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Acids and Bases: Strengths and Characteristics - Prof. Andrew S. Osborne, Study notes of Chemistry

An overview of acids and bases, distinguishing between strong and weak acids and bases. It explains how strong acids completely ionize in water, while weak acids only slightly dissociate. The document also covers the role of acid dissociation constants (ka) in determining the strength of an acid.

Typology: Study notes

2009/2010

Uploaded on 05/03/2010

ewhaley
ewhaley 🇺🇸

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Chapter 10 Acids and Bases
10.2
Strengths of Acids and Bases
General, Organic, and Biological Chemistry
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1

Chapter 10 Acids and Bases

Strengths of Acids and Bases

General, Organic, and Biological Chemistry

 (^) Strong acids completely ionize (100%) in aqueous solutions. HCl( g ) + H 2 O( l ) H 3 O+( aq ) + Cl−( aq )  (^) Weak acids dissociate only slightly in water to form a solution of mostly molecules and a few ions. H 2 CO 3 ( aq ) + H 2 O( l ) H 3 O+( aq ) + HCO 3 −( aq )

Strengths of Acids

Weak Acids

Weak acids dissolved in water  (^) have only a few molecules dissociated  (^) remain mostly as the undissociated (molecular) form  (^) have low concentrations of H 3 O+^ and anion (A–) HA( aq ) + H 2 O( l ) H 3

O

( aq ) + A − ( aq )

5

Comparing Strong and Weak

Acids

In solution,  (^) HCl, a strong acid , dissociates 100%  (^) acetic acid (CH 3 COOH), a weak acid , is mostly molecules and only a few ions General, Organic, and Biological Chemistry

Weak acids  (^) make up most of the acids  (^) have strong conjugate bases

Weak Acids

8

Strong Bases

Strong bases  (^) are formed from metals of Groups 1A(1) and 2A(2)  (^) include LiOH, NaOH, KOH, and Ca(OH) 2  (^) dissociate completely in water KOH( s ) K+( aq ) + OH−( aq ) General, Organic, and Biological Chemistry

Learning Check

Identify each of the following as a strong or weak acid or base. A. HBr B. HNO 2 C. NaOH D. H 2 SO 4 E. Cu(OH) 2

Solution

Identify each of the following as a strong or weak acid or base. A. HBr strong acid B. HNO 2 weak acid C. NaOH strong base D. H 2 SO 4 strong acid E. Cu(OH) 2 weak base

Solution

Identify the stronger acid in each pair: A. HNO 2 B. HBr C. H 3 O+

Acid Dissociation Constant, K

a  (^) In a weak acid, the rate of the dissociation of the acid is equal to the rate of the association. HA + H 2 O H 3 O+^ + A–  (^) The equilibrium expression is: K eq = [H 3 O+][A–] [HA][H 2 O]  (^) Because the concentration of H 2 O is constant, the^ K a expression for a weak acid is: K a = [H 3 O+][A–] [HA]

16

Some Acid Dissociation Constants

 K

a values for some weak acids are shown in Table 10.4. General, Organic, and Biological Chemistry

Writing K

a

for a Weak Acid

Write the K a for H 2 S.

  1. Write the equation for the dissociation of H 2 S: H 2 S( aq ) + H 2 O( l ) H 3 O+( aq ) + HS−( aq )
  2. Set up the K a expression: K a = [H 3 O+][HS−] [H 2 S]

Solution

  1. Write the equation for the dissociation of HCN: HCN( aq ) + H 2 O( l ) H 3 O+( aq ) + CN( aq )
  2. Set up the K a expression: K a = [H 3 O+][CN] [HCN]