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

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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 )

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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

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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]

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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]