Chapter 16 11

Chapter 16Acid-Base Equilibria

CHEMISTRY The Central Science

9th Edition

Chapter 16 21

• Acids: taste sourcause dyes to change color

• Bases: taste bitterfeel soapy

• Arrhenius: acids increase [H+], bases increase [OH-] in AQUEOUS solution

• Arrhenius: acid + base salt + water

• Problem: the definition confines us to aqueous solutions

16.1: Acids and Bases: A 16.1: Acids and Bases: A Brief ReviewBrief Review

Chapter 16 31

The H+ Ion in Water• The H+

(aq) ion is a proton with no electrons

• In water, the H+(aq) form clusters

• The simplest cluster is H3O+(aq)

• A hydrated proton• Larger clusters are H5O2

+ and H9O4+

• Generally H+(aq) and H3O+

(aq) are used interchangeably

16.2: Br16.2: Brøønstead-Lowry nstead-Lowry Acids and BasesAcids and Bases

Chapter 16 41

Proton Transfer Reactions• Brønsted-Lowry theory:

• Acid donates H+ and base accepts H+

• Brønsted-Lowry base does not need to contain OH-

• HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)– HCl donates a proton to water (HCl is an acid)– H2O accepts a proton from HCl (H2O is a base)

• Water can behave as either an acid or a base• Amphoteric substances can behave as acids and bases

• With a stronger base, it is an acid (and vice versa)

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Conjugate Acid-Base Pairs

• Whatever is left of the acid after the proton is donated is called its conjugate base

• Whatever remains of the base after it accepts a proton is called a conjugate acid

– After HA (acid) loses its proton it is converted into A- (base) – HA and A- are conjugate acid-base pairs

– After H2O (base) gains a proton it is converted into H3O+ (acid)

– H2O and H3O+ are conjugate acid-base pairs

• Conjugate acid-base pairs differ by only one proton

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

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Relative Strengths of Acids and Bases

• The stronger the acid, the weaker the conjugate base

Text, P. 618

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Generalizations about Acid and Base Behavior

1. Strong acids completely transfer their protons to water• The conjugate base of a strong acid has negligible

acid-base properties• The conjugate acid of a strong base has negligible

acid-base properties

2. Weak acids partly dissociate in aqueous solution• The conjugate bases of weak acids are weak bases

3. Substances with negligible acidity (CH4) contain H but don’t behave as acids in water• Their conjugate bases are strong bases (form (OH)- in

water)

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Proton Transfer Reactions• Strong acids and bases react with water to liberate H+

(aq) and (OH)-

(aq) ions

• The leveling effect of water: one can’t distinguish among the strengths of the strong acids or strong bases• Place HCl, HBr or HI in dilute acetic acid and there

is a noticeable difference in their strengths

• H+ is the strongest acid that can exist in equilibrium in aqueous solution

• OH- is the strongest base that can exist in equilibrium in aqueous solution

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Proton Transfer Reactions• In acid-base reactions, the equilibrium favors transfer of

the proton from the stronger acid to the stronger base

• Strong acid in water: Equilibrium favors the right sideHSO4

-(aq) + CO3

-2(aq) SO4

-2(aq) + HCO3

-(aq)

Compare bases: CO3-2 is from HCO3

-

SO4-2 is from HSO4

-

Use chart ( P. 618) to determine which base is stronger (CO3-2)

• The stronger base and acid are consumed• Proton is transferred to the stronger base

• Production of the weaker electrolytes is favored

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• The reaction favors the

consumption of the stronger acid and stronger base and the formation of the weaker acid and weaker base (the weaker electrolytes)

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Proton Transfer Reactions

• Weak acid in water: Equilibrium favors the left side• The anion of the acid is a stronger base, so it accepts

a proton from the H3O+

HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2

-(aq)

• Use chart ( P. 618) to determine which base is stronger (C2H3O2-)

• The stronger base and acid are consumed • Proton is transferred to the stronger base

Chapter 16 121

The reaction favors the consumption of the stronger acid and stronger base and the formation of the weaker acid and weaker base (the weaker electrolytes)

Complete Sample Problems:

# 5, 7, 9, 11, 15