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8.2 Chapter 16 Acids and Bases (6 per page)

# 8.2 Chapter 16 Acids and Bases (6 per page) - Objectives...

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07/03/2011 1 Chapter 16: Acids and Bases Objectives Compare Arrhenius, Brønsted-Lowry and Lewis theories of acids & bases. Study strong versus weak acids & bases Define ionization constants K a & K b . Calculate pH, pK a , pK b . Study polyprotic acids. Molecular determinants of acid & base strength. 16-1 Arrhenius Theory Svante Arrhenius 1859-1927 acids: H + donor base: OH - donor neutralization: H + + OH - H 2 O 16-1 Arrhenius Theory HCl(g H + (aq) + C (aq H 2 O Consider the neutralization reaction between HCl(aq) and NaOH(aq): NaOH(s) OH - (aq) + Na + (aq) H 2 O HCl(g) (aq) + Cl - (aq) H 2 O(l) NaCl(aq) 16-1 Arrhenius Theory + - hydrogen atom = proton + electron hydrogen ion = proton + H + ions do not actually exist in aqueous solution. Combine with water to form the hydronium ion: H O + From Sections 5-1 and 16-3 hydronium ion: H 3 O + +

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07/03/2011 2 16-1 Arrhenius Theory High concentration of hydronium ions H 3 O + : acidic O : High concentration of hydroxide ions OH - : basic 16-1 Arrhenius Theory Works well for many systems, but there is a small problem: How do you explain the fact that ammonia (NH 3 ) behaves as a base? WhereistheOH? Where is the OH? 16-1 Arrhenius Theory Almost a good answer: NH 3 (g) + H 2 O(l) NH 4 OH(aq) NH OH(aq) NH + (aq +OH - (aq NH 4 OH(aq) 4 (aq) + OH (aq) Provides an explanation for OH - and basic properties. Postulates existence of NH 4 OH - not actually observed, unsatisfactory Johannes Nicolaus Brønsted Thomas Martin Lowry 16-2 Brønsted-Lowry Theory (1923) Acids: proton donors Bases: proton acceptors A definition widely used today 16-2 Brønsted-Lowry Theory Substances that are acids/bases according to Arrhenius Theory are also acids/bases according to Brønsted-Lowry Theory. 16-2 Brønsted-Lowry Theory Arrhenius treatment of NH 3 NH 3 + H 2 O NH 4 OH NH 4 + + OH - Brønstead-Lowry treatment of NH 3 NH 3 + H 2 O NH 4 + + OH -
07/03/2011 3 16-2 Brønsted-Lowry Theory - + NH 4 + OH - Hydrogen ion is positively charged; transfer generates charged ions. 16-2 Brønsted-Lowry Theory NH 3 + H 2 O NH 4 + + OH - In the forward direction, NH 3 accepts a proton: base Water donates a proton: acid 16-2 Brønsted-Lowry Theory NH 3 + H 2 O NH 4 + + OH - In the reverse direction, NH 4 + donates a proton: acid OH - accepts a proton: base 16-2 Brønsted-Lowry Theory NH +H O NH + +OH conjugate pair NH 3 + H 2 O 4 + OH - conjugate pair 16-2 Brønsted-Lowry Theory NH 4 + is the conjugate acid of NH 3. NH 3 + H 2 O NH 4 + + OH - OH - is the conjugate base of H 2 O H O OH 16-2 Brønsted-Lowry Theory Sometimes water can act as a base: CH 3 COOH + H 2 O CH 3 COO - + H 3 O + acetic acid: vinegar

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07/03/2011 4 16-2 Brønsted-Lowry Theory - CH 3 COO - H 3 O + + 16-2 Brønsted-Lowry Theory CH 3 COOH + H 2 O CH 3 COO - + H 3 O + acid base base acid Definition acid: H 2 O amphiprotic : A substance that can react as either an acid or a base, for example water.
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8.2 Chapter 16 Acids and Bases (6 per page) - Objectives...

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