4.6 Acids and Bases: General Principles TABLE 4. 2 Acid Dissociation Constants Ka and pka Values for Some Bronsted acids* Dissociation Conjugate Ac Formulat constant k base Hydrogen iodide HI 101 10 Hydrogen bromide HB ≈10 Br Hydrogen chloride HCI ≈10 Sulfuric acid HOSO,OH 1.6×10 4.8 HOSO2O Hydronium ion H-OH 55 1.7 Hydrogen fluorid 3.5×10-4 3.5 Acetic acid H3COH 18×10-5 CHCO Ammonium ion H—NH 5.6×10 9.2 Water 1.8×10-16 15.7 Methanol CH3O Ethano CH3CH2OH CH3 CH2O isopropyl alcohol (CH3)2 CHOH tert-Butyl alcohol (CH3)3COH (CH3)3CO Ammonia Dimethylamine (CH3)2NH 36 (CH3)2N .Acid strength decreases from top to bottom of the table. Strength of conjugate base increases from top proton-the one that is lost on ionizatio Thetrue", for water is 1 x 10. Dividing this value by 55.5(the number of moles of water in 1 L of s in he table. a paper in the May 1990 issue of the Journal of Chemical Education(. 386)outlines the or this approach. For a dissenting view, see the March 1992 issue of the Journal of Chemical Education(p. 255) PROBLEM 4.8 As noted in Problem 4.7, hydrogen cyanide(HCn)has a pka of 9.1. Is cyanide ion (CN")a stronger base or a weaker base than hydroxide ion (HO)? In any proton-transfer process the position of equilibrium favors formation of the weaker acid and the weaker base Stronger acid stronger base weaker acid weaker base Table 4.2 is set up so that the strongest acid is at the top of the acid column, with the chemistry strongest base at the bottom of the conjugate base column. An acid will transfer a pro- ton to the conjugate base of any acid that lies below it in the table, and the equilibrium constant for the reaction will be greater than one Table 4.2 contains both inorganic and organic compounds Organic compounds are similar to inorganic ones when the functional groups responsible for their acid-base prop- erties are the same. Thus, alcohols(rod) are similar to water(HOh) in both their bror sted acidity(ability to donate a proton from oxygen) and Bronsted basicity(ability to accept a proton on orygen). Just as proton transfer to a water molecule gives oxonium ion(hydronium ion, H30), proton transfer to an alcohol gives an alkyloxonium ion ROH,) Back Forward Main Menu Study Guide ToC Student OLC MHHE WebsitePROBLEM 4.8 As noted in Problem 4.7, hydrogen cyanide (HCN) has a pKa of 9.1. Is cyanide ion (CN) a stronger base or a weaker base than hydroxide ion (HO)? In any proton-transfer process the position of equilibrium favors formation of the weaker acid and the weaker base. Table 4.2 is set up so that the strongest acid is at the top of the acid column, with the strongest base at the bottom of the conjugate base column. An acid will transfer a pro￾ton to the conjugate base of any acid that lies below it in the table, and the equilibrium constant for the reaction will be greater than one. Table 4.2 contains both inorganic and organic compounds. Organic compounds are similar to inorganic ones when the functional groups responsible for their acid–base prop￾erties are the same. Thus, alcohols (ROH) are similar to water (HOH) in both their Brøn￾sted acidity (ability to donate a proton from oxygen) and Brønsted basicity (ability to accept a proton on oxygen). Just as proton transfer to a water molecule gives oxonium ion (hydronium ion, H3O), proton transfer to an alcohol gives an alkyloxonium ion (ROH2 ). Stronger acid stronger base weaker acid weaker base K 1 4.6 Acids and Bases: General Principles 135 TABLE 4.2 Acid Dissociation Constants Ka and pKa Values for Some Brønsted Acids* HI HBr HCl HOSO2OH H±NH3 HOH CH3OH CH3CH2OH (CH3)2CHOH (CH3)3COH H2NH (CH3)2NH Formula† CH3COH O X H±OH2 HF Acid Hydrogen iodide Hydrogen bromide Hydrogen chloride Sulfuric acid Hydronium ion Hydrogen fluoride Acetic acid Ammonium ion Water Methanol Ethanol Isopropyl alcohol tert-Butyl alcohol Ammonia Dimethylamine 10 9 7 4.8 1.7 3.5 4.7 9.2 15.7 16 16 17 18 36 36 pKa CH3CO O X I  Br Cl HOSO2O H2O F NH3 HO CH3O CH3CH2O (CH3)2CHO (CH3)3CO H2N (CH3)2N Conjugate base 1010 109 107 1.6  105 1.8  105 5.6  1010 1.8  1016‡ 1016 1016 1017 1018 1036 1036 Dissociation constant, Ka 55 3.5  104 *Acid strength decreases from top to bottom of the table. Strength of conjugate base increases from top to bottom of the table. † The most acidic proton—the one that is lost on ionization—is highlighted. ‡ The “true” Ka for water is 1  1014. Dividing this value by 55.5 (the number of moles of water in 1 L of water) gives a Ka of 1.8  1016 and puts water on the same concentration basis as the other substances in the table. A paper in the May 1990 issue of the Journal of Chemical Education (p. 386) outlines the justification for this approach. For a dissenting view, see the March 1992 issue of the Journal of Chemical Education (p. 255). This is one of the most important equations in chemistry. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website