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17 Acid and Base 1 17 Acids and Bases A group of chemical properties is related to acidity. According to these properties, a substance can be called an.

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Presentation on theme: "17 Acid and Base 1 17 Acids and Bases A group of chemical properties is related to acidity. According to these properties, a substance can be called an."— Presentation transcript:

1 17 Acid and Base 1 17 Acids and Bases A group of chemical properties is related to acidity. According to these properties, a substance can be called an acid or a base. The word acid comes from Latin acere meaning sour. A base is an alkaline, which is derived from Arabic alqali. Presence of acids and bases have been recognized by ancient people. Lavoisier named oxygen as the element from which acids are derived. Liebig (German) proposed that an acid contains hydrogen. Many chemical reactions are called acid-base reactions ; they are not necessarily neutralizations. The acid-base concept is interesting, especially the way it evolved. Understand acid or base, important properties of substances.

2 17 Acid and Base 2 Answer these questions What are acids? What are bases? What are acid-base reactions ? What are neutralization reactions? What are the relationships between acids and bases? What is the role of the solvent in acid or base solutions? How does the acid-base concept evolve and why? What are Arrhenius acid, Bronsted-Lowry acid and Lewis acids? What is the relationship between conjugate acids and bases? What does pH mean and how is the pH scale related to acidity? What are salts? Understand important properties of substances

3 17 Acid and Base 3 Arrhenius acids and bases The fundamental concept: (Text PHH: 17-1) Acid - any substance which delivers hydrogen ion (H + ) to the solution. HA  H + + A¯ Base - any substance which delivers hydroxide ion (OH¯) to the solution. BOH  X + + OH¯ Sevante Arrhenius proposed that substances exists as ions in solution in his dissertation, which was awarded a fourth class ( D ) in He was unable to find a job in his native Sweden. He was awarded the Nobel Prize in 1903 for his electrolytic dissociation theory.

4 17 Acid and Base 4 Problems with Arrhenius theory Like Dalton’s atomic theory, Arrhenius theory have problems today: Acidity did not show in other solvent. What is the solvent role? Some salts produce acidic or basic solutions, not neutral. Why? Which is the base, NH 3 or NH 4 OH? Is OH¯ really the only base? How can H + be stable? Are proton donated? Some chemists want students to learn Brønsted-Lowry theory of acids and bases before learning Arrhenius theory, because the former is more general. Question the established theory.

5 17 Acid and Base 5 Brønsted-Lowry theory of acids and bases An acid is a substance from which a proton can be removed. Acids are proton donors. A base is a substance that can remove a proton from an acid. Bases are proton acceptors. Because of strong desire for protons, bases rip protons off acids. Acid-base reactions are competitions for protons. HCl + H 2 O  H 3 O + + Cl¯ HNO 3 + H 2 O  H 3 O + + NO 3 ¯ HC 2 H 3 O 2 + H 2 O  H 3 O + + C 2 H 3 O 2 ¯ H 2 O + NH 3  NH OH¯ conjugate acids and bases 17–2

6 17 Acid and Base 6 Problems with B-L theory The theory works very nicely in all protic solvent, but fails to explain acid-base behavior in aprotic solvents and non-solvent situations. A more general concept on acid and base was proposed by G.N. Lewis at about the same time Bronsted-Lowry theory was proposed. Question the established theory.

7 17 Acid and Base 7 Evolution of the acid-base concept yearthinkerAcid Base acid-base reaction 1884Arrhenius ionizeionize H + + OH¯ = HOH H + OH¯ 1923 Bronsted-Proton proton HA + B = HB + A Lowry Donor acceptorconjugation 1923 Lewiselectrophil nucleophilE + Nu = E:Nu

8 17 Acid and Base 8 Lewis acids and bases Gilbert Newton Lewis ( ) influential American chemist. His theories include the Lewis dot structure taught in Chem120 and covalent bond theories. Lewis acids are electrophils: H +, Na +, BF 3, Lewis bases are nucleophils: NH 3, H 2 O, PH 3 Acid base reactions: BF 3 + : NH 3  F 3 B : NH 3 Lewis at his desk. He generalized an idea. 17–9

9 17 Acid and Base 9 Relative strengths of acids and bases HClO 4 ClO 4 ¯ H 2 SO 4 HSO 4 ¯ HII¯ HBrBr¯ HClCl¯ HNO 3 NO 3 ¯ H 3 O+H 2 O HSO 4 ¯ SO 4 2 ¯ H 2 SO 3 HSO 3 ¯ H 3 PO 4 H 2 PO 4 ¯ HNO 2 NO 2 ¯ HF F ¯ HC 2 H 3 O 2 C 2 H 3 O 2 ¯ Al(H 2 O) 6 3+ Al(OH)(H 2 O) 5 2+ H 2 CO 3 HCO 3 ¯ H 2 SHS ¯ HClOClO¯ HBrO BrO¯ NH 4 + NH 3 HCN CN ¯ HCO 3 ¯ CO 3 2 ¯ H 2 O 2 HO 2 HS ¯ S 2 ¯ H 2 O OH¯ ROHRO¯ 17–4 & 17–5 Table 17–3 The stronger the acid, the weaker its conjugate base.

10 17 Acid and Base 10 Skills regarding strength of acids What are strong acids? What are strong bases? Which is the strongest acid, HF, HCl, HBr, or HI? How about HNO 3 and HNO 2 ? H 2 SO 4, H 2 SO 3 ? How about HClO 4, HClO 3, HClO 2, HClO, and HCl? What is the strongest acid in an aqueous solution? What is leveling effect? Order a given list according to the strength. Interpret reactions as due to strength of acids and bases. Explain strength with related acids and bases.

11 17 Acid and Base 11 Molecular structure and acid strength A strong acid loses its proton easily. A strong base holds onto a proton tightly. The more polarized is the H—X bond in an acid, the easier the molecule releases H + thus a stronger acid. The weaker the H —X bond, the easier the molecule releases H + thus a stronger acid. The bond strength and the polarity are related to electronegativity and electrostatic interactions – size of the ions. 17–8 Able to predict acidity from molecular structure.

12 17 Acid and Base 12 Self-ionization of water Water molecules autoionize 2 H 2 O (l) = H 3 O + (aq) + OH¯ (aq) [H 3 O + ] [OH¯] K c = ———————— [H 2 O](= 1000/18 = 55.6) K w = [H 2 O] K c = 1e–14 only at 25 o C, it’s T dependent. The ion product, K w increases as T increases, and its value remains the same in the presence of acid or base. 17 – 3 The molecule of life, H 2 O, and its acidity.

13 17 Acid and Base 13 Solutions of strong acids and bases Strong acids and strong bases completely ionize in their solutions. HCl (aq) + H 2 O (l)  H 3 O + (aq)+ Cl – (aq) KOH (aq)  K + (aq)+ OH – (aq) In a M HCl or HNO 3 solution, [H + ] = M and [OH – ] = 1e–14 / = 1e–13 M at 25 o C In a M NaOH or KOH solution, [OH – ] = M and [H + ] = 1e–14 / = 1e–13 M at 25 o C In a M Ca(OH) 2 solution, [OH – ] = 2*0.010 = M [H + ] = 1e–14 / = 5e–13 M at 25 o C

14 17 Acid and Base 14 Some strong acids and bases Strong acids Hydrogen halides HCl HBr HI Oxyacids of halogens HClO 3 HBrO 3 HIO 3 HClO 4 HBrO 4 HIO 4 Other oxyacids H 2 SO 4 HNO 3 Strong Bases Hydroxides of alkali metals NaOH KOH CsOH Hydroxides of alkali earth Ca(OH) 2 Sr(OH) 2 17 – 4

15 17 Acid and Base 15 Neutralization Reactions The neutralization reaction between strong acids and strong bases has the net ionic reaction H 3 O + (aq)+ OH – (aq) = 2 H 2 O The anions are bystander ions. They do not participate the reaction. These ions are I –, Br –, Cl –, NO 3 –, HSO 4 –, CClO 4 –, Na +, K +, Cs +, Ca 2+ etc. When dried, the ions left behind in neutralization reactions form salts.

16 17 Acid and Base 16 The pH scale Sorensen introduced the pH scale in 1909 using the symbol p H. The p is from the German word potenz, power of (10). pH = – log [H + ]; [H + ] = 10 –pH pOH = – log [OH – ]; [OH – ] = 10 –pOH p K = – log [ K ]; K = 10 –p K pH = [H + ] = (not linear; need not copy, figure out yourself) For aqueous solution; K w = [H + ] [OH – ] - log K w = pH + pOH = 14 only at 25 o C 17–3, p–672

17 17 Acid and Base 17 pH meter and pH electrodes The pH meter is based on the principle to be discussed in electrochemistry. This topic is also related to the equilibrium constant K and Gibbs free energy,  G.

18 17 Acid and Base 18 Neutrality In a neutral solution, [H + ] = [OH – ]. (Different from Fig. 17-5) In an acidic solution, [H + ] > [OH – ], and a basic solution, [H + ] < [OH – ]. In a saturated CaO (same as Ca(OH) 2 ) solution, [OH – ] = Calculate pH, [H + ], [Ca 2+ ], and pOH at 25 o C. Solution : pOH = – log = 1.60 pH = – 1.60 = only applicable at 25 o C [H + ] = 10 –12.40 = 4.0e–13 M Note that 4.0e-13*0.025 = 1e-14 [Ca 2+ ] = [OH – ] / 2 = M

19 17 Acid and Base 19 Answer these questions – review What are acid-base reactions ? What are neutralization reactions? What is the role of the solvent in acidity? What does pH mean and how is the pH scale related to acidity? What are salts?

20 17 Acid and Base 20 Acid-Base Equilibria Strong acids and bases completely ionize in their aqueous solutions. HCl  H + + Cl – KOH  K + + OH – Weak acids and bases ionize but not completely in their solutions. HCH 3 COO  H + + CH 3 COO – acetic acid, vinegar spirit HNic  H + + Nic – HNic = Niacine, C 5 NH 5 COOH, a water-soluble vitamin required by the body for health, growth and reproduction; part of the vitamin B complex. It was first prepared in pure form by oxidizing nicotine using conc. HNO 3. Nicotine is a major chemical in tobacco. N N CH 3 N COOH <= Nicotine Niacine => Vitamine B3

21 17 Acid and Base 21 Caffeine Caffeine C 8 H 10 N 4 O 2 is a weak base, ( pH of 1% soln 6.9 ) taste bitter smell like tea, a cardiac stimulant, (boost of energy), mild diuretic, addictive, operates using the same mechanisms that amphetamines, cocaine and heroin use to stimulate the brain. Crystals are hexagonal prisms by sublimation, mp 238°C. Sublimes 178°. Fast sublimation is obtained at ° under 1 mm Hg pressure. Many organic bases are interesting drugs, and their chemistry is fascinating.

22 17 Acid and Base 22 Ionization of weak acids Ionization of acetic acid, HCH 3 COO, HA HA = H + + A – same as HA + H 2 O = H 3 O + + A – [H + ] [A – ] A – = CH 3 COO – (acetate) K a = ————— K a acid ionization constant, important for an acid [HA] K a equilibrium constant with solvent effect The p K a is defined similar to the pH, p K a = – log K a K a = 10 –p K a See Table 17-3 on page 678

23 17 Acid and Base 23 Some weak acids and bases Common Weak Acids Formic acid HCOOH Acetic acidCH 3 COOH Trichloroacetic acid CCl 3 COOH HydrofluoricHF Hydrocyanic HCN Hydrogen sulfide H 2 S Water H 2 O Conjugate acids of weak bases NH 4 + Common Weak Bases ammonia NH 3 trimethyl ammonia N(CH 3 ) 3 pyridine C 5 H 5 N ammonium hydroxide NH 4 OH water H 2 O HS - ion HS - conjugate bases of weak acids e.g.:HCOO -

24 17 Acid and Base 24 Determine K a and percent ionization Nicotinic acid, HNic, is a monoprotic acid. A solution containing M HNic, has a pH of What is its K a ? What is the percent of ionization? Solution : HNic = H + + Nic – x x x x = [H + ] = 10 –3.39 = 4.1e-4 [HNic] = – = (4.1e-4) 2 K a = ————— = 1.4e Degree of ionization = / = = 3.4%

25 17 Acid and Base 25 Application of K a The K a of nicotinic acid, HNic, is 1.4e-5. A solution containing 0.22 M HNic. What is its pH? What is the degree of ionization? Solution : HNic = H + + Nic – 0.22-x x x x 2 K a = ———— = 1.4e – x (use approximation, small indeed) x =  (0.22*1.4e-5) = pH = – log (0.0018) = 2.76 Degree of ionization = / 0.22 = = 0.79% compare with 3.4% when the solution was M

26 17 Acid and Base 26 pH of a weak acid The pH of C M acid HA ( K a ) solution. Method : HA = H + + A – C- x x x x 2 K a = ———— C – x x 2 + K a x – C K a = 0 – K a +  K a C K a x = ——————————— 2 pH = – log x The pOH of C M base BOH ( K b ) solution. Method : BOH = B + + OH – C- y y y y 2 K b = ———— C – y y 2 + K b x – C K b = 0 – K b +  K b C K b y = ——————————— 2 pOH = -log y

27 17 Acid and Base 27 Using the quadratic formula The K a of nicotinic acid, HNic, is 1.4e-5. A solution containing M HNic. What is its pH? What is the degree of ionization? Solution : HNic = H + + Nic – x x x x 2 K a = —————— = 1.4e-5 x e-5 x – 1.4e-8 = – x –1.4e–5 +  (1.4e–5) 2 + 4*1.4e-8 x = —————————————————— = M 2 pH = – log ( ) = 3.95 Degree of ionization = / = = 11.1% Deg.’f ioniz’n % % % x 2 + K a x – C K a = 0 – K a +  K a C K a x = ——————————— 2

28 17 Acid and Base 28 Degree of or percent ionization Deg.’f ioniz’n % % % The degree or percent of ionization of a weak acid always decreases as its concentration increases, as shown from the table given earlier. Concentration of acid % ionization

29 17 Acid and Base 29 Polyprotic acids Polyprotic acids such as sulfuric and carbonic acids have more than one hydrogen to donate. H 2 SO 4 = H + + HSO 4 – K a1 very large completely ionized HSO 4 – = H + + SO 4 2– K a2 = H 2 CO 3 = H + + HCO 3 – K a1 = 4.3e-7 HCO 3 – = H + + CO 3 2– K a2 = 4.8e-11 Ascorbic acid (vitamin C) is a diprotic acid, abundant in citrus fruit. Others : H 2 S, H 2 SO 3, H 3 PO 4, H 2 C 2 O 4 (oxalic acid) … C

30 17 Acid and Base 30 Aspartame - nutrasweet A dipeptide methyl ester : L-aspartyl-L-phenylalanine methyl ester C 14 H 18 N 2 O 5 (molar mass = ) Aspartame has two ionizable protons Jim Schlatter synthesized it and discovered it sweet leaving no bitter after tast.

31 17 Acid and Base 31 Species concentrations of diprotic acids Evaluate concentrations of species in a 0.10 M H 2 SO 4 solution. Solution : H 2 SO 4 = H + + HSO 4 – completely ionized (0.1–0.1) 0.10+y 0.10-y HSO 4 – = H + + SO 4 2– K a2 = –y 0.10+yyAssume y = [SO 4 2– ] (0.10+y) y ————— = (0.10-y) [SO 4 2– ] = y = 0.01M [H+] = = 0.11 M; [HSO 4 – ] = = 0.09 M Y y – =  * y = —————————————— = If concentration’f H 2 SO 4 = 1.0 M, what doUdo?

32 17 Acid and Base 32 Species concentrations of weak diprotic acids Evaluate concentrations of species in a 0.10 M H 2 S solution. Solution : H 2 S = H + + HS – K a1 = 1.02e-7 (0.10–x) x+y x-yAssume x = [HS – ] HS – = H + + S 2– K a2 = 1.0e-13 x–y x+y yAssume y = [S 2– ] (x+y) (x-y) (x+y) y ————— = 1.02e-7 ———— = 1.0e-13 (0.10-x)(x-y) [H 2 S] = 0.10 – x = 0.10 M [HS – ] = [H + ] = x  y = 1.0e–4 M; [S 2– ] = y = 1.0e-13 M 0.1>> x >> y: x+ y = x-y = x x =  0.1*1.02e-7 = 1.00e-4 y = 1e-13 See Example 16.4

33 17 Acid and Base 33 Beware of Misconceptions These equations show dynamic equilibria at the molecular level in a system. These equations are not separate reactions, but they indicate possible combination and dissociation in both directions. H 2 S = H + + HS – K a1 = 1.02e-7 HS – = H + + S 2– K a2 = 1.0e-13 H 2 O = H + + OH - K w = 1e-14 H2OH2O H2OH2O H2SH2S HS - H+H+ H+H+ H+H+ H2SH2S H2OH2O H2OH2O H2SH2S OH - S 2- OH - S 2- H+H+ H 2 S (g) HS -

34 17 Acid and Base 34 Base-ionization constant For acid, HA HA = H + + A – [H + ] [A – ] K a = ————— [HA] The p K a is defined similar to the pH, p K a = – log K a K a = 10 –p K a From slide 3 For a base BOH, BOH = B + + OH – [B + ] [OH – ] Kb = ——————— [BOH] The p K b is defined similar to p K a p K b = – log K b, K b = 10 –p K b Whatever you have learned for weak acids apply to weak bases June 18

35 17 Acid and Base 35 Common weak bases Substance Formula K b Ammonia NH 3 1.8e-5 aniline C 6 H 5 NH 2 4.2e-10 dimethylamine (CH 3 ) 2 NH 5.1e-4 ethylamine C 2 H 5 NH 2 4.7e-4 Hydrozine N 2 H 4 1.7e-6 Hydroxylamine H 2 NOH1.1e-8 methylamineCH 3 NH 2 4.4e-4 Pyridine C 5 H 5 N1.4e-9 Urea NH 2 CONH 2 1.5e-14 no need to copy Table 16.2 Many drugs are salts of weak bases, such as Advil Pseudoephedrine HCl Buckley's mixture Dextromethorphan hydrobromide Dristan Traxodone HCl


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