Chapter 17 - Brady(1)

Chapter 17 - Brady(1) - Introduction to General and Organic...

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1 Chapter 17 Acids and Bases Dr. David P. Brown Department of Chemistry Introduction to General and Organic Chemistry II
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2 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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3 • 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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4 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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5 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 2 3 3 2 2 3 3 ) ( ) ( ) ( : base a As ) ( ) ( ) ( : acid an As + + + + + - + - - Amphoteric Substances
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6 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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7 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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8 • 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 is a stronger base) The position of an acid-base equilibrium favors the weaker acid and base base stronger acid stronger base aker we acid weaker 2 3 2 3 2 2 3 2 ) ( ) ( ) ( aq O H C aq O H O H aq O H HC - + + +
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9 In general: The stronger a Brønsted acid is, the weaker is its conjugate base The weaker a Brønsted acid is, the stronger is its conjugate base These ideas can be applied to the binary acids (acids made from hydrogen and one other element) Binary Acids
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This note was uploaded on 04/06/2008 for the course CHEM 10765 taught by Professor Brown during the Spring '08 term at St. Johns Duplicate.

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Chapter 17 - Brady(1) - Introduction to General and Organic...

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