Properties of Acids

1
Properties of Acids

• sour taste
• react with active metals
• i.e., Al, Zn, Fe, but not Cu, Ag, or Au
• e.g. 2 Al 6 HCl 2 AlCl3 3 H2
• corrosive
• react with carbonates, producing CO2
• marble, baking soda, chalk, limestone
• CaCO3 2 HCl CaCl2 CO2 H2O
• change color of vegetable dyes
• blue litmus (from a lichen) turns red
• react with bases to form ionic salts

2
Structures of Acids

• binary acids have acid hydrogens attached to a
  nonmetal atom
• HCl, HF

• oxy acids have acid hydrogens attached to an
  oxygen atom
• H2SO4, HNO3

• carboxylic acids have COOH group
• HC2H3O2, H3C6H5O7
• only the first H in the formula is acidic
• the H is on the COOH

3
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 salts
• neutralization

4
Structure of Bases

• most ionic bases contain OH ions
• NaOH, Ca(OH)2
• some contain CO32 ions
• CaCO3, NaHCO3
• molecular bases contain structures that react
  with H
• mostly amine groups

Indicators

• chemicals which change color depending on the
  acidity/basicity
• many vegetable dyes are indicators
• anthocyanins
• litmus
• from Spanish moss
• red in acid, blue in base
• phenolphthalein
• found in laxatives
• red in base, colorless in acid

5
Arrhenius Theory

• bases dissociate in water to produce OH ions and
  cations
• ionic substances dissociate in water
• NaOH(aq) ? Na(aq) OH(aq)
• acids ionize in water to produce H ions and
  anions
• because 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)

6
Hydronium Ion

• the H ions produced by the acid are so reactive
  they cannot exist in water
• H 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

7
Arrhenius Acid-Base Reactions

• the H from the acid combines with the OH- from
  the base to make a molecule of H2O
• it 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)

Problems with Arrhenius Theory

• doesnt explain why molecular substances, like
  NH3, in water form basic solutions even though
  they dont have OH ions
• doesnt explain how some ionic compounds, like
  Na2CO3, in water form basic solutions even
  though they dont have OH ions
• doesnt explain why molecular substances, like
  CO2, in water form acidic solutions even though
  they dont have H ions
• doesnt explain acid-base reactions that take
  place outside aqueous solution

8
Brønsted-Lowry Theory

• in a Brønsted-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 molecule
• HA B D A HB

9
Brønsted-Lowry Acids

• Brønsted-Lowry acids are H donors
• any material that has H can potentially be a
  Brønsted-Lowry acid
• because 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 ions
• water acts as base, accepting H

HCl(aq) H2O(l) ? Cl(aq) H3O(aq) acid
base
10
Brønsted-Lowry Bases

• Brønsted-Lowry bases are H acceptors
• any material that has atoms with lone pairs can
  potentially be a Brønsted-Lowry base
• because 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) D NH4(aq) OH(aq) base
acid
11
Amphoteric Substances

• amphoteric substances can act as either an acid
  or a base
• have both transferable H and atom with lone pair
• water acts as base, accepting H from HCl
• HCl(aq) H2O(l) ? Cl(aq) H3O(aq)
• water acts as acid, donating H to NH3
• NH3(aq) H2O(l) D NH4(aq) OH(aq)

Brønsted-Lowry Acid-Base Reactions

• one of the advantages of Brønsted-Lowry theory is
  that it allows reactions to be reversible
• HA B D 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 D HA B

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Conjugate Pairs

• In a Brønsted-Lowry Acid-Base reaction, the
  original base becomes an acid in the reverse
  reaction, and the original acid becomes a base in
  the reverse process
• each reactant and the product it becomes is
  called a conjugate pair
• the original base becomes the conjugate acid and
  the original acid becomes the conjugate base

HA B D A HB
acid base conjugate conjugate base
acid
HCHO2 H2O D CHO2 H3O acid
base conjugate conjugate base acid
H2O NH3 D HO NH4 acid
base conjugate conjugate base
acid
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Example Identify the Brønsted-Lowry Acids and
Bases and Their Conjugates in the Reaction
H2SO4 H2O D HSO4 H3O
When the H2SO4 becomes HSO4?, it lost an H ? so
H2SO4 must be the acid and HSO4? its conjugate
base
When the H2O becomes H3O, it accepted an H ? so
H2O must be the base and H3O its conjugate acid
H2SO4 H2O D HSO4 H3O acid
base conjugate conjugate base
acid
14
Example Identify the Brønsted-Lowry Acids and
Bases and Their Conjugates in the Reaction
HCO3 H2O D H2CO3 HO
When the HCO3? becomes H2CO3, it accepted an H ?
so HCO3? must be the base and H2CO3 its conjugate
acid
When the H2O becomes OH?, it donated an H ? so
H2O must be the acid and OH? its conjugate base
HCO3 H2O D H2CO3 HO base
acid conjugate conjugate acid
base
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Practice Write the formula for the conjugate
acid of the following
H2O NH3 HCO32- H2PO41-
Practice Write the formula for the conjugate
base of the preceding
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Arrow Conventions

• chemists commonly use two kinds of arrows in
  reactions to indicate the degree of completion of
  the reactions
• a single arrow indicates all the reactant
  molecules are converted to product molecules at
  the end

• a double arrow indicates the reaction stops when
  only some of the reactant molecules have been
  converted into products

Strong or Weak

• a strong acid is a strong electrolyte
• practically all the acid molecules ionize, ?
• a strong base is a strong electrolyte
• practically all the base molecules form OH ions,
  either through dissociation or reaction with
  water, ?
• a weak acid is a weak electrolyte
• only a small percentage of the molecules ionize,
  D
• a weak base is a weak electrolyte
• only a small percentage of the base molecules
  form OH ions, either through dissociation or
  reaction with water, D

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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 harder
• H2SO4 is a stronger acid than HSO4

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Increasing Basicity
Increasing Acidity
19
Strengths of Acids Bases

• commonly, acid or base strength is measured by
  determining the equilibrium constant of a
  substances reaction with water
• HAcid H2O D Acid1 H3O1
• Base H2O D HBase1 OH1
• 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

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General Trends in Acidity

• the stronger an acid is at donating H, the weaker
  the conjugate base is at accepting H
• higher oxidation number stronger oxyacid
• H2SO4 gt H2SO3 HNO3 gt HNO2
• cation stronger acid than neutral molecule
  neutral stronger acid than anion
• H3O1 gt H2O gt OH1 NH41 gt NH3 gt NH21
• base trend opposite

Acid Ionization Constant, Ka

• acid strength measured by the size of the
  equilibrium constant when react with H2O
• HAcid H2O D Acid1 H3O1
• the equilibrium constant is called the acid
  ionization constant, Ka
• larger Ka stronger acid

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Autoionization of Water

• Water is actually an extremely weak electrolyte
• therefore there must be a few ions present
• about 1 out of every 10 million water molecules
  form ions through a process called autoionization
• H2O D H OH
• H2O H2O D H3O OH
• all aqueous solutions contain both H3O and OH
• the concentration of H3O and OH are equal in
  pure water
• H3O OH 107M _at_ 25C

23
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 OH Kw 1 x 10-14 _at_ 25C
• if you measure one of the concentrations, you can
  calculate the other
• as H3O increases the OH must decrease so
  the product stays constant (inversely
  proportional)

Acidic and Basic Solutions

• all aqueous solutions contain both H3O and OH
  ions
• neutral solutions have equal H3O and OH
• H3O OH 1 x 107
• acidic solutions have a larger H3O than OH
• H3O gt 1 x 107 OH lt 1 x 107
• basic solutions have a larger OH than H3O
• H3O lt 1 x 107 OH gt 1 x 107

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Example Calculate the OH at 25C when the
H3O 1.5 x 109 M, and determine if the
solution is acidic, basic, or neutral
H3O 1.5 x 109 M OH?
Given Find
Concept Plan Relationships
Solution
Check
The units are correct. The fact that the H3O
lt OH means the solution is basic
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pH

• the acidity/basicity of a solution is often
  expressed as pH
• pH logH3O, H3O 10pH
• pHwater log107 7
• need to know the H concentration to find pH
• pH lt 7 is acidic pH gt 7 is basic, pH 7 is
  neutral
• DO NOT REPORT MORE THAN 2 DECIMAL PLACES FOR
  pH!!!

• the lower the pH, the more acidic the solution
  the higher the pH, the more basic the solution
• 1 pH unit corresponds to a factor of 10
  difference in acidity
• normal range 0 to 14
• pH 0 is H 1 M, pH 14 is OH 1 M
• pH can be negative (very acidic) or larger than
  14 (very alkaline)

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pH of Common Substances
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pOH

• another way of expressing the acidity/basicity of
  a solution is pOH
• pOH logOH, OH 10pOH
• pOHwater log107 7
• need to know the OH concentration to find pOH
• pOH lt 7 is basic pOH gt 7 is acidic, pOH 7 is
  neutral

• the sum of the pH and pOH of a solution 14.00
• at 25C
• can use pOH to find pH of a solution

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pK

• a way of expressing the strength of an acid or
  base is pK
• pKa log(Ka), Ka 10pKa
• pKb log(Kb), Kb 10pKb
• the stronger the acid, the smaller the pKa
• larger Ka smaller pKa
• because it is the log

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Finding the pH of a Strong Acid

• there are two sources of H3O in an aqueous
  solution of a strong acid the acid and the
  water
• for the strong acid, the contribution of the
  water to the total hydronium ion concentration,
  H3O, is negligible
• shifts the Kw equilibrium to the left so far that
  H3Owater is too small to be significant
• except in very dilute solutions, generally lt 1 x
  104 M
• for a monoprotic strong acid H3O HAcid
• for polyprotic acids, the other ionizations can
  generally be ignored
• 0.10 M HCl has H3O 0.10 M and pH 1.00

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Finding the pH of a Weak Acid

• there are also two sources of H3O in and aqueous
  solution of a weak acid the acid and the water
• however, finding the H3O is complicated by the
  fact that the acid only undergoes partial
  ionization
• calculating the H3O requires solving an
  equilibrium problem for the reaction that defines
  the acidity of the acid
• HAcid H2O D Acid H3O

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Example - Find the pH of 0.200 M HNO2(aq)
solution _at_ 25C
Ka for HNO2 4.6 x 10-4
HNO2 H2O D NO2 H3O
Practice Find the pH of a 0.012 M nicotinic
acid solution, HC6H4NO2 _at_ 25C (Ka 1.4 x 105).
Practice Find the pH of 0.100 M HClO2(aq)
solution _at_ 25C (Ka for HClO2 1.1 x 102).
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Example - What is the Ka of a weak acid if a
0.100 M solution has a pH of 4.25?
HA H2O D A H3O
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Percent Ionization

• another way to measure the strength of an acid is
  to determine the percentage of acid molecules
  that ionize when dissolved in water this is
  called the percent ionization
• the higher the percent ionization, the stronger
  the acid

• since ionized acidequil H3Oequil

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Relationship Between H3Oequilibrium
HAinitial

• the reaction for ionization of a weak acid is
• HA(aq) H2O(l) D A-(aq) H3O(aq)
• according to Le Châteliers Principle, if we
  reduce the concentrations of all the (aq)
  components, the equilibrium should shift to the
  right to increase the total number of dissolved
  particles
• we can reduce the (aq) concentrations by using a
  more dilute initial acid concentration
• the result will be a larger H3O in the dilute
  solution compared to the initial acid
  concentration
• this will result in a larger percent ionization

• increasing the initial concentration of acid
  results in increased H3O concentration at
  equilibrium
• increasing the initial concentration of acid
  results in decreased percent ionization

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Strong Bases

• the stronger the base, the more willing it is to
  accept H
• use water as the standard acid
• for strong bases, practically all molecules are
  dissociated into OH or accept Hs
• strong electrolyte
• multi-OH strong bases completely dissociated
• OH strong base x ( OH)

Weak Bases

• in weak bases, only a small fraction of molecules
  accept Hs
• weak electrolyte
• most of the weak base molecules do not take H
  from water
• much less than 1 ionization in water
• OH ltlt weak base
• finding the pH of a weak base solution is similar
  to finding the pH of a weak acid

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Practice - Calculate the pH of a 0.0010 M Ba(OH)2
solution.
Practice Find the pH of a 0.0015 M morphine
solution, Kb 1.6 x 10-6
38
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 basic
• NaHCO3 solutions are basic
• Na is the cation of the strong base NaOH
• HCO3- 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 acidic
• NH4Cl solutions are acidic
• NH4 is the conjugate acid of the weak base NH3
• Cl- is the anion of the strong acid HCl

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Anions as Weak Bases

• every anion can be thought of as the conjugate
  base of an acid
• therefore, every anion can potentially be a base
• A-(aq) H2O(l) D HA(aq) OH-(aq)
• the stronger the acid is, the weaker the
  conjugate base is
• an anion that is the conjugate base of a strong
  acid is pH neutral
• Cl-(aq) H2O(l) ? HCl(aq) OH-(aq)
• since HCl is a strong acid, this equilibrium lies
  practically completely to the left
• an anion that is the conjugate base of a weak
  acid is basic
• F-(aq) H2O(l) D HF(aq) OH-(aq)
• since HF is a weak acid, the position of this
  equilibrium favors the right

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Example - Use the table below to determine if the
given anion is basic or neutral

• NO3-
• the conjugate base of a strong acid, therefore
  neutral
• NO2-
• the conjugate base of a weak acid, therefore
  basic

41
Relationship between Ka of an Acid and Kb of Its
Conjugate Base

• many reference books only give tables of Ka
  values because Kb values can be found from them

when you add equations, you multiply the Ks
42
Polyatomic Cations as Weak Acids

• some cations can be thought of as the conjugate
  acid of a base
• others are the counterions of a strong base
• therefore, some cation can potentially be an acid
• MH(aq) H2O(l) D M(aq) H3O(aq)
• the stronger the base is, the weaker the
  conjugate acid is
• a cation that is the counterion of a strong base
  is pH neutral
• a cation that is the conjugate acid of a weak
  base is acidic
• NH4(aq) H2O(l) D NH3(aq) H3O(aq)
• since NH3 is a weak base, the position of this
  equilibrium favors the right

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) D Al(H2O)5(OH)2 (aq)
  H3O(aq)

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Example - Determine if the Given Cation Is Acidic
or Neutral

• C5N5NH2
• the conjugate acid of a weak base, therefore
  acidic
• Ca2
• the counterion of a strong base, therefore
  neutral
• Cr3
• a highly charged metal ion, therefore acidic

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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 solution
• NaCl 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 solution
• NaF Ca(C2H3O2)2 KNO2

• 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 solution
• NH4Cl
• 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 solution
• Al(NO3)3

• 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 base

45
Example - Determine whether a solution of the
following salts is acidic, basic, or neutral

• SrCl2
• Sr2 is the counterion of a strong base, pH
  neutral
• Cl- is the conjugate base of a strong acid, pH
  neutral
• AlBr3
• Al3 is a small, highly charged metal ion, weak
  acid
• Cl- is the conjugate base of a strong acid, pH
  neutral
• CH3NH3NO3
• CH3NH3 is the conjugate acid of a weak base,
  acidic
• NO3- is the conjugate base of a strong acid, pH
  neutral
• NaCHO2
• Na is the counterion of a strong base, pH
  neutral
• CHO2- is the conjugate base of a weak acid,
  basic
• NH4F
• NH4 is the conjugate acid of a weak base,
  acidic
• F- is the conjugate base of a weak acid, basic
• Ka(NH4) gt Kb(F-) solution will be acidic

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Strengths of Binary Acids

• the more d H-X d- polarized the bond, the more
  acidic the bond
• the stronger the H-X bond, the weaker the acid
• binary acid strength increases to the right
  across a period
• H-C lt H-N lt H-O lt H-F
• binary acid strength increases down the column
• H-F lt H-Cl lt H-Br lt H-I

Strengths of Oxyacids, H-O-Y

• the more electronegative the Y atom, the stronger
  the acid
• helps weakens the H-O bond
• the more oxygens attached to Y, the stronger the
  acid
• weakens and polarizes the H-O bond

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Lewis Acid - Base Theory

• electron sharing
• electron donor Lewis Base nucleophile
• must have a lone pair of electrons
• electron acceptor Lewis Acid electrophile
• electron deficient
• when Lewis Base gives electrons from lone pair to
  Lewis Acid, a covalent bond forms between the
  molecules
• Nucleophile Electrophile D NucleophileElectrop
  hile
• product called an adduct
• other acid-base reactions also Lewis

48
Practice - Complete the Following Lewis
Acid-Base ReactionsLabel the Nucleophile and
Electrophile

• BF3 HF D
• CaO SO3 D
• KI I2 D