acids & bases chapter 16 (& part of chapter 17)

Acids & BasesCHAPTER 16

(& part of CHAPTER 17)

Chemistry: The Molecular Nature of Matter, 6th editionBy Jesperson, Brady, & Hyslop

2

CHAPTER 16: Acids & Bases

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Learning Objectives:

Define Brønsted-Lowry Acid/Base Define Lewis Acid/Base Evaluate the strength of acids/bases

Strong vs weak acids/bases Periodic trends Conjugate acids/bases

Identify likely compounds that will form acids and bases from the periodic table Acidic metal ions

Acid/Base equilibrium: pH, pOH Ka, Kb, pKa, pKb Kw of water

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CHAPTER 16: Acids & Bases


Lecture Road Map:

① Brønsted-Lowry Acids/Bases

② Trends in acid strength

③ Lewis Acids & Bases

④ Acidity of hydrated metal ions

⑤ Acid/Base equilibrium

4Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Brønsted-Lowry Acid/Base

CHAPTER 16 Acids & Bases

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Arrhenius Acid/Base Definition


Acid produces H3O+ in waterBase gives OH–

Acid-base neutralization – Acid and base combine to produce water and a salt.

e.g. HCl(aq) + NaOH(aq) H2O + NaCl(aq)

H3O+(aq) + Cl–(aq) + Na+(aq) + OH–(aq) 2H2O + Cl–(aq) + Na+(aq)

• Many reactions resemble this without forming H3O+ or OH– in solution

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Arrhenius Acid/Base Definition


Gas Phase Acid/Base chemistry not covered by Arrhenius definition

e.g. NH3(g) + HCl(g) NH4Cl(s)

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Brønsted-Lowry Definition


• Acid = proton donor• Base = proton acceptor • Allows for gas phase acid-base reactions

e.g. HCl + H2O H3O+ + Cl–

– HCl = acid • Donates H+

– Water = base • Accepts H+

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Brønsted-Lowry Conjugate Acid-Base Pair


• Species that differ by H+ e.g. HCl + H2O H3O+ + Cl–

• HCl = acid • Water = base • H3O+

– Conjugate acid of H2O• Cl–

– Conjugate base of HCl

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Brønsted-Lowry Example: Formic Acid


• Formic acid (HCHO2) is a weak acid • Must consider equilibrium

– HCHO2(aq) + H2O CHO2–(aq) + H3O+(aq)

• Focus on forward reaction

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Brønsted-Lowry Example: Formic Acid


Now consider reverse reaction:• Hydronium ion transfers H+ to CHO2

–

• Formate Ion is the Brønsted Base

H3O+ + CHO2HCHO2 + H2O

conjugate pair

conjugate pair

acid base acid base

GroupProblem

conjugate base

conjugate acid

HClNH3

HC2H3O2

CN–

HF

• Identify the conjugate partner for each


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GroupProblem


12

Write a reaction that shows that HCO3– is a

Brønsted acid when reacted with OH–

Write a reaction that shows that HCO3– is a

Brønsted base when reacted with H3O+(aq)

GroupProblem


13

In the following reaction, identify the acid/base conjugate pairs. (CH3)2NH + H2SO4 → (CH3)2NH+ + HSO4

–

A. (CH3)2NH / H2SO4 (CH3)2NH+ / HSO4–

B. (CH3)2NH / (CH3)2NH+ H2SO4 / HSO4–

C. H2SO4 / HSO4– (CH3)2NH+ / (CH3)2NH

D. H2SO4 / (CH3)2NH (CH3)2NH+ / HSO4–

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Brønsted-Lowry Amphoteric Substances


• Can act as either acid or base– Can be either molecules or ions

e.g. Hydrogen carbonate ion:– Acid

HCO3–(aq) + OH–(aq) CO3

2–(aq) + H2O– Base

HCO3–(aq) + H3O+(aq) H2CO3(aq) + H2O

[Amphiprotic substances can donate or accept a proton. This is a subtle but important difference from the word amphoteric]

GroupProblem


15

Which of the following can act as an amphoteric substance?A. CH3COOHB. HClC. NO2

–

D. HPO42–


Trends in Acid/Base Strength


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Acid/Base Trends Strengths of Acids & Bases


Strength of Acid – Measure of its ability to transfer H+

– Strong acids • React completely with water e.g. HCl and HNO3

– Weak acids • Less than completely ionized e.g. CH3COOH and

CHOOH

Strength of Base classified in similar fashion:– Strong bases

• React completely with water e.g. Oxide ion (O2–) and OH–

– Weak bases • Undergo incomplete reactionse.g. NH3 and NRH2 (NH2CH3, methylamine)

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Acid/Base Trends Strength in Water


• Strongest acid = hydronium ion, H3O+

– If more powerful H+ donor added to H2O– Reacts with H2O to produce H3O+

Similarly, • Strongest base is hydroxide ion (OH–)

– More powerful H+ acceptors – React with H2O to produce OH–

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Acid/Base Trends Acid/Base Equilibrium


• Acetic acid (HC2H3O2) is weak acid– Ionizes only slightly in water

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

weaker acid weaker base stronger acid stronger base

• Hydronium ion– Better H+ donor than acetic acid – Stronger acid

• Acetate ion – Better H+ acceptor than water – Stronger base

• Position of equilibrium favors weaker acid and base


In the reaction: HCl + H2O → H3O+ + Cl–

which species is the weakest base ?A. HClB. H2OC. H3O+

D. Cl–

GroupProblem

GroupProblemIdentify the preferred direction of the following reactions:

H3O+(aq) + CO32–(aq) HCO3

–(aq) + H2O

Cl–(aq) + HCN(aq) HCl(aq) + CN–

(aq) Jesperson, Brady, Hyslop. Chemistry: The

Molecular Nature of Matter, 6E 21

GroupProblem

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Acid/Base Trends General Trends


• Stronger acids and bases tend to react with each other to produce their weaker conjugates– Stronger Brønsted acid has weaker

conjugate base– Weaker Brønsted acid has stronger

conjugate base• Can be applied to binary acids (acids made

from hydrogen and one other element)

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Acid/Base Trends Binary Acid Trends


Binary Acids = HnX X = Cl, Br, P, As, S, Se, etc.

1. Acid strength increases from left to right within same period (across row)

– Acid strength increases as electronegativity of X increases

e.g. HCl is stronger acid than H2S which is stronger acid than PH3

– or PH3 < H2S < HCl

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Acid/Base Trends Binary Acid Trends


Binary Acids = HnX X = Cl, Br, P, As, S, Se, etc.

2. Acid strength increase from top to bottom within group

– Acid strength increases as size of X and bond length increases

e.g. HCl is weaker acid than HBr which is weaker acid than HI

– or HCl < HBr < HI

GroupProblem

Which is stronger?• H2S or H2O

• CH4 or NH3

• HF or HI

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Acid/Base Trends Oxoacid Trends


Oxoacids (HnX Om)– Acids of H, O, and one other element– HClO, HIO4, H2SO3, H2SO4, etc.

1. Acids with same number of oxygen atoms and differing Xa. Acid strength increases from bottom to top within group

• HIO4 < HBrO4 < HClO4 b. Acid strength increases from left to

right within period as the electronegativity of the central atom increases H3PO4 < H2SO4 < HClO4

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Acid/Base Trends Definition


Oxoacids (HnXOm)2. For same X

– Acid strength increases with number of oxygen atoms • H2SO3 < H2SO4 • More oxygens, remove more electron density from

central atom, weakening O—H bond make H more acidic

GroupProblem

Which is the stronger acid in each pair?• H2SO4 or H3PO4

• HNO3 or H3PO3

• H2SO4 or H2SO3

• HNO3 or HNO2Jesperson, Brady, Hyslop. Chemistry: The

Molecular Nature of Matter, 6E 28

GroupProblem

Which corresponds to the correct order of acidity from weakest to strongest acid ?A. HBrO3, HBrO, HBrO2

B. HBrO, HBrO2, HBrO3

C. HBrO, HBrO3, HBrO2

D. HBrO3, HBrO2, HBrO

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Acid/Base Trends Basicity


• Acid strength can be analyzed in terms of basicity of anion formed during ionization

• Basicity – Willingness of anion to accept H+ from H3O+

• Consider HClO3 and HClO4:

OClO OH

OClO

O OH

HClO3 HClO4

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Acid/Base Trends Basicity


• Lone oxygens carry most of the negative charge– ClO4

– has 4 O atoms, so each has –¼ charge – ClO3

– has 3 O atoms, so each has –1/3 charge • ClO4

– weaker base than ClO3–

– Thus conjugate acid, HClO4, is stronger acid• HClO4 stronger acid as more fully ionized

OClO OH

OClO

O OH

HClO3 HClO4

GroupProblem

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Acid/Base Trends Organic Acid Trends


• Organic acid —COOH • Presence of electronegative atoms (halide, nitrogen or

other oxygen) near —COOH group – Withdraws electron density from O—H bond– Makes organic acid, stronger acidse.g.

CH3CO2H < CH2ClCO2H < CHCl2CO2H < CCl3CO2H

GroupProblem

OHCO

CHH

H

OHCO

CFH

H

OHCO

CBrH

HOHC

OCIH

H

OHCO

CClH

H

A B C

D E

Which of the following is the strongest organic acid?


Lewis Acid/Base


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Lewis Acid/Base Definition


• Broadest definition of species that can be classified as either acid or base

• Definitions based on electron pairs • Lewis acid

– Any ionic or molecular species that can accept pair of electrons

– Formation of coordinate covalent bond• Lewis base

– Any ionic or molecular species that can donate pair of electrons

– Formation of coordinate covalent bond

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Lewis Acid/Base Lewis Neutralization


• Formation of coordinate covalent bond between electron pair donor and electron pair acceptor

• NH3BF3 = addition compound– Made by joining two smaller molecules

Addition Compound

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Lewis Acid/Base Lewis Acid-Base Reaction


Electrons in coordinate covalent bond come from O in hydroxide ion

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Lewis Acid/Base Lewis Acids


1. Molecules or ions with incomplete valence shells e.g. BF3 or H+

2. Molecules or ions with complete valence shells, but with multiple bonds that can be shifted to make room for more electrons

e.g. CO2

3. Molecules or ions that have central atoms that can expand their octets

– Capable of holding additional electrons– Usually, atoms of elements in Period 3 and belowe.g. SO2

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O2–

Lewis Acid/Base Lewis Acid Example: SO2


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Lewis Acid/Base Lewis Bases


• Molecules or ions that have unshared electron pairs and that have complete shells – e.g. O2– or NH3

Lewis Definition is Most General– All Brønsted acids and bases are Lewis acids and

bases– All Arrhenius acids and bases are Brønsted acids

and bases

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Lewis Acid/Base Proton (H+) Transfer


H2O—H+ + NH3 H2O + H+—NH3

GroupProblem

Identify the Lewis acid and base in the following:• NH3 + H+ NH4

+

• F– + BF3 BF4

–

• SeO3 + O2– SeO42–

GroupProblem

Which of the following species can act as a Lewis base ?A. Cl–

B. Fe2+

C. NO2–

D. O2–


Acidity of Oxides & Hydrates


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Acidic Metal Ions Acid-Base Properties of Elements &

their Oxides


Nonmetal oxides – React with H2O to form acids– Upper right hand corner of periodic table– Acidic Anhydrides– Neutralize bases– Aqueous solutions red to litmus– SO3(g) + H2O H2SO4(aq) – N2O5(g) + H2O 2HNO3(aq) – CO2(g) + H2O H2CO3(aq)

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Acidic Metal Ions


Acid-Base Properties of Elements & their Oxides

Metal oxides – React with H2O to form hydroxide (Base)– Group 1A and 2A metals (left hand side of periodic table)– Basic Anydrides– Neutralize acids– Aqueous solutions blue to litmus– Na2O(s) + H2O 2NaOH(aq) – CaO(s) + H2O Ca(OH)2(aq)

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Acidic Metal Ions Metal Oxides


• Solids at room temperature• Many insoluble in H2O

• Why? – Too tightly bound in crystal– Can't remove H+ from H2O– Do dissolve in solution of strong acid– Now H+ free, can bind to O2– and remove

from crystalFe2O3(s) + 6H+(aq) 2Fe3+(aq) + 3H2O

GroupProblem

What is the acid formed by P2O3 when it reacts with water ?A. H2PO4

B. H2PO2

C. H3PO4

D. H3PO3

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Acidic Metal Ions Metal Ions in Solution


• Exist with sphere of water molecules with their negative poles directed toward Mn+

• Mn+(aq) + mH2O M(H2O)mn+(aq) Lewis Acid Lewis Base hydrated

metal ion = addition compound

– n = charge on metal ion = 1, 2, or 3 depending on metal atom

– For now assume m = 1 (monohydrate)

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Acidic Metal Ions Metal Hydrates are Weak Brønsted Acids


M(H2O)n+(aq) + H2O M(OH)n+(aq) + H3O+(aq)

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Acidic Metal Ions


• Electron deficiency of metal cations causes them to induce electron density towards metal from water of hydration

• Higher charge density = more acidic metal

• Acidity increases left to right across period• Acidity decreases top to bottom down group

volumeioniccharge ionicdensity charge

Metal Hydrates are Weak Brønsted Acids

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Acidic Metal Ions Acidity of Hydrated Metal Ions


Degree to which M(H2O)mn+ produces acidic solutions

depends on: 1. Charge on Cation: As charge increases on Mn+, acidity

increases– Increases metal ion’s ability to draw electron density to itself and

away from O—H bond

2. Cation’s Size: As size of cation decreases, acidity increases– Smaller, more concentrated charge– Means greater pull of electron density from O—H bond

Net result: Very small, highly charged cations are very acidic[Al(H2O)6]3+(aq) + H2O [Al(H2O)5(OH)]2+(aq) + H3O+ (aq)

GroupProblem

In the following list of pairs of ions, which is the more acidic ? Fe2+ or Fe3+; Cu2+ or Cu+; Co2+ or Co3+

A. Fe3+, Cu+, Co2+

B. Fe2+, Cu2+, Co3+

C. Fe3+, Cu2+, Co3+

D. Fe2+, Cu2+, Co2+

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Acidic Metal Ions Trends in Acidity of Mn+


• Acidity increases up group (column) as cation size decreases

• Acidity increases across period (row) as cation size decreases

Alkali Metal Ions (Li+, Na+, K+, Rb+, Cs+)

All weak (+1, large size)

Be2+ Moderately weakOther Alkaline earth metals (Ba2+, Ca2+ Sr2+, Mg2+) Very Weak

Quite acidicTransition metal ions, Al3(often +3, +4 charges)


Identify each of the following as acidic or basic and give their reaction with water:• P2O5

• MnO2

GroupProblem

acids & bases chapter 16 (& part of chapter 17)

Documents

molecular nature of

brnsted acid

acid water

h2so4 hso4 ch32nh ch32nh

ch32nh h2so4 ch32nh

ch32nh ch32nh h2so4

formic acid hcho2

gas phase acidbase reactions