18 - CHAPTER 18 ACIDBASE EQUILIBRIA 18.1 18.2 The Arrhenius...

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CHAPTER 18 ACID–BASE EQUILIBRIA 18.1 The Arrhenius definition classified substances as being acids or bases by their behavior in the solvent water. 18.2 All Arrhenius acids contain hydrogen and produce hydronium ion (H 3 O + ) in aqueous solution. All Arrhenius bases contain an OH group and produce hydroxide ion (OH ) in aqueous solution. Neutralization occurs when each H 3 O + molecule combines with an OH molecule to form 2 molecules of H 2 O. Chemists found that the Δ H rxn was independent of the combination of strong acid with strong base. In other words, the reaction of any strong base with any strong acid always produced 56 kJ/mol ( Δ H = –56 kJ/mol). This was consistent with Arrhenius’s hypothesis describing neutralization, because all other counter ions (those present from the dissociation of the strong acid and base) were spectators and did not participate in the overall reaction. 18.3 The Arrhenius acid-base definition is limited by the fact that it only classified substances as an acid or base when dissolved in the single solvent water. The anhydrous neutralization of NH 3 ( g ) and HCl( g ) would not be included in the Arrhenius acid/base concept. In addition, it limited a base to a substance that contains OH in its formula. NH 3 does not contain OH in its formula but produces OH ions in H 2 O. 18.4 Strong acids and bases dissociate completely into their ions when dissolved in water. Weak acids only partially dissociate. The characteristic property of all weak acids is that a significant number of the acid molecules are not dissociated. For a strong acid, the concentration of hydronium ions produced by dissolving the acid is equal to the initial concentration of the undissociated acid. For a weak acid, the concentration of hydronium ions produced when the acid dissolves is less than the initial concentration of the acid. 18.5 a) Water, H 2 O, is an Arrhenius acid because it produces H 3 O + ion in aqueous solution. Water is also an Arrhenius base because it produces the OH ion as well. b) Calcium hydroxide, Ca(OH) 2 is a base, not an acid. c) Phosphorous acid, H 3 PO 3 , is a weak Arrhenius acid . It is weak because the number of O atoms equals the number of ionizable H atoms. d) Hydroiodic acid, HI, is a strong Arrhenius acid . 18.6 (b) H 2 O and (d) H 2 NNH 2 both are very weak Arrhenius bases. 18.7 a) HNO 2 ( aq ) + H 2 O( l ) ' H 3 O + ( aq ) + NO 2 ( aq ) K a = [] 23 2 NO H O HNO −+ ⎡⎤ ⎣⎦ b) CH 3 COOH( aq ) + H 2 O( l ) ' H 3 O + ( aq ) + CH 3 COO ( aq ) K a = 33 3 CH COO H O CH COOH c) HBrO 2 ( aq ) + H 2 O( l ) ' H 3 O + ( aq ) + BrO 2 ( aq ) K a = 2 BrO H O HBrO 18.8 a) H 2 PO 4 ( aq ) + H 2 O( l ) ' H 3 O + ( aq ) + HPO 4 2– ( aq ) K a = 2 43 24 HPO H O HPO 18-1
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b) H 3 PO 2 ( aq ) + H 2 O( l ) ' H 3 O + ( aq ) + H 2 PO 2 ( aq ) K a = 22 3 32 HPO HO −+ ⎡⎤ ⎣⎦ c) HSO 4 ( aq ) + H 2 O( l ) ' H 3 O + ( aq ) + SO 4 2– ( aq ) K a = 2 43 4 SO H O HSO 18.9 Appendix C lists the K a values. The larger the K a value, the stronger the acid. Hydroiodic acid, HI, is not shown in Appendix C because K a approaches infinity for strong acids and is not meaningful.
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18 - CHAPTER 18 ACIDBASE EQUILIBRIA 18.1 18.2 The Arrhenius...

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