3- H+ + H NO3- NO3- - + H NO3 H+ H+ Ca2+ OH- Ca2+ OH- OH- OH- Ca2+ OH- Ca2+ OH- OH- - OH Ca2+ OH- OH- The combined sol'ns H2O Ca2= H2O Ca2= NO3- H2O NO3- NO3- NO3- H2O Ca2= NO3- H2O NO3- 11 The complete ionic equation reflects the ions in the reactants and the products. 2 H+(aq) + 2NO3-(aq) + Ca2+ (aq) + 2OH-(aq) 2H2O (l) + Ca2+ (aq) + 2 NO3- (aq) The net ionic equation reflects the reaction that has taken place--the driving force of the reaction. To write the net equation, all spectator ions are eliminated from the complete ionic equation. Spectator ions are those that are in the same ionic state on both sides of the complete ionic equation. 2H2O (l) + Ca2+ (aq) + 2 NO3- (aq) 2 H+(aq) + 2NO3-(aq) + Ca2+ (aq) + 2OH-(aq) When the spectator ions are removed, the net equation is written as follows: 2 H+ (aq) + 2OH- (aq) 2H2O (l) The equation can be further reduced by dividing both sides by 2: H+ (aq) + OH- (aq) H2O (l) (The net ionic) Weak acid and Strong base neutralization reaction: Molecular (formula) chemical equation: HF (aq) + NaOH (aq) NaF (aq) + H2O (l) The strong bases drives the rxn to completion. HF sol'n NaOH sol'n Na+ OH- Na+ OH- Na+ OH- Na+ OH- OH- Na+ + - Na OH Na+ + - Na OH Na+ HF HF HF HF HF HF HF HF HF HF HF H+product HF F- HF HF HF HF HF The combined sol'ns F- Na+ H2O F- H2O F- Na+ - F Na+ F- H2O Na+ F- Na+ H2O H2O Na+ F- H2O Na+ F- 12 Complete ionic chemical equation: HF(aq) + Na+(aq) + OH- (aq) Na+(aq) + F-(aq) + H2O (l) Net ionic equation: (Remove spectator ions) HF(aq) + OH- (aq) F-(aq) + H2O (l) Strong acid and Weak base neutralization reaction: H2SO4 (aq) + 2NH3 (aq) The strong acid drives the rxn to completion. H2SO4 sol'n (NH4)2SO4 (aq) NH3 sol'n SO42- H+ H+ H+ SO42- 2- SO4 H+ 2- product SO4 H + H+ NH3 NH3 NH3 NH3 NH3 NH3 NH4+ OH- NH3 NH3 NH3 The combined sol'ns NH4+ SO42- NH4+ NH4+ SO4 NH4+ NH4+ 2SO4 NH4+ SO42- NH4+ 2- NH4+ Complete ionic equation: 2H+(aq) + SO42-(aq) + 2NH3 (aq) Net ionic equation: (reduced) H+(aq) + NH3 (aq) NH4+(aq) 2NH4+(aq) + SO42-(aq) 13 Acidic Salts and Basic Salts: Monoprotic acids only have one hydrogen capable of forming H+. HCl, HBr, HI, HNO3, HClO3, HF, HClO4, etc... Diprotic acids have two hydrogens that are capable of forming H+ H2SO4, H2SeO4, H2S, H2CO2, etc... Polyprotic acids have more than two hydrogens that are capable of forming H+ H3PO4, H3AsO4, H6TeO6, etc... If a strong base is added to a polyprotic acid in stoichiometric amounts, all the hydrogens will participate in the neutralization process. However, if there is not enough base to react with all of the hydrogens in a polyprotic acid, the resulting product will be an acidic salt. The same process occurs when an inadequate amount of strong acid is added to molecules of a base that are polyhydroxy (more than two hydroxides are on the molecule.) If the amount of acid is not sufficient to react with all the hydroxides on the molecule, a basic salt will be produced. (pages 386-387) Another concept that is mentioned in this chapter, which is important to realize (especially for CH302), is that salts of weak acids and salts of weak bases produce basic and acidic solutions, respectively. For example: NaF (the salt of the weak acid, HF) NaF (aq) + H2O (l) The complete ionic: Na+ (aq) + F-(aq) + H2O (l) HF(aq) + Na+(aq) + OH-(aq) HF (aq) + Na +(aq) + OH-(aq) Remember that the F- is a strong base. It wants very much to have hydrogen, therefore; it will react with water to remove a hydrogen to produce hydroxide (OH-) ions. The net: (remove the spectator ions) F-(aq) + H2O (l) HF(aq) + OH-(aq) Since HF is in greater concentration, the reaction favors the product side of this reaction. 14 The same reaction occurs with salts of weak bases. For example: NH4Cl NH4Cl (aq) + H2O (l) Complete ionic: NH4 +(aq) + Cl- (aq) + H2O (l) H3O+ (aq) + Cl- (aq) + NH3(aq) H3O+ (aq) + Cl- (aq) + NH3(aq) Remember that the NH4+ is now a strong acid. It would like very much to get rid of one hydrogen. Since the chloride ion is completely soluble and will not react with water, nor will it accept hydrogen, the ammonium gives one hydrogen to water, producing the hydronium ion. The resulting solution becomes acidic. The net reaction: NH4 +(aq) + H2O (l) H3O+(aq) + NH3(aq) Since NH3 is in greater concentration, the reaction favors the product side of this reaction. These reactions of the salts of weak acids and bases are called, hydrolysis. (Hydro- meaning water, lysismeaning breaking.) 10-10 The Lewis Theory an acid is a substance that accepts an electron pair a base is a substance that donates an electron pair the reaction is a coordinate covalent bond The classic example is given on page 388. .. Cl3B:NH3 BCl3 (g) + NH3 acid base neutralization product .. H--O: H acid + .. H--O: H base .. .. + H--O--H + :O--H .. H neutralization products All Br nsted-Lowry acids/bases can be described by the Lewis theory, but not all of the Lewis acids/bases are described by Br nsted-Lowry. We will omit section 10-11.

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