Acids and Bases. Arrhenius Acids and Bases

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2. Show the chemical reaction to which Ka applies for H2PO41-

4. Determine the Kb value for HPO42-.

5. Which direction is the reaction favored?

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Acid-Base Reactions of Salts (Ions as Acids and Bases)

A. Salts

1. Ionic compounds (salts) formed from acid-base reaction.

HA + MOH  M+A- + H2O

acid base salt

2. The salt (MA) could leave the resulting aqueous solution to be neutral, acidic, or basic.

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B. Hydrolysis of Salts

Salts may react with water (hydrolysis reaction involving the splitting of a water molecule) to produce acidic or basic solutions.

For hypothetical ionic compound MA, consisting of

Mx+ cation and Ax- anion, consider possible

hydrolysis reactions and possible consequences:

Mx+ + H2O  M(OH)(x-1)+ + H+ acidic solution

Ax- + H2O  HA(x+1)- + OH- basic solution

If both cation and anion hydrolyze, compare Ka and Kb

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C. Predicting If Hydrolysis Occurs

1. Write the hypothetical hydrolysis rxns for salt.

(one for cation and one for anion)

2. Look at the hypothetical hydrolysis products.

3. If a strong acid or strong base was hypothetically

produced, these hydrolysis reactions DO NOT

occur. No acid or base properties upon

hydrolysis. Recall Table of Strong Acids and Bases

4. If a weak acid or base was produced, these hydrolysis reactions DO occur and produce acid or base properties. For hydrolysis products:

H+ : acidic OH- : Basic

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D. Example Problems

Will an aqueous solution of the salt be acidic,

basic, or neutral?

1) NaCl

2) NH4Cl

3) NH4CN

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Cumulative Example Problem

The cyanate ion, OCN-, is a weak base. The Ka for HOCN (cyanic acid) is 3.4 x 10-8. Calculate the pH of a 0.200 M solution of NaOCN.

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Lewis Acids and Bases

A. Some Acid-Base Reactions Do Not Fit

Either Arrehenius or Bronsted –Lowry

Definitions.

Consider:

HCl + NaOH  NaCl + H2O

Al3+ + 6 H2O  Al (H2O)63+

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B. Lewis Acids and Bases

1. Lewis Acid: electron pair acceptor

(Accepts a pair of electrons to form a new bond)

** Usually has an incomplete octet of electrons

2. Lewis Base: electron pair donor

(Donates a pair of electrons to form a new bond)