Chapter 17 Ionic Equilibria: Acids and Bases A Review of Strong Electrolytes We must distinguish weak acids and bases from strong electrolytes. Weak acids and bases ionize or dissociate much less than 0%. In this chapter we will see that it is often less than %! Strong electrolytes ionize or dissociate completely. Strong electrolytes approach 0% dissociation in aqueous solutions. 1 A Review of Strong Electrolytes There are three classes of strong electrolytes. 1 Strong Water Soluble Acids Remember the list of strong acids from Chapter 4. NO ( l) O NO 0% ( l) or 0% ( l) O NO NO A Review of Strong Electrolytes Strong Water Soluble Bases The entire list of these bases was also introduced in Chapter 4. O Sr(O) O 0% (s) O 0% (s) Sr O O 4 A Review of Strong Electrolytes Most Water Soluble Salts The solubility guidelines from Chapter 4 will help you remember these salts. NaCl Ca(NO O 0% (s) Na O 0% ) ( s) Cl Ca NO ACIDBASE TEORIES The most general theory for common aqueous acids and bases is the LOWRY theory BRØNSTED LOWRY ACIDS DONATE IONS BASES ACCEPT IONS 5 6 1
ACIDBASE TEORIES The Brønsted definition means N is a BASE in water and water is itself an ACID ACIDBASE TEORIES N is a BASE in water and water is itself an ACID N O Base Acid N 4 O Acid Base N / N 4 is a conjugate pair related by the gain or loss of Every acid has a conjugate base and viceversa. versa. 7 8 Conjugate Pairs More About Water O can function as both an ACID and a BASE. In pure water there can be AUTOIONIZATION Equilibrium constant for autoion w w [ O ] [O ] 1.00 x 14 at 5 o C 9 The Autoionization of Water The p and po scales We can write the autoionization of water as a dissociation reaction similar to those previously done in this chapter. O( l) O( l) O O Because the activity of pure water is 1, the equilibrium constant for this reaction is: [ ][ ] O O c 11 A convenient way to express the acidity and basicity of a solution is the p and po scales. The p of an aqueous solution is defined as: [ p log O ] 1
The p and po scales The p and po scales A convenient relationship between p and po may be derived for all dilute aqueous solutions at 5 0 C. [O ][O ] 1.0 14 Taking the logarithm of both sides of this equation gives: [ O ] log[ O ] 14. 00 log [ O ] ( log[ O ]) 14. 00 Multiplying both sides of this equation by 1 gives: log Which can be rearranged to this form: p po 14.00 1 14 The p and po scales Remember these two expressions!! [ O ][ O] 1.0 p po 14.00 14 The Autoionization of Water Example: Calculate the concentrations of O and O in 0.050 M Cl. 15 16 Calculating [ O ] & [O ] You add 0.00 mol of NaO to 1.0 L of pure water. Calculate [ O ] and [O ]. [ O ], [O ] and p What is the p of the 0.00 M NaO solution? General conclusion Basic solution p > 7 Neutral p 7 Acidic solution p < 7 17 18
The p Scale [ O ], [O ] and p If the p of Coke is.1, it is. Active Figure 17. 19 0 The p and po scales To help develop familiarity with the p and po scale we can look at a series of solutions in which [ O ] varies between 1.0 M and 1.0 x 14 M. Let s look at the dissolution of acetic acid, a weak acid, in water as an example. The equation for the ionization of acetic acid is: [ O ] 1.0 M 1.0 x M 1.0 x 7 M.0 x 1 M 1.0 x 14 M [O ] 1.0 x 14 M 1.0 x 11 M 1.0 x 7 M 5.0 x M 1.0 M p 0.00.00 7.00 11.70 14.00 po 14.00 11.00 7.00.0 0.00 [ ][ COO ] The equilibrium constant for this ionization is expressed as: O C c C [ COO] 1 We can define a new equilibrium constant for weak acid equilibria that uses the previous definition. This equilibrium constant is called the acid ionization constant. The symbol for the ionization constant is a. a [ O ][ C COO ] [ C COO] for acetic acid 1.8 5 The ionization constant values for several acids are given below. Which acid is the strongest? Acid Formula a value Acetic Nitrous ydrofluoric ypochlorous ydrocyanic C COO NO F ClO CN 1.8 x 5 4.5 x 4 7. x 4.5 x 8 4.0 x 4 4
From the above table we see that the order of increasing acid strength for these weak acids is: F > NO > CCOO> ClO> CN The order of increasing base strength of the anions (conjugate bases) of these acids is: F < NO < C COO < ClO < CN Example: Write the equation for the ionization of the weak acid CN and the expression for its ionization constant. a CN CN [ ][ CN ] [ CN] 4.0 x 5 6 Example: The p of a 0. M solution of a weak monoprotic acid, A, is found to be.97. What is the value for its ionization constant? Example: Calculate the concentrations of the various species in 0.15 M acetic acid, C COO, solution. It is always a good idea to write down the ionization reaction and the ionization constant expression. C COO a O O C [ O ][ C COO ] [ C COO ] COO 1.8 5 Do problem 7 8 Equilibria Involving A Weak Acid Consider the approximate expression a 1.8 x 5 x 1.00 For many weak acids x [ O ] [ a 1.00] 1/ [ O ] [conj. base] [ a C o ] 1/ where C 0 initial conc. of acid Useful Rule of Thumb: If 0 a < C o, then [ O ] [ a C o ] 1/ Example: Calculate the concentrations of the species in 0.15 M hydrocyanic acid, CN, solution. a 4.0 x for CN 9 0 5
Equilibria Involving A Weak Acid Calculate the p of a 0.00 M solution of formic acid, CO. CO O a 1.8 x 4 CO O Let s look at the percent ionization of two weak acids as a function of their ionization constants. Solution 0.15 M acetic acid 0.15 M CN a 1.8 x 5 4.0 x [ ] 1.6 x 7.7 x 6 p.80 5.11 % ionization 1.1 0.0051 Note that the [ ] in 0.15 M acetic acid is times greater than for 0.15 M CN. 1 All of the calculations and understanding we have at present can be applied to weak acids and weak bases! Equilibria Involving A Weak Base N O N 4 O b 1.8 x 5 Example: Calculate the concentrations of the various species in 0. M aqueous ammonia and the p. 4 Equilibrium Constants for Weak Acids Equilibrium Constants for Weak Bases Weak acid has a < 1 Leads to small [ O ] and a p of 7 Weak base has b < 1 Leads to small [O ] and a p of 1 7 5 6 6
Ionization Constants for Acids/Bases Acids Increase strength Conjugate Bases Relation of a, b, [ O ] and p Increase strength 7 8 and AcidBase Reactions ACIDS STRONG weak CONJUGATE BASES weak STRONG Reactions always go from the stronger AB B pair (larger ) to the weaker AB A pair (smaller ). and AcidBase Reactions A strong acid is 0% dissociated. Therefore, a STRONG ACID a a good donor must have a WEA CONJUGATE BASE a a poor acceptor. NO O(liq) O NO STRONG A base acid weak B Every AB A B reaction has two acids and two bases. Equilibrium always lies toward the weaker pair. ere is very large. 9 40 and AcidBase Reactions BASE NO O STRONG ACID ACID O NO WEA BASE We know from experiment that NO is a strong acid. 1. It is a stronger acid than O. O is a stronger base than NO. for this reaction is large 41 and AcidBase Reactions Acetic acid is only 0.4% ionized when [OAc[ OAc] ] 1.0 M. It is a WEA ACID OAc O O OAc WEA A base acid Because [ O ] is small, this must mean 1. O is a stronger acid than OAc. OAc is a stronger base than O. for this reaction is small STRONG B 4 7
Types of Acid/Base Reactions Types of Acid/Base Reactions Strong acid (Cl( Cl) ) Strong base (NaO( NaO) Cl Na O O Na Cl Net ionic equation ) O ) O(liq) 1/ w 1 x 14 Weak acid (acetic ac.) Strong base (NaO( NaO) C CO O O C CO This is the reverse of the reaction of C CO (conjugate base) with O. O stronger base than C CO 1/ b 1/(5.6 x ) 1.8 x 9 CO Mixing equal molar quantities of a strong acid and strong base produces a neutral solution. 4 Mixing equal molar quantities of a weak acid and strong base produces the acid s conjugate base. The solution is basic. 44 Types of Acid/Base Reactions Strong acid (Cl( Cl) ) Weak base (N ) O N O N 4 This is the reverse of the reaction of N 4 (conjugate acid of N ) with O. O stronger acid than N 4 1/ a 1.8 x 9 Mixing equal molar quantities of a strong acid and weak base produces the bases s conjugate acid. The solution is acid. Polyprotic Acids Many weak acids contain two or more acidic hydrogens. Examples include PO 4 and. The calculation of equilibria for polyprotic acids is done in a stepwise fashion. There is an ionization constant for each step. Consider arsenic acid,, which has three ionization constants. 1 a1.5 x 4 a 5.6 x 8 a.0 x 1 45 46 Polyprotic Acids Example: Calculate the concentration of all species in 0.0 M arsenic acid,, solution. 1 Write the first ionization step and represent the concentrations. Approach this problem exactly as previously done. AsO 4 AsO4 ( 0.0x ) M xm xm Polyprotic Acids A comparison of the various species in 0.0 M solution follows. Species O Concentration 0.095 M 0.0049 M 0.0049 M 5.6 x 8 M.4 x 18 M.0 x 1 M 47 48 8
AcidBase Properties of Salts AcidBase Properties of Salts Calculate the p of a 0. M solution of Na CO. Na O > > neutral CO O CO O base acid acid base b.1 x 4 49 50 Salts of Weak Bases and Strong Acids Example: Calculate [ ], p, and percent hydrolysis for the ammonium ion in 0. M ammonium bromide, N 4 Br, solution. Salts of Weak Bases and Weak Acids The fluoride ion hydrolyzes to produce O ions. Its hydrolysis constant is (base dissociation): F O F O [ F][ O ] [ F ] b b. 7. 14 4 Because the a for (C ) N ions is one order of magnitude larger than the b for F ions, ions are produced in excess making the solution acidic. w a for F 14. 11 51 5 Salts of Weak Bases and Weak Acids Summary of the major points of hydrolysis up to now. 1 The reactions of anions of weak monoprotic acids (from a salt) with water to form free molecular acids and O. A O A O b w a( A) Salts of Weak Bases and Weak Acids. The reactions of anions of weak monoprotic acids (from a salt) with water to form free molecular acids and O. B O B O a w b( B) B weak base 5 54 9
Salts of Weak Bases and Weak Acids Aqueous solutions of salts of strong acids and strong bases are neutral. Aqueous solutions of salts of strong bases and weak acids are basic. Aqueous solutions of salts of weak bases and strong acids are acidic. Aqueous solutions of salts of weak bases and weak acids can be neutral, basic or acidic. The values of a and b determine the p. Lewis Acids & Bases Lewis acid a substance that accepts an electron pair Lewis base a substance that donates an electron pair 55 56 Reaction of a Lewis Acid and Lewis Base New bond formed using electron pair from the Lewis base. Coordinate covalent bond Lewis Acids & Bases Formation of hydronium ion is also an excellent example. ACID O BASE Electron pair of the new O O bond originates on the Lewis base. O 57 58 Lewis Acid/Base Reaction Lewis Acids & Bases Other good examples involve metal ions. O Co O Co ACID BASE 59 60
Lewis Acids & Bases The combination of metal ions (Lewis acids) with Lewis bases such as O and N > COMPLEX IONS Lewis AcidBase Interactions in Biology eme group The heme group in hemoglobin can interact with O and CO. The Fe ion in hemoglobin is a Lewis acid O and CO can act as Lewis bases 61 6 Why? Why are some compounds acids? Why are some compounds bases? Why do acids and bases vary in strength? Can we predict variations in acidity or basicity? Why is C CO an Acid? 1. The electronegativity of the O atoms causes the attached to O to be highly positive.. The O O bond is highly polar.. The atom of O O is readily attracted to polar O. Figure 17.9 6 64 Basicity of Oxoanions Acetic acid a 1.8 x 5 a 0. Trichloroacetic acid Trichloroacetic acid is a much stronger acid owing to the high electronegativity of Cl. Cl withdraws electrons from the rest of the molecule. This makes the O O bond highly polar. The of O O is very positive. NO CO These ions are BASES. They become more and more basic as the negative charge increases. As the charge goes up, they interact more strongly with polar water molecules. PO 4 65 66 11