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Química inorgánica ácidos y bases

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  • Chapter 15Acids and

    Bases

    2008, Prentice Hall

    Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro

    Roy KennedyMassachusetts Bay Community College

    Wellesley Hills, MA

  • Tro, Chemistry: A Molecular Approach 2

    Properties of Acids sour taste react with active metals i.e., Al, Zn, Fe, but not Cu, Ag, or Au

    2 Al + 6 HCl AlCl3 + 3 H2 corrosive

    react with carbonates, producing CO2marble, baking soda, chalk, limestone

    CaCO3 + 2 HCl CaCl2 + CO2 + H2O change color of vegetable dyes blue litmus turns red

    react with bases to form ionic salts

  • Tro, Chemistry: A Molecular Approach 3

    Common AcidsChemical Name Formula Uses Strength

    Nitric Acid HNO3 explosive, fertilizer, dye, glue Strong

    Sulfuric Acid H2SO4 explosive, fertilizer, dye, glue,

    batteries Strong

    Hydrochloric Acid HCl metal cleaning, food prep, ore

    refining, stomach acid Strong

    Phosphoric Acid H3PO4 fertilizer, plastics & rubber,

    food preservation Moderate

    Acetic Acid HC2H3O2plastics & rubber, food preservation, Vinegar

    Weak

    Hydrofluoric Acid HF metal cleaning, glass etching Weak

    Carbonic Acid H2CO3 soda water Weak

    Boric Acid H3BO3 eye wash Weak

  • Tro, Chemistry: A Molecular Approach 4

    Structures of Acids

    binary acids have acid hydrogens attached to a nonmetal atomHCl, HF

  • Tro, Chemistry: A Molecular Approach 5

    Structure of Acids oxy acids have acid hydrogens attached to

    an oxygen atomH2SO4, HNO3

  • Tro, Chemistry: A Molecular Approach 6

    Structure of Acids carboxylic acids have

    COOH groupHC2H3O2, H3C6H5O7

    only the first H in the formula is acidic the H is on the COOH

  • Tro, Chemistry: A Molecular Approach 7

    Properties of Bases also known as alkalis taste bitter alkaloids = plant product that is alkaline

    often poisonous solutions feel slippery change color of vegetable dyes different color than acid red litmus turns blue

    react with acids to form ionic saltsneutralization

  • Tro, Chemistry: A Molecular Approach 8

    Common BasesChemical

    Name Formula

    Common Name

    Uses Strength

    sodium hydroxide

    NaOH lye,

    caustic soda soap, plastic,

    petrol refining Strong

    potassium hydroxide

    KOH caustic potashsoap, cotton, electroplating

    Strong

    calcium hydroxide

    Ca(OH)2 slaked lime cement Strong

    sodium bicarbonate

    NaHCO3 baking soda cooking, antacid Weak

    magnesium hydroxide

    Mg(OH)2 milk of

    magnesia antacid Weak

    ammonium hydroxide

    NH4OH, {NH3(aq)}

    ammonia water

    detergent, fertilizer,

    explosives, fibers Weak

  • Tro, Chemistry: A Molecular Approach 9

    Structure of Bases

    most ionic bases contain OH ionsNaOH, Ca(OH)2

    some contain CO32- ionsCaCO3 NaHCO3

    molecular bases contain structures that react with H+

    mostly amine groups

  • Tro, Chemistry: A Molecular Approach 10

    Indicators chemicals which change color depending on

    the acidity/basicity many vegetable dyes are indicatorsanthocyanins

    litmus from Spanish mossred in acid, blue in base

    phenolphthaleinfound in laxativesred in base, colorless in acid

  • Tro, Chemistry: A Molecular Approach 11

    Arrhenius Theory bases dissociate in water to produce OH- ions and

    cationsionic substances dissociate in water

    NaOH(aq) Na+(aq) + OH(aq) acids ionize in water to produce H+ ions and anionsbecause molecular acids are not made of ions, they cannot

    dissociate they must be pulled apart, or ionized, by the water

    HCl(aq) H+(aq) + Cl(aq)in formula, ionizable H written in front

    HC2H3O2(aq) H+(aq) + C2H3O2(aq)

  • Tro, Chemistry: A Molecular Approach 12

    Arrhenius Theory

    HCl ionizes in water,producing H+ and Cl ions

    NaOH dissociates in water,producing Na+ and OH ions

  • Tro, Chemistry: A Molecular Approach 13

    Hydronium Ion the H+ ions produced by the acid are so reactive they

    cannot exist in waterH+ ions are protons!!

    instead, they react with a water molecule(s) to produce complex ions, mainly hydronium ion, H3O+

    H+ + H2O H3O+ there are also minor amounts of H+ with multiple water

    molecules, H(H2O)n+

  • Tro, Chemistry: A Molecular Approach 14

    Arrhenius Acid-Base Reactions

    the H+ from the acid combines with the OH-from the base to make a molecule of H2Oit is often helpful to think of H2O as H-OH

    the cation from the base combines with the anion from the acid to make a salt

    acid + base salt + water

    HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

  • Tro, Chemistry: A Molecular Approach 15

    Problems with Arrhenius Theory does not explain why molecular substances, like

    NH3, dissolve in water to form basic solutions even though they do not contain OH ions

    does not explain how some ionic compounds, like Na2CO3 or Na2O, dissolve in water to form basic solutions even though they do not contain OHions

    does not explain why molecular substances, like CO2, dissolve in water to form acidic solutions even though they do not contain H+ ions

    does not explain acid-base reactions that take place outside aqueous solution

  • Tro, Chemistry: A Molecular Approach 16

    Brnsted-Lowry Theory in a Brnsted-Lowry Acid-Base reaction, an

    H+ is transferred does not have to take place in aqueous solution broader definition than Arrhenius

    acid is H donor, base is H acceptor base structure must contain an atom with an

    unshared pair of electrons in an acid-base reaction, the acid molecule

    gives an H+ to the base moleculeHA + :B :A + HB+

  • Tro, Chemistry: A Molecular Approach 17

    Brnsted-Lowry Acids Brnsted-Lowry acids are H+ donorsany material that has H can potentially be a

    Brnsted-Lowry acidbecause of the molecular structure, often one H in

    the molecule is easier to transfer than others

    HCl(aq) is acidic because HCl transfers an H+ to H2O, forming H3O+ ionswater acts as base, accepting H+

    HCl(aq) + H2O(l) Cl(aq) + H3O+(aq)acid base

  • Tro, Chemistry: A Molecular Approach 18

    Brnsted-Lowry Bases Brnsted-Lowry bases are H+ acceptorsany material that has atoms with lone pairs can

    potentially be a Brnsted-Lowry basebecause of the molecular structure, often one atom

    in the molecule is more willing to accept H+ transfer than others

    NH3(aq) is basic because NH3 accepts an H+from H2O, forming OH(aq)water acts as acid, donating H+

    NH3(aq) + H2O(l) NH4+(aq) + OH(aq)base acid

  • Tro, Chemistry: A Molecular Approach 19

    Amphoteric Substances

    amphoteric substances can act as either an acid or a basehave both transferable H and atom with lone pair

    water acts as base, accepting H+ from HClHCl(aq) + H2O(l) Cl(aq) + H3O+(aq)

    water acts as acid, donating H+ to NH3NH3(aq) + H2O(l) NH4+(aq) + OH(aq)

  • Tro, Chemistry: A Molecular Approach 20

    Brnsted-Lowry Acid-Base Reactions

    one of the advantages of Brnsted-Lowry theory is that it allows reactions to be reversible

    HA + :B :A + HB+ the original base has an extra H+ after the

    reaction so it will act as an acid in the reverse process

    and the original acid has a lone pair of electrons after the reaction so it will act as a base in the reverse process

    :A + HB+ HA + :B

  • Tro, Chemistry: A Molecular Approach 21

    Strong or Weak a strong acid is a strong electrolytepractically all the acid molecules ionize,

    a strong base is a strong electrolytepractically all the base molecules form OH ions,

    either through dissociation or reaction with water,

    a weak acid is a weak electrolyteonly a small percentage of the molecules ionize,

    a weak base is a weak electrolyteonly a small percentage of the base molecules form

    OH ions, either through dissociation or reaction with water,

  • Tro, Chemistry: A Molecular Approach 22

    Strong Acids The stronger the acid, the

    more willing it is to donate H use water as the standard base

    strong acids donate practically all their Hs 100% ionized in water strong electrolyte

    [H3O+] = [strong acid]

    HCl H+ + Cl-HCl + H2O H3O+ + Cl-

  • Tro, Chemistry: A Molecular Approach 23

    Weak Acids weak acids donate a small

    fraction of their Hsmost of the weak acid

    molecules do not donate H to water

    much less than 1% ionized in water

    [H3O+]

  • Tro, Chemistry: A Molecular Approach 24

    Polyprotic Acids often acid molecules have more than one ionizable H

    these are called polyprotic acids the ionizable Hs may have different acid strengths or be equal 1 H = monoprotic, 2 H = diprotic, 3 H = triprotic

    HCl = monoprotic, H2SO4 = diprotic, H3PO4 = triprotic polyprotic acids ionize in steps each ionizable H removed sequentially

    removing of the first H automatically makes removal of the second H harderH2SO4 is a stronger acid than HSO4

  • Tro, Chemistry: A Molecular Approach 25

    Strengths of Acids & Bases commonly, acid or base strength is measured by

    determining the equilibrium constant of a substances reaction with water

    HAcid + H2O Acid-1 + H3O+1Base: + H2O HBase+1 + OH-1

    the farther the equilibrium position lies to the products, the stronger the acid or base

    the position of equilibrium depends on the strength of attraction between the base form and the H+ stronger attraction means stronger base or weaker acid

  • Tro, Chemistry: A Molecular Approach 26

    Acid Ionization Constant, Ka acid strength measured by the size of the

    equilibrium constant when react with H2OHAcid + H2O Acid-1 + H3O+1

    the equilibrium constant is called the acid ionization constant, Kalarger Ka = stronger acid

    [HAcid]

    ]O[H][Acid 131

    a

    K

  • 27

  • Name Formula Ka1 Ka2 Ka3 Benzoic C6H5COOH 6.14 x 10

    -5

    Propanoic CH3CH2COOH 1.34 x 10-5

    Formic HCOOH 1.77 x 10-5

    Acetic CH3COOH 1.75 x 10-5

    Chloroacetic ClCH2COOH 1.36 x 10-5

    Trichloroacetic Cl3C-COOH 1.29 x 10-4

    Oxalic HOOC-COOH 5.90 x 10-2 6.40 x 10-5

    Nitric HNO3 strong

    Nitrous HNO2 4.6 x 10-4

    Phosphoric H3PO4 7.52 x 10-3 6.23 x 10-8 2.2 x 10-13

    Phosphorous H3PO3 1.00 x 10-2 2.6 x 10-7

    Arsenic H3AsO4 6.0 x 10-3 1.05 x 10-7 3.0 x 10-12

    Arsenious H3AsO3 6.0 x 10-10 3.0 x 10-14 very small

    Perchloric HClO4 > 108

    Chloric HClO3 5 x 102

    Chlorous HClO2 1.1 x 10-2

    Hypochlorous HClO 3.0 x 10-8

    Boric H3BO3 5.83 x 10-10

    Carbonic H2CO3 4.45 x 10-7 4.7 x 10-11

  • Tro, Chemistry: A Molecular Approach 29

    Autoionization of Water Water is actually an extremely weak electrolytetherefore there must be a few ions present

    about 1 out of every 10 million water molecules form ions through a process called autoionization

    H2O H+ + OHH2O + H2O H3O+ + OH

    all aqueous solutions contain both H3O+ and OHthe concentration of H3O+ and OH are equal in water[H3O+] = [OH] = 10-7 M a 25C

  • Tro, Chemistry: A Molecular Approach 30

    Ion Product of Water the product of the H3O+ and OH

    concentrations is always the same number the number is called the ion product of

    water and has the symbol Kw [H3O+] x [OH] = Kw = 1 x 10-14 at 25Cif you measure one of the concentrations, you

    can calculate the other

    as [H3O+] increases the [OH] must decrease so the product stays constantinversely proportional

  • Tro, Chemistry: A Molecular Approach 31

    Acidic and Basic Solutions

    all aqueous solutions contain both H3O+ and OH ions

    neutral solutions have equal [H3O+] and [OH][H3O+] = [OH] = 1 x 10-7

    acidic solutions have a larger [H3O+] than [OH][H3O+] > 1 x 10-7; [OH] < 1 x 10-7

    basic solutions have a larger [OH] than [H3O+][H3O+] < 1 x 10-7; [OH] > 1 x 10-7

  • Tro, Chemistry: A Molecular Approach 32

    pH the acidity/basicity of a solution is often

    expressed as pH pH = -log[H3O+], [H3O+] = 10-pHexponent on 10 with a positive signpHwater = -log[10-7] = 7need to know the [H+] concentration to find pH

    pH < 7 is acidic; pH > 7 is basic, pH = 7 is neutral

  • 33

    pH the lower the pH, the more acidic the solution; the

    higher the pH, the more basic the solution1 pH unit corresponds to a factor of 10 difference

    in acidity normal range 0 to 14pH 0 is [H+] = 1 M, pH 14 is [OH] = 1 MpH can be negative (very acidic) or larger than 14

    (very alkaline)

  • 34

    pH of Common SubstancesSubstance pH

    1.0 M HCl 0.0

    0.1 M HCl 1.0

    stomach acid 1.0 to 3.0

    lemons 2.2 to 2.4

    soft drinks 2.0 to 4.0

    plums 2.8 to 3.0

    apples 2.9 to 3.3

    cherries 3.2 to 4.0

    unpolluted rainwater 5.6

    human blood 7.3 to 7.4

    egg whites 7.6 to 8.0

    milk of magnesia (satd Mg(OH)2) 10.5

    household ammonia 10.5 to 11.5

    1.0 M NaOH 14

  • Tro, Chemistry: A Molecular Approach 35

    pOH another way of expressing the acidity/basicity of

    a solution is pOH pOH = -log[OH], [OH] = 10-pOHpOHwater = -log[10-7] = 7need to know the [OH] concentration to find pOH

    pOH < 7 is basic; pOH > 7 is acidic, pOH = 7 is neutral

  • Tro, Chemistry: A Molecular Approach 36

    Relationship between pH and pOH the sum of the pH and pOH of a solution = 14.00at 25Ccan use pOH to find pH of a solution

    14.00pOHpH

    00.14]-[OHlog]OH[log

    100.1log]-][OHOH[log

    100.1]-][OHOH[

    3

    143

    14w3

    K

  • Tro, Chemistry: A Molecular Approach 37

    NaOH Na+ + OH-

    Strong Bases

    the stronger the base, the more willing it is to accept Huse water as the standard acid

    for strong bases, practically all molecules are dissociated into OH or accept Hsstrong electrolytemulti-OH strong bases

    completely dissociated

    [HO] = [strong base] x (# OH)

  • Tro, Chemistry: A Molecular Approach 38

    Weak Bases in weak bases, only a small

    fraction of molecules accept Hsweak electrolytemost of the weak base molecules

    do not take H from watermuch less than 1% ionization in

    water [HO]

  • Tro, Chemistry: A Molecular Approach 39

  • Tro, Chemistry: A Molecular Approach 40

    Structure of Amines

  • Tro, Chemistry: A Molecular Approach 41

    Acid-Base Properties of Salts salts are water soluble ionic compounds salts that contain the cation of a strong base and an

    anion that is the conjugate base of a weak acid are basicNaHCO3 solutions are basic

    Na+ is the cation of the strong base NaOHHCO3 is the conjugate base of the weak acid H2CO3

    salts that contain cations that are the conjugate acid of a weak base and an anion of a strong acid are acidicNH4Cl solutions are acidic

    NH4+ is the conjugate acid of the weak base NH3Cl is the anion of the strong acid HCl

  • Tro, Chemistry: A Molecular Approach 42

    Metal Cations as Weak Acids cations of small, highly charged metals are weakly

    acidic alkali metal cations and alkali earth metal cations pH neutral cations are hydrated

    Al(H2O)63+(aq) + H2O(l) Al(H2O)5(OH)2+ (aq) + H3O+(aq)

  • Tro, Chemistry: A Molecular Approach 43

    Classifying Salt Solutions asAcidic, Basic, or Neutral

    if the salt cation is the counterion of a strong base and the anion is the conjugate base of a strong acid, it will form a neutral solutionNaCl Ca(NO3)2 KBr

    if the salt cation is the counterion of a strong base and the anion is the conjugate base of a weak acid, it will form a basic solutionNaF Ca(C2H3O2)2 KNO2

  • Tro, Chemistry: A Molecular Approach 44

    Classifying Salt Solutions asAcidic, Basic, or Neutral

    if the salt cation is the conjugate acid of a weak base and the anion is the conjugate base of a strong acid, it will form an acidic solutionNH4Cl

    if the salt cation is a highly charged metal ion and the anion is the conjugate base of a strong acid, it will form an acidic solutionAl(NO3)3

  • Tro, Chemistry: A Molecular Approach 45

    Classifying Salt Solutions asAcidic, Basic, or Neutral

    if the salt cation is the conjugate acid of a weak base and the anion is the conjugate base of a weak acid, the pH of the solution depends on the relative strengths of the acid and baseNH4F since HF is a stronger acid than NH4+, Ka of

    NH4+ is larger than Kb of the F; therefore the solution will be acidic

  • Tro, Chemistry: A Molecular Approach 46

    Polyprotic Acids since polyprotic acids ionize in steps, each H has a

    separate Ka Ka1 > Ka2 > Ka3 generally, the difference in Ka values is great enough so

    that the second ionization does not happen to a large enough extent to affect the pHmost pH problems just do first ionization except H2SO4 use [H2SO4] as the [H3O+] for the second

    ionization

    [A2-] = Ka2 as long as the second ionization is negligible

  • 47

  • Tro, Chemistry: A Molecular Approach 48

    What Is Acid Rain?

    natural rain water has a pH of 5.6naturally slightly acidic due mainly to CO2

    rain water with a pH lower than 5.6 is called acid rain

    acid rain is linked to damage in ecosystems and structures

  • Tro, Chemistry: A Molecular Approach 49

    What Causes Acid Rain? many natural and pollutant gases dissolved in the air

    are nonmetal oxidesCO2, SO2, NO2

    nonmetal oxides are acidicCO2 + H2O H2CO3

    2 SO2 + O2 + 2 H2O 2 H2SO4 processes that produce nonmetal oxide gases as waste

    increase the acidity of the rain natural volcanoes and some bacterial actionman-made combustion of fuel

    weather patterns may cause rain to be acidic in regions other than where the nonmetal oxide is produced