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Química inorgánica ácidos y bases
Citation preview
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