• Document: NH 3 + H 2 O + OH - NH 4. Acid-Base Concepts -- Chapter 15 + H + Conjugate Acid-Base Pairs: - H + base. acid
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Acid-Base Concepts -- Chapter 15 1. Arrhenius Acid-Base Concept (last semester) Acid: H+ supplier Base: OH- supplier 2. Brønsted-Lowry Acid-Base Concept (more general) (a) Definition (H+ transfer) Acid: H+ donor Base: H+ acceptor + H+ Conjugate Acid-Base Pairs: Base Acid - H+ e.g., conjugate pair NH3 + H2O NH4+ + OH- base acid acid base conjugate pair more examples: conjugate acids NH2- NH3 NH4+ OH- H2O H3O+ O2- OH- H2O HSO4- H2SO4 H3SO4+ CH3- CH4 " CH5+ " conjugate bases 1 (b) Amphoteric Substances -- molecules or ions that can function as both acids and bases (e.g., H2O itself !) e.g., the bicarbonate ion, HCO3- HCO3- + OH- → H2O + CO32- acid base HCO3- + HCl → H2CO3 + Cl- base acid (c) Relative Strengths of Brønsted Acids Binary Acids e.g., HCl, HBr, H2S, etc. Acid Strength Increases Periodic Table e.g., relative acidity: HCl > H2S (across a period) HI > HBr > HCl > HF (up in a group) 2 Oxo acids e.g., HNO3, H2SO4, H3PO4, etc. 1. for same central element, acid strength increases with # of oxygens Acid Strength Increases HClO < HClO2 < HClO3 < HClO4 2. for different central element, but same # oxygens, acid strength increases with electronegativity Acid Strength Increases Periodic Table e.g., H2SO4 > H2SeO4 > H2TeO4 (d) Relative strengths of conjugate acid-base pairs For example, HF + H2O H3O+ + F- acid base acid base In this case, the equilibrium lies mainly on reactant side. Therefore, " HF is a weaker acid than H3O+ " In general, weak Brønsted acids have strong conjugate bases. (vice versa) 3 3. Lewis Acid-Base Concept (most general) (a) Definition (electron pair transfer) Acid: e- pair acceptor Base: e- pair donor Lewis acids -- electron deficient molecules or cations. Lewis bases -- electron rich molecules or anions. (have one or more unshared e- pairs) (b) Lewis acid-base reactions (i.e., all non-redox reactions!) OH- + NH4+ → H2O + NH3 H + H .. H N H H O : + H N H .. H H .. - .. : H O Lewis acids .. .. - - Lewis H O: H O: .. .. base :O C O : :O C O : : O C O: .. .. .. .. .. .. OH- + CO2 → HCO3- 4 4. Auto-ionization of Water and the pH Scale (a) water undergoes self-ionization to slight extent: H2O + H2O H3O+(aq) + OH-(aq) H3O+ = hydronium ion OH- = hydroxide ion or, in simplified form: H2O(l) H+(aq) + OH-(aq) equilibrium constant: Kc = [H+] [OH-] / [H2O] but, [H2O] ∼ constant ∼ 55.6 mole/L at 25°C so, instead, use the "ion product" for water = Kw Kw = [H+] [OH-] = 1.0 x 10-14 (at 25°C) in pure water: [H+] = [OH-] = 1.0 x 10-7 M (b) the p

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