Chapter 15 - Introduction to General and Organic Chemistry...

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1 Chapter 15 Acids and Bases Dr. David P. Brown Department of Chemistry Introduction to General and Organic Chemistry II
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2 Learning Objectives I - Be able to identify a Bronsted acid and a Bronsted base . - Understand the concept of conjugate acids and bases. - Understand what is meant by complete and partial dissociation. - Understand periodic trends in acid and base strengths. - Have a thorough understanding of redox reactions.
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3 Introduction Acid-base reactions involve proton (hydrogen ion, H + ) transfer The generalization of the Arrhenius definition of acids and bases is called the Brønsted- Lowry definitions: An acid is a proton donor A base is a proton acceptor This allows for gas phase acid-base reactions
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4 • Species that differ by a proton, like H 2 O and H 3 O + , are called conjugate acid-base pairs HCl is the acid because it donates a proton. Water is the base because it accepts a proton . The reaction of HCl and H 2 O
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5 Members of a conjugate acid-base pair differ by a single proton ! Thus, the conjugate acid of NH 3 is NH 4 + . Conjugate Acid-Base Pairs
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6 15.1. Brønsted-Lowry acids and bases exchange protons Learning Check conjugate base conjugate acid HCl NH 3 HC 2 H 3 O 2 CN - HF Cl - NH 4 + C 2 H 3 O 2 - HCN F - Identify the Conjugate Partner for Each
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7 15.1. Brønsted-Lowry acids and bases exchange protons Your Turn! How many of the following combinations are conjugate pairs: i. HCN/CN - ii. HCl/Cl - iii. H 2 S/S 2- A. 1 B. 2 C. 3 D. None of them is a conjugate pair.
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8 An amphoteric substances can act as either an acid or base These are also called amphiprotic , and can be either molecules or ions For example, the hydrogen carbonate ion: O H aq CO H aq O H aq HCO O H aq CO aq OH aq HCO 2 3 2 3 2 2 3 ) ( ) ( ) ( ) ( ) ( ) ( + + + + + - - - - 3 3 : base a As : acid an As Amphoteric Substances
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9 15.1. Brønsted-Lowry acids and bases exchange protons Your Turn! How many of the following are amphoteric? HC 2 H 3 O 2 , HPO 4 2- , HSO 4 - A. 1 B. 2 C. 3 D. None of these
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10 The strength of an acid is a measure of its ability to transfer a proton Acids that react completely with water (like HCl and HNO 3 ) are classified as strong Acids that are less than completely ionized are called weak acids Bases can be classified in a similar fashion: Strong bases, like the oxide ion, react completely – Weak bases, like NH 3 , undergo incomplete reactions Strengths of Acids & Bases
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11 The strongest acid in water is the hydronium ion If a more powerful proton donor is added to water, it quantitatively reacts with water to produce H 3 O + Similarly, the strongest base that can be found in water is the hydroxide ion, because more powerful proton acceptors react quantitatively with water to produce OH -
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12 • Acetic acid (HC 2 H 3 O 2 ) is a weak acid It ionizes only slightly in water The hydronium ion is a better proton donor than acetic acid (it is a stronger acid) The acetate ion is a better proton acceptor than water (it
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Chapter 15 - Introduction to General and Organic Chemistry...

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