Chapter 15 Acids- Bases. The Nature of Acids and Bases. Some Common Acid. Principles of Chemistry A Molecular Approach, 1 st Ed. Nivaldo J.

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1 Chapter 15 -Acids and Bases Principles of Chemistry A Molecular Approach, 1 st Ed. Nivaldo J. Tro Dr. Azra Ghumman Memorial University of Newfoundland The Nature of Acids and Bases Properties of Acids: Sour taste, dissolve many metals e.g., Al, Zn, Fe, but not Cu, Ag, or Au corrosive React with bases to neutralize them React with carbonates, producing CO 2 marble, baking soda, chalk, limestone change color of certain dyes blue litmus turns red Structures of acids: Binary, oxoacids and organic acids(-cooh) Only the first H in the formula of is acidic 2 Some Common Acid 3 1

2 Properties of Bases Bases: bitter taste, feel slippery Also known as alkalies alkaloids = plant product that is alkaline often poisonous change color of certain dyes(different than acid) red litmus turns blue react with acids to neutralize them form ionic salts 4 Common Bases 5 Indicators Indicators -chemicals that change colour depending on the solution s acidity or basicity Many vegetable dyes are indicators. Red cabbage juice green in basic solution (left beaker) but turns red when dry ice, or CO 2, is added (center beaker) to give an acidic solution (right beaker). litmus paper red in acid, blue in base phenolphthalein red in base, colorless in acid 6 2

3 15.3 Acid Base concepts Three main concepts for Acids and bases: Arrhenius Acid and Base Svante Arrhenius (1884) explained the actual cause of acidity and basicity in terms of the effect these compounds have on water Bronsted-Lowery Acid and Base Lewis Acid and Base 7 Arrhenius Concept of Acids and Bases Arrhenius Acid: A substance that, when dissolved in water, produces H + ions (H 3 O + ) e.g. HCl(aq) HCl(aq) H + (aq) + Cl (aq) Remember, however, that the aqueous hydrogen ion is actually chemically bonded to water, that is, H 3 O +. Arrhenius Base: A substance that, when dissolved in water, produces OH - ions in solution e.g. NaOH(aq) NaOH(aq) Na + (aq) + OH (aq) 8 Hydronium Ion The H + ions produced by the acid are very reactive and produce complex ions with water, mainly H 3 O +. H + (aq) + H 2 O (l) H 3 O + (aq) There are also minor amounts of H + with multiple water molecules, H(H 2 O) n+. 9 3

4 Limitations of Arrhenius Theory Limitations of Arrhenius Theory It does not explain; why molecular and ionic substances, that do not contain OH ions can form basic solution in water e.g. NH 3 and Na 2 CO 3 or Na 2 O why molecular substances, like CO 2, dissolve in water to form acidic solutions even though they do not contain H + ions does not explain acid base reactions that take place outside aqueous solutions 10 Brønsted-Lowry Concept of Acids and Bases Broader definition than Arrhenius -In the Brønsted- Lowry concept: 1.Acids and bases can be ions as well as molecular substances. 2.Acid-base reactions are not restricted to aqueous solution. 3.Some species can act as either acids or bases depending on what the other reactant is. 11 The Brønsted Lowry Definition Focus on the transfer of H + ion in an acid-base reaction Brønsted Lowry Acid: A Proton (H + ion) donor e.g. HCl HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) Brønsted Lowry Base: A Proton (H + ion) acceptor NH 3 (aq) is basic because NH 3 accepts an H + from H 2 O, forming OH (aq). + NH 3 (aq ) + H 2O (l) NH 4 (aq ) + OH (aq ) base acid In any reversible acid-base reaction, both forward and reverse reactions involve proton transfer. 12 4

5 Amphoteric Species Amphoteric species is a species that can act either as an acid or a base (it can gain or lose a proton). E.g H 2 O acts as an acid and a base. For example, HCO 3- acts as a proton donor (an acid) in the presence of OH - and can act as a proton acceptor (a base) in the presence of HF. 2 HCO 3 (aq) + OH (aq ) CO 3 (aq ) + H 2O(l) H + HCO 3 (aq ) + HF (aq ) H 2CO 3 (aq ) + F (aq ) H + 13 Conjugate Acid-Base Pairs Conjugate Pairs: Two substances related to each other by the transfer of a proton A base accepts a proton and becomes a conjugate acid An acid donate a proton and becomes a conjugate base Water can react as a base with the acid HF and also as an acid with base NH 3 14 Sample problems Label conjugate acid base pairs in the following reactions. HCHO 2 + H 2 O CHO 2 + H 3 O + Write the equations for the following reacting with water and acting as a monoprotic-accepting base and label the conjugate acid and base H 2 O NH 3 CO 3 2 H 2 PO

6 Examples: Conjugate acid Conjugate base HF F- HSO - 4 SO 2-4 NH + 4 NH 3 HCN CN - NH 3 and NH 4+ constitute a base conjugate acid pair. 16 Common Strong Acids and Bases Strong acids and bases are strong electrolytes In the Arrhenius concept, A strong acid is a substance that ionizes completely in aqueous solution to give H 3 O + (aq) and an anion e.g. perchloric acid, HClO 4. HClO4 (aq) + H2O(l) H3O (aq) + ClO4 (aq) Single arrow indicates complete ionization Six strong Acids; HClO 4, H 2 SO 4, HI, HBr, HCl, HNO A Strong Base A strong Arrhenius base is a substance that ionizes completely in aqueous solution to give OH - (aq) and a cation. An example is sodium hydroxide, NaOH. H 2 O NaOH(s) Na (aq) + OH + (aq) Strong bases include NaOH, LiOH, KOH, Ca(OH) 2, Sr(OH) 2, and Ba(OH) 2. Mostly strong bases are hydroxide of Group 1A and Group IIA elements. 18 6

7 Neutralization Reaction The neutralization of a strong acid by strong base is essentially the reaction of H 3 O + (aq) and OH - (aq). For the neutralization of HCl(aq) with NaOH(aq), the net ionic equation is as follows H 3 O + (aq) + OH - (aq) 2 H 2 O(l) Polyprotic acids:acids containing more than one ionizable proton e.g. H 2 SO 4 is a diprotic acid 19 Weak Acids and Bases Most other acids and bases that we encounter are weak (weak electrolyte) Weak Acids and Weak bases are not completely ionized and exist in equilibrium with the corresponding ions. e.g. Acetic acid, HC 2 H 3 O 2 is a weak cid. HC2 H 3O2(aq) + H 2O(l) + H3O (aq) + C2H3O2 (aq) e.g. weak base ammonia NH 3 H 2 O(l) + NH 3 (aq) HO (aq) + NH 4 + (aq) 20 Trends in Acidity The stronger an acid is the weaker is its conjugate base. cation is stronger acid than neutral molecule; neutral molecule is stronger acid than anion H 3 O + > H 2 O > OH ; trend in base strength opposite NH 2 > NH 3 > NH 4 + Strongest bases have the weakest conjugate acid and the reaction will go towards weak acid 21 7

8 Increasing acidity Increasing basicity 22 Comparing Arrhenius Theory and Brønsted Lowry Theory Arrhenius theory Brønsted Lowry theory HCl(aq) HCl(aq) + H 2 O(l) H + (aq) + Cl (aq) Cl (aq) + H 3 O + (aq) HF(aq) HF(aq) + H 2 O(l) H + (aq) + F (aq) F (aq) + H 3 O + (aq) NaOH(aq) NaOH(aq) Na + (aq) + OH (aq) Na + (aq) + OH (aq) NH 4 OH(aq) NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH (aq) NH 4+ (aq) + OH (aq) 23 Lewis Concept of Acids and Bases A Lewis acid is an electron pair acceptor. This concept broadened the view of an acid include reactions that did not involve H +. The Lewis concept embraces many reactions that we might not think of as acid-base reactions e.g. Na 2 O(s) + SO 3 (g) Na 2 SO 4 (s) A Lewis base is an electron pair donor. The base donates a pair of electrons to the acid. 24 8

9 Lewis Concept of Acids and Bases The reaction of boron trifluoride with ammonia is an example Boron trifluoride accepts the electron pair, so it is a Lewis acid. Ammonia donates the electron pair, so it is the Lewis base. Arrhenius and Brønsted Lowry acid base reactions are also Lewis 25 Formation of hydrated metal ion A Lewis Acid has an empty orbital (or can rearrange eletrons to create an empty orbital)that can accept an electron. The formation of complex ions can also be looked as Lewis acid-base reactions 26 Strengths of Acids and Bases Acid or base strength is measured by determining the equilibrium constant of a substance s reaction with water. HF + H 2 O F + H 3 O + B: + H 2 O HB + + OH The strong acid or strong base favours the products in the equilibrium The position of equilibrium depends on the strength of attraction between the base form and the H +. Stronger attraction means stronger base or weaker acid. 27 9

10 Consider the equilibrium below: + HC2 H3O 2(aq) + H2O(l) H3O (aq) + C2H 3O2 (aq) w. acid w. base s. acid s.base conjugate acid-base pairs H 3 O + is the stronger of the two acids. Consequently, the equilibrium is skewed toward reactants. 28 Acid Ionization Constant, K a Relative strength of weak acid is quantified with acid ionization constant K a HA(aq) + H 2 O(l) A (aq)+ H 3 O + (aq) The equilibrium constant is called the acid ionization constant, K a. larger K a = stronger acid + [ H 3O ][ A ] K a = [ HA] Note conc. of H 2 O is excluded from Ka expression 29 Autoionization of Water and ph Autoionization is a reaction in which two like molecules react to give ions. Water is extremely weak electrolyte. About 1 out of every 10 million water molecules form ions through autoionization Water is amphoteric + H 2O(l) + H 2O(l) H 3O (aq ) + OH (aq ) The equilibrium-constant expression for this system is: + Kw = [H3O ][OH ] We call the equilibrium value for the ion product [H 3 O + ][OH - ] the ion-product constant for water( K w ) At 25 C, the value of K w = 1.0 x Like any equilibrium constant, K w varies with temperature 30 10

11 Ionization constant of Water Example: Using K w calculate the concentrations of H + and OH - ions in pure water. These ions are produced in equal numbers in pure water neutral water, so if we let x = [H + ] = [OH - ] o = (x )( x ) at 25 C x = = If you add acid or base to water [H + ] and [OH - ] are no longer equal but the K w expression still holds. All aqueous solutions contain both H 3 O + and OH ions. 1.0 x Neutral solutions [H 3 O + ] = [OH ] = 1 x 10 7 Acidic solutions [H 3 O + ] > 1 x 10 7 ; [OH ] < 1 x 10 7 Basic solutions [H 3 O + ] < 1 x 10 7 ; [OH ] > 1 x10 7 H+ Chemand 1011OH- in pure water Chapter e both 15 Acids- 1.0 Bases x 10-7 M. 31 Using K w in calculations Calculate [OH - ] or [[H 3 O + ] at 25 C for each of the following solution and determine whether solution is acidic, basic or neutral. [H 3 O + ]= 7.5 x10-3 mol.l -1 [H 3 O + ]= 1.5 x10-9 mol.l -1 [OH - ]= 1.5 x10-2 mol.l The ph scale The acidity/basicity of a solution is often expressed as ph. S.P.L.Sorensen devised the ph scale, Logarithmic scale: 1-14 ph is the negative log of H + ion concentration need to know the [H + ] concentration to find ph ph = log[h 3 O + ]; [H 3 O + ] = 10 ph change of 1 ph unit = 10-fold change in [H 3 O + ] 33 11

12 Sig. Figs. and Logs Only the numbers to the right of the decimal point are significant in a logarithm 34 Measuring ph 1. ph meter translate H + ion concentrate of a solution into an electrical signal that is converted into either a digital display or a deflection on a meter that reads ph directly. 2. Acid-base indicators. Universal indicator is a mixture of acid base indicators. It is deep red, in acidic solution and it changes to yellow and green at ph 6-8 deep violet in strongly basic solution. 3. ph paper strips or litmus paper are coated with a mixture of ph sensitive dyes used to measure ph of food, ground water and biological stuff over a wide range of ph. 35 Practice Problems 1. Calculate the ph at 25 C when [OH ] = 1.3 x 10 2 M, and determine whether the solution is acidic, basic, or neutral. Plan: 2. Determine [OH ] of a solution with a ph of Plan: 36 12

13 poh and pka Scales poh Scale-Another way of expressing the acidity/basicity of a solution poh = log[oh ] or [OH ] = 10 poh poh water = log[10 7 ] = 7 need to know [OH ] to find poh poh <7 is basic; poh >7 is acidic; poh = 7 is neutral At 25 C: ph + poh = can use poh to find ph of a solution pk a Scale: pk a = log K a K a = 10 pka The smaller the pk a value the stronger the acid larger K a = smaller pk a because it is the Chapter log15 Acids- Bases 37 Relationship between ph and poh Problem: Determine the 25 ºC of a solution that has [H 3 O + ] = 2.5 x 10 9 M 38 Finding the [H 3 O + ] and ph of Strong Acid and Strong base Solution Two sources of H 3 O + in an aqueous solution of a strong acid the acid and the water. There are two sources of OH in an aqueous solution of a strong base the base and the water. For a strong acid or base, the contribution of the water to the total [H 3 O + ] or [OH ] is negligible. shifts the K w equilibrium so far that [H 3 O + ] water and [OH ] is too small to be significant except in very dilute solutions, generally <1 x 10 4 M 39 13

14 Finding ph of the Strong Acid or Strong Base Solution For a monoprotic strong acid, [H 3 O + ] = [HA]. For polyprotic acids, the other ionizations can generally be ignored M HCl has [H 3 O + ] = 0.10 M and ph = 1.00 For a strong ionic base, [OH ] = (number OH ) x [Base] M Ca(OH) 2 has [OH ] = 0.20 M and ph = For molecular bases with multiple lone pairs available, the other reactions can generally be ignored. 40 Finding the ph of a Weak Acid Weak Acids:Less than < 1% Ionize. Two sources of H 3 O + in an aqueous solution the acid and the water However, finding [H 3 O + ] is complicated as the acid undergoes only partial ionization requires solving an equilibrium problem for the reaction that defines the acidity of the acid. HA(aq) + H 2 O(l) A (aq)+ H 3 O + (aq) Assumed the initial conc. Of [H 3 O + ] 0 Set the ICE table Solve the quadratic equation in x Usually Assumption of x << the initial conc. of weak acid works well Otherwise solve quadratic equation or use method of successive approximation 41 Example 15.6 Find the ph of M HNO 2 (aq) 25 C. Solution: Write the balance eq. for the ionization of the Acid Assume [H 3 O + ]= 0 ignoring the autoionization of H 2 O Set the ICE table, expressing the the D[ ] in terms of x Write the K a expression for Acid ionization and substitute the values for K a and equilibrium concentrations Assume x is small (in denominator) and solve for x Check that the x is small approximation is valid by computing the ratio of X/initial conc. Of the weak acid The ratio should be less than 5% (<0.05) 42 14

15 Sample Problems Finding the ph of a Weak Acid where approximation doesnot work Finding Equilibrium constant from ph What is the K a of a weak acid if a M solution has a ph of 4.25? 43 Percent Ionization We can quantify the ionization of a weak acid based on percentage of acid molecules that ionize when dissolved in water Percent ionization of a weak acid is defined as: molarity of ionized acid Percent Ionization = 100 initial molarity of acid since [ionized acid] equil = [H 3 O + ] equil The higher the percent ionization, the stronger the acid 44 Example 15.9 What is the percent ionization of a 2.5 M HNO 2 solution? Points to remember The equilibrium [H 3 O + ] of a weak acid increases with increasing the initial concentration of weak acid The percent ionization of a weak acid decreases with increasing concentration of the acid 45 15

16 Base solutions Strong bases, practically all molecules are dissociated into OH or accept H s. strong electrolyte multi-oh strong bases completely dissociated [HO ] = [strong base] x (# OH) e.g. Ca(OH) 2 (aq) Ca 2+ (aq) +2 OH - (aq) [HO ] = 2 x [strong base] Example: Calculate the ph at 25 C of a M Sr(OH) 2 solution and determine whether the solution is acidic, basic, or neutral 46 Weak Bases Weak bases, only a small fraction of molecules accept H and release OH - in solution weak electrolyte much less than 1% ionization in water NH 3 + H 2 O NH 4+ + OH [HO ] << [weak base] The extent of ionization of weak base is quantified by base ionization constant K b The smaller the value of K b, the weaker the base Finding the ph of a weak base solution is similar to finding the ph of a weak acid. Most weak bases are amines 47 Finding the [OH - ] and poh of basic solutions Analogous to finding the [H 3 O + ] and ph of weak acids We can neglect the contribution of autoionization of water to the [OH - ] Set the ICE table and use the value of and expression for K b for calculating [OH - ] :B + H 2 O H:B + + OH 48 16

17 49 Practice problem Find the ph of M NH 3 (aq). K b for NH 3 = 1.76 x 10 5 NH 3 (aq)+ H 2 O (l) NH 4+ (aq) + OH (aq) [NH 3 ] [NH 4+ ] [OH ] initial change x +x +x equilibrium x x x Write expression for K b and substitute the equilibrium conc. In the expression Assume that x is very small compared to the initial conc. Of weak base Find x = [OH - ] and then [H+] and ph or find poh and then ph from ph + poh = Acid Base Properties of Salts Salts are water-soluble ionic compounds. Salts that contain the cation of a strong base and an anion that is the conjugate base of a weak acid are basic. NaHCO 3 solutions are basic. Na + is the cation of the strong base NaOH. HCO 3 is the conjugate base of the weak acid H 2 CO 3. Salts that contain cations that are the conjugate acid of a weak base and an anion of a strong acid are acidic. NH 4 Cl solutions are acidic. NH 4+ is the conjugate acid of the weak base NH 3. Cl is the anion of the strong acid HCl

18 Acid Base Properties of Ions and Salts Anions as weak bases: You can think any anion as the conjugate base of an acid An anion that is the conjugate base of a weak acid is itself a weak base F (aq) + H 2 O(l) HF(aq) + OH (aq) An anion that is the conjugate base of a strong acid is ph neutral(forms a solution that are neither acidic or basic) Cl (aq) + H 2 O(l) HCl(aq) + OH (aq) 52 Example: Use the table to determine whether the given anion is basic or neutral a) NO 3 b) NO 2 53 Relationship between K a of an Acid and K b of Its Conjugate Base Many reference books give tables of only K a values because K b values can be found from them 54 18

19 Determining the ph of a solution containing an anion acting as base Problem: Find the ph of a M NaCHO 2 (aq) solution. CHO 2 + H 2 O HCHO 2 + OH 55 Cations as Weak Acids Cations that are counterions of a strong base are ph neutral e.g. Na +, K +, and Ca 2+ A cation that is the conjugate acid of a weak base is acidic. NH 4+ (aq) + H 2 O(l) NH 3 (aq) + H 3 O + (aq) Since NH 3 is a weak base, the position of this equilibrium favors the right (NH 3 ) stronger the base, the weaker the conjugate acid Cations that are small and highly charged metal ion are acidic as they form weakly acidic solution(al 3+ ) Example-Determine whether the given cation is acidic or neutral. a) C 5 N 5 NH 2 + Ca 2+ Cr Classifying Salt Solutions as Acidic, Basic, or Neutral Salts in which neither the cation nor anion acts as an acid or base form ph neutral solution e.g. NaCl, Ca(NO 3 ),KBr A salt in which cation is the counterion of a strong base and the anion is the conjugate base of a weak acid, it will form a basic solution. NaF Ca(C 2 H 3 O 2 ) 2 KNO

20 A salt in which cation acts as acid and anion does not act as a base will form acidic solution e.g. NH 4 Cl or Al(NO 3 ) 3 A salt in which cation acts as an acid and the anion acts as a base form a solution in which the ph depends on the relative strengths of the acid and base. NH 4 F: Since HF is a stronger acid than NH 4+, the K a of NH 4+ is larger than the K b of the F ; therefore, the solution will be acidic 58 Example Determine whether a solution of the following salts is acidic, basic, or neutral. a) SrCl 2, a) AlBr 3, b) CH 3 NH 3 NO 3 c) NaCHO 2 d) NH 4 F 59 Operational Skills Identifying acid and base species Identifying Lewis acid and base species Deciding whether reactants or products are favored in an acid-base reaction Calculating the concentration of H 3 O + and OH - in solutions of strong acid or base Calculating the ph from the hydronium-ion concentration, and vice versa 60 20