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acids and bases - 15-1 ACIDS AND BASES Acids taste sour e.g...

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15-1 ACIDS AND BASES Acids : taste sour, e.g. , vinegar and lemons (acetic and citric acids, respectively), and cause plant dyes to change color. Aqueous solutions of acids conduct electricity Bases : taste bitter and feel soapy (used in cleaners, and cause plant dyes to change color. Aqueous solutions of bases conduct electricity Arrhenius definitions (1884): acids increase [H + ] while bases increase [OH - ] in solution. Question: What is the difference between a strong and weak acid, or a strong and weak base? What is the problem with the Arrhenius definition and the characteristics of strong and weak acids and bases? (Hint, NH 3 is a base) Compare, through the following examples, the Arrhenius definition acids and bases with two others developed in the 1920s: the Brønsted- Lowry and the Lewis definitions. HCl (aq) + NaOH (aq) H 2 O (l) + NH 3(aq) BF 3(g) + NH 3(g)
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15-2 Brønsted-Lowry Acid – Base Theory Reactions between Brønsted-Lowry acids and bases are proton transfer reactions An acid donates H + (a proton) and a base accepts H + A B-L base does not need to contain OH - Water can behave as either an acid or a base and is considered amphoteric - a substance that can behave as an acid or a base The portion of an acid remaining after a proton is donated is its conjugate base Similarly, the product formed after a base accepts a proton is its conjugate acid Example : Identify the acid, base, and conjugate acid and base HClO 2 + H 2 O OCl + H 2 O HCl + H 2 PO 4 NH 3 + H 2 PO 4
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15-3 Lewis Acid – Base Theory Provides a more universal definition of acids and bases than the Brønsted-Lowry description Acid – Base – Can be used for solid and gas phase reactions as well as those in solution Lewis acids and bases do not need to contain protons (H + ) or hydroxide (OH - ) Lewis acids generally have an incomplete octet and are electron deficient ( e.g., BF 3 ) Transition metal ions are generally Lewis acids Lewis acids must have a vacant orbital (into which the electron pairs can be donated) Is NaOH a Lewis base? Is NH 3 a Lewis base?
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15-4 Example : Identify the Lewis acid and base for each and show the transfer of electrons: BF 3 + F BF 4 SnCl 4 + 2 Cl SnCl 6 2– N 2 H 4 + HNO 3 N 2 H 5 + + NO 3 B(OH) 3 + OH B(OH) 4 SO 3 + H 2 O H 2 SO 4 Compounds with π -bonds can act as Lewis acids: H 2 O (l) + CO 2(g) H 2 CO 3(aq) Mechanism:
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15-5 Strong Acids The most common strong acids are HCl, HBr, HI, HNO 3
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