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Chapter 16: Acids and Bases, A Molecular Look. Chemistry: The Molecular Nature of Matter, 6E Jespersen/Brady/Hyslop. Arrhenius Acids and Bases. Acid produces H 3 O + in water Base gives OH – Acid-base neutralization Acid and base combine to produce water and a salt.

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Chapter 16 acids and bases a molecular look

Chapter 16: Acids and Bases, A Molecular Look

Chemistry: The Molecular Nature of Matter, 6E


Arrhenius acids and bases
Arrhenius Acids and Bases

Acid produces H3O+ in water

Basegives 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

  • Gas phase acid base
    Gas Phase Acid-Base

    • Not covered by Arrhenius definition

      e.g. NH3(g) + HCl(g) NH4Cl(s)

    Br nsted lowry definition
    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+

    Conjugate acid base pair
    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

    Formic acid is bronsted acid
    Formic Acid is Bronsted Acid

    • Formic acid (HCHO2) is a weak acid

    • Must consider equilibrium

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

    • Focus on forward reaction

    Formate ion is bronsted base
    Formate Ion is Bronsted Base

    • Now consider reverse reaction

    • Hydronium ion transfers H+ to CHO2–

    Learning check

    Identify the conjugate partner for each

    Learning Check






    Learning check1
    Learning Check

    • Write a reaction that shows that HCO3– is a Brønsted acidwhen reacted with OH–

    • HCO3–(aq)+ OH–(aq)

    • Write a reaction that shows that HCO3– is a Brønsted basewhen reacted with H3O+(aq)

    • HCO3–(aq) + H3O+(aq)

    H2O + CO32–(aq)

    H2CO3(aq) + H2O

    Your turn
    Your Turn!

    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–

    Amphoteric substances
    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) CO32–(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]

    Your turn1
    Your Turn!

    Which of the following can act as an amphoteric substance?

    A. CH3COOH

    B. HCl

    C. NO2–

    D. HPO42–

    Strengths of acids and bases
    Strengths of Acids and Bases

    Strength of Acid

    • Measure of its ability to transfer H+

    • Strongacids

      • React completely with water e.g.HCl and HNO3

    • Weak acids

      • Less than completely ionized e.g.CH3COOH and CHOOH

        Strength of Baseclassified in similar fashion:

    • Strong bases

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

    • Weak bases

      • Undergo incomplete reactions

        e.g.NH3 and NRH2 (NH2CH3, methylamine)

    Reactions of strong acids and bases
    Reactions of Strong Acids and Bases

    In water

    • Strongest acid= hydronium ion, H3O+

      • If more powerful H+ donor added to H2O

      • Reacts with H2O to produce H3O+


    • Strongest baseis hydroxide ion (OH–)

      • More powerful H+ acceptors

      • React with H2O to produce OH–

    Position of acid base equilibrium
    Position of Acid-Base Equilibrium

    • Acetic acid (HC2H3O2) is weak acid

      • Ionizes only slightly in water

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

        weaker acidweaker basestronger acidstronger base

    • Hydronium ion

      • Better H+ donor than acetic acid

      • Stronger acid

    • Acetate ion

      • Better H+ acceptor than water

      • Stronger base

    • Position of equilibrium favors weakeracid and base

    Your turn2
    Your Turn!

    In the reaction:

    HCl + H2O → H3O+ + Cl–

    which species is the weakest base ?

    A. HCl

    B. H2O

    C. H3O+

    D. Cl–

    In general
    In General

    • 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)

    Learning check2
    Learning Check

    Identify the preferred direction of the following reactions:

    H3O+(aq)+ CO32–(aq) HCO3–(aq)+ H2O

    Cl–(aq)+ HCN(aq) HCl(aq)+ CN–(aq)

    Trends in binary acid strength
    Trends in Binary Acid Strength

    Binary Acids = HnX

    X = Cl, Br, P, As, S, Se, etc.

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

      • Acid strength increases as electronegativity of Xincreases

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

      • or PH3 < H2S < HCl

    Trends in binary acid strength1
    Trends in Binary Acid Strength

    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 Xand bond length increases

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

    • or HCl < HBr < HI

    Learning check3
    Learning Check

    • Which is stronger?

    • H2S or H2O

    • CH4 or NH3

    • HF or HI

    • H2S

    • NH3

    • HI

    Trends in oxoacid strength
    Trends in Oxoacid Strength

    Oxoacids (HnX Om)

    • Acids of H, O, and one other element

    • HClO, HIO4, H2SO3, H2SO4, etc.

  • Acids with same number of oxygen atoms and differing X

    • Acid strength increasesfrom

      bottom to top within group

      • HIO4 < HBrO4 < HClO4

    • Acid strength increases from left to right within period as the electronegativity of the central atom increases H3PO4 < H2SO4 < HClO4

  • Trends in oxoacid strength1
    Trends in Oxoacid Strength

    Oxoacids (HnXOm)

    • 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

    Learning check4
    Learning Check

    Which is the stronger acid in each pair?

    • H2SO4 or H3PO4

    • HNO3 or H3PO3

    • H2SO4 or H2SO3

    • HNO3 or HNO2





    Your turn3
    Your Turn!

    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

    Alternate definition of acid strength
    Alternate Definition of Acid Strength

    • 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:

    Comparing basicity
    Comparing 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

    Learning check5
    Learning Check

    • Arrange the following in order of increasing acid strength:

    • HBr, AsH3, H2Se

      • AsH3 < H2Se < HBr

    • H2SeO4, H2SO4, H2TeO4

      • H2TeO4 < H2SeO4 < H2SO4

    • HBrO3, HBrO, HBrO4, HBrO2

      • HBrO < HBrO2 < HBrO3 < HBrO4

    Strength of organic acids
    Strength of Organic Acids

    • 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 acids

        e.g.CH3CO2H < CH2ClCO2H < CHCl2CO2H < CCl3CO2H

    Your turn4
    Your Turn!

    Which of the following is the strongest organic acid?






    Lewis definition of acid and base
    Lewis Definition of Acid and Base

    • 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 acceptpair of electrons

      • Formation of coordinate covalent bond

    • Lewis base

      • Any ionic or molecular species that can donatepair of electrons

      • Formation of coordinate covalent bond

    Lewis neutralization
    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

    Lewis acid base reaction
    Lewis Acid-Base Reaction

    • Electrons in coordinate covalent bond come from O in hydroxide ion

    Lewis acids
    Lewis Acids:

    • Molecules or ions with incomplete valence shells

      e.g. BF3 or H+

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

      e.g. CO2

    • 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 below

        e.g. SO2

    So 2 as lewis acid
    SO2 as Lewis Acid


    Lewis bases
    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

    H transfer from lewis perspective
    H+ Transfer from Lewis Perspective

    e.g.H2O—H+ + NH3 H2O + H+—NH3

    Learning check6
    Learning Check

    Identify the Lewis acid and base in the following:

    • NH3 + H+NH4+


    • F– + BF3BF4–


    • SeO3 + O2–SeO42–


    Your turn5
    Your Turn!

    Which of the following species can act as a Lewis base ?

    A. Cl–

    B. Fe2+

    C. NO2–

    D. O2–

    Acid base properties of elements and their oxides
    Acid-Base Properties of Elements and Their Oxides

    Nonmetal oxides

    • React with H2O to form acids

    • Upper right hand corner of periodic table

    • Acidic Anhydrides

    • Neutralize bases

    • Aqueous solutions redto litmus

    • SO3(g) + H2O  H2SO4(aq)

    • N2O5(g) + H2O  2HNO3(aq)

    • CO2(g) + H2O  H2CO3(aq)

    Acid base properties of elements and their oxides1
    Acid-Base Properties of Elements and Their Oxides

    Metal oxides

    • React with H2O to form hydroxide (Base)

    • Group 1A and 2A metals (left hand side of periodic table)

    • BasicAnydrides

    • Neutralize acids

    • Aqueous solutions blue to litmus

    • Na2O(s) + H2O  2NaOH(aq)

    • CaO(s) + H2O  Ca(OH)2(aq)

    Metal oxides m x o y
    Metal Oxides MxOy

    • 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 crystal

        Fe2O3(s) + 6H+(aq) 2Fe3+(aq) + 3H2O

    Your turn6
    Your Turn!

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

    A. H2PO4

    B. H2PO2

    C. H3PO4

    D. H3PO3

    • P2O3 + 3H2O → 2H3PO3

    Metal ions in solution once anion is removed
    Metal Ions in Solution (Once Anion is Removed)

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

    • Mn+(aq) + mH2O M(H2O)mn+(aq)

      Lewis AcidLewis Basehydrated metal ion = addition compound

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

      • For now assume m = 1 (monohydrate)

    Hydrated metal ions weak br nsted acids
    Hydrated Metal Ions = Weak Brønsted Acids

    M(H2O)n+(aq) + H2OM(OH)n+(aq) + H3O+(aq)

    Your turn7
    Your Turn!

    The following reactions:

    Al(OH)3 + 3H+ → Al3+ + H2O

    Al(OH)3 + OH– → Al(OH)4–

    illustrate the concept of

    A. neutralization

    B. amphoteric property of Al(OH)3

    C. oxidation of Al

    D. reduction of OH–

    Hydrated metal ions can act as weak acids
    Hydrated Metal Ions Can Act as Weak Acids

    • 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

    Acidity of hydrated metal ions
    Acidity of Hydrated Metal Ions

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

      • Charge on cation

      • Cation's size

        1. As charge increases on Mn+, acidity increases

      • Increases metal ion’s ability to draw electron density to itself and away from O—H bond

    Acidity of hydrated metal ions1
    Acidity of Hydrated Metal Ions

    2. 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)

    Your turn8
    Your Turn!

    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+

    Trends in acidity of m n
    Trends in Acidity of Mn+

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

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


    Moderately weak

    Other Alkaline earth metals (Ba2+, Ca2+ Sr2+, Mg2+)

    Very Weak

    Transition metal ions, Al3(often +3, +4 charges)

    Quite acidic

    Learning check7
    Learning Check

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

    • P2O5

      P2O5(s) + 3H2O2H3PO4(aq)

      2H3PO4(aq) 2H+(aq) + 2H2PO4–(aq)

    • MnO2

      MnO2(s) + 2H2OMn2+(aq) + 4OH–(aq)



    Ceramic materials
    Ceramic Materials

    • Date back to prehistoric times

      • Pottery as far back as 13,000 years old

    • Today found in brick, cement, and glass

      • Porcelain dinnerware, tiles, sinks, toilets, artistic pottery and figurines

      • Composed of Silicates — compounds containing anions composed of silicon and oxygen

    • Advanced ceramic materials

      • Made in chemistry laboratories

      • High-tech applications

      • Found in cell phones and diesel engines

    Traditional ceramic synthesis
    Traditional Ceramic Synthesis

    • Pulverize components of ceramic into fine powders

    • Mix with

      • Water and pour into mold or

      • Binder and press into desired shape

    • Heat in kiln, tC > 1000 ˚C

      • Sintering – particles fuse together to form ceramic

    • Problems:

      • Hard to form uniform, very small particles

      • Ceramics formed often have small crackswhich decreases their strength

      • Composition not easily reproducible

    Sol gel process
    Sol-Gel Process

    • Synthesis of ceramics that avoid problems of particle size and uniformity

    • Based on acid–base reactions

    • Starting materials are

      • Metal salts or

      • Compounds where metal or metalloid (e.g., Si) is bonded to some number of alkoxide groups

    Sol gel process1
    Sol-Gel Process

    • Metal alkoxide salts generally soluble in alcohols

    • Alcohols are very weak acids

      • Essentially no tendency to lose H+

    • Alkoxide ions very strong bases

      • React with water to form alcohol and OH–

      • C2H5O– + H2O  C2H5OH + OH–

    • Basis of sol-gel process


    Your turn9
    Your Turn!

    Which of the following is an example of an alkoxide ion ?

    A. CH3OCH2+

    B. CH3CH2CHO–

    C. CHCOO–

    D. OH–

    Sol gel process2
    Sol-Gel Process

    • Gradually add water to alcohol solution of alkoxide salts

    • Alkoxide ions gradually replaced by OH– ions

      Hydrolysis reaction

      Zr(C2H5O)4 + H2O  Zr(C2H5O)3OH + C2H5OH

    • When two Zr(C2H5O)3OH encounter each other, they undergo an acid-base reaction and lose H2O

    Sol gel process3
    Sol-Gel Process

    • As more H2O is added:

      • More alkoxide ions converted to alcohols

      • Form more oxide linkages bridging Zr ions

      • Result is very fine particles of metal oxides with residual OH ions suspended in alcohol (gel-like)

    • Sol-gel used in various ways

      • Dip coated on surface yields thin film ceramics

      • Cast into mold produces semisolid gelatin-like material = wet gel

        • Dry wet gel by evaporation gives porous gel = xerogel

        • Heating xerogel yields dense ceramic or glass with uniform structure

    Sol gel process4
    Sol-Gel Process

    • Sol-gel used in various ways

      • Remove solvent from wet gel at temperature above critical temperature of solvent yields very porous and extremely low density solid = aerogel

      • Adjust viscosity of gel suspension and spinning yields ceramic fibers

      • Precipitation of sol-gels yields ultrafine and uniform ceramic powders

    TiN coating

    Ceramic heat tiles