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LectureNote5 - Acids and Bases pH a measure of acidity The...

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1 pH – a measure of acidity The pH scale is useful in that it lets us express acidity by numbers that are not exponentials Acids and Bases
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2 The strength of acids and bases A strong acid is one that is completely ionized in water. A weak acid is one that ionizes in water to the extent of less than 100%.
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3 Historical development 1661 - Robert Boyle Characterized acids and alkalies ( bases ) as the following: Acids: Sour taste; Corrosive Change litmus ( 石蕊 ) (dye extracted from lichens) from blue to red Become less acidic when combined with alkalies. Alkalies (Bases): Feel slippery Change litmus from red to blue Become less alkaline when combined with acids. 1778 - Antoine Lavoisier (He stated the first version of the law of conservation of mass) Believed that all acids contained oxygen after studying several acids e.g. H 2 SO 4 - sulfuric acid, HNO 3 - nitric acid
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4 1884 – Svante Arrhenius acids and bases: Acids : compounds that produce hydrogen ions (H + or H 3 O + ) in water Bases : compounds that form OH - when they dissolve in water 1811 - Humphry Davy Questioned Lavoisier's theory, noting that hydrochloric acid (HCl) did not contain oxygen yet is an acid. Soon thereafter, several more acids without oxygen were found. e.g. HBr, HF, HI 1838 - Justig Liebig Suggested that acids contain one or more hydrogen atoms which can be replaced by metal atoms to produce salts. e.g. HSCN is an acid because the H atom can be replace by a metal to form a salt, such as NaSCN.
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5 Bronsted-Lowry acids and bases An acid is a proton donor A base is a proton acceptor hydronium ion 1923 - Johannes Brønsted and Thomas Lowry
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6 A base is a proton acceptor ammonium ion Identifying the acid-base pair Questions: A. Which of the following compounds can function as Bronsted acid? NaH H 2 S Fe(H 2 O) 6 ] 3+ B. Which of the following compounds can function as Bronsted base? CH 3 OH Cl - PPh 3
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7 Water as an acid and a base
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10 Generalizations The stronger the acid, the weaker the conjugate base. Hydronium ion is the strongest acid that can exist in aqueous solution. Hydroxide ion is the strongest base that can exist in aqueous solution. The position of equilibrium lies to the side of the weaker acid
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12 Weak acids and acid ionization constants
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15 Weak bases and base ionization constant
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16 The relationship between the ionization constants of acids and their conjugate bases
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Some typical Bronsted acids Non-oxygen containing acids, e.g. HX (X = halide) Aqua acids: Fe(H 2 O) 6 ] 3+ Which one is a stronger Bronsted acid? (charge effect) Ca(H 2 O) 6 2+ v.s. Mg(H 2 O) 6 2+ Fe(H 2 O) 6 2+ v.s. Fe(H 2 O) 6 3+ Oxoacids and hydroxoacids S O O OH OH Si OH HO OH OH B OH HO OH H 2 SO 4 and B(OH) 3 (or Si(OH) 4 ): which one is more acidic? H
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This note was uploaded on 04/04/2010 for the course CHEM 100 taught by Professor Linzhenyang during the Spring '10 term at HKUST.

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LectureNote5 - Acids and Bases pH a measure of acidity The...

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