Acids and Bases

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Acids and Bases
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Properties of Acids

• Sour taste
• React w/ metals to form H2
• Most contain hydrogen
• Are electrolytes
• Change color in the presence of indicators (turns
  litmus red)
• Has a pH lower than 7

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Two Types of Acids

• Strong acids
• Any acid that dissociates completely in aqueous
  soln
• Weak acids
• Any acid that partially dissociates in aqueous
  soln

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Properties of Bases

• Bitter taste
• Slippery feel
• Are electrolytes
• Change color in the presence of indicators (turns
  litmus blue)
• Has a pH higher than 7

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Types of Bases

• Strong Base
• Any base that dissociates completely in aqueous
  soln
• Weak Base
• Any base that partially dissociates in aqueous
  soln

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Neutralization

• Neutralization rxn a rxn of an acid and a base
  in aqueous soln to produce a salt and water
• Salt compound formed from the positive ion of a
  base and a negative ion of an acid
• Properties of the acid and base cancel each other

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Arrhenius Model of Acids and Bases

• Proposed the model in 1887
• Acid any compound that produces H ions in
  aqueous (water) soln
• Base any compound that produces OH-
  (hydroxide) ion in aqueous soln
• Offers an explanation of why acids and bases
  neutralize each other (H OH- H2O)

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Problems with Model

• Restricts acids and bases to water solns
  (similar reactions occur in the gas phase)
• Does not include certain compounds that have
  characteristics of bases (e.g., ammonia)

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Brønsted-Lowry Model of Acids and Bases

• Brønsted acid a hydrogen ion donor (H, or
  proton)
• Brønsted base a hydrogen ion acceptor
• Defines acids and bases independently of how they
  behave in water
• Amphiprotic having the property of behaving as
  an acid and a base
• Also called amphoteric, e.g., water

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Lewis Model

• More general definition than either Arrhenius or
  Bronsted-Lowry
• Includes substances that are not classified as
  acids or bases under the other definitions
• Acid - a substance that can accept a pair of
  electrons to form a covalent bond
• Base - a substance that can donate a pair of
  electrons to form a covalent bond

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Conjugate Acid-Base Pairs

• The rxn between Brønsted-Lowry acids and bases
  can proceed in the reverse direction (reversible
  reactions)
• HX (aq) H2O (l) ? H3O (aq) X- (aq)
• The water molecule becomes a hydronium ion
  (H3O), and is an acid because it has an extra H
  to donate
• The acid HX, after donating the H, becomes a
  base X-

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Conjugate Acids and Bases

• HX (aq) H2O (l) ? H3O (aq) X- (aq)

Base
Conjugate Acid
Acid
Conjugate Base
Forward reaction Acid and base Reverse
reaction Conjugate acid and conjugate base
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Hydronium Ions - H3O ion

• In water - H ions are strongly attracted to the
  electrons surrounding water molecules
• When one compound in a reaction acts as an acid
  (donate an H ion) the other acts as a base
  (accepts an H ion)

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Types of Acids

• Monoprotic acids acids that contain only 1
  hydrogen e.g., HCl
• Diprotic acids acids that contain 2 hydrogens
  e.g. H2CO3
• Triprotic acids acids that contain 3 hydrogens
  e.g. H3PO4

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Strengths of Acids and Bases

• Strong acid/base acids/bases that dissociate
  completely in water 
• Strong Acids - HCl, H2SO4, HBr , HNO3
• HI , HClO4
• Strong Bases LiOH, NaOH, Ca(OH)2
• KOH, Sr(OH)2 , RbOH, Ba(OH)2 , CsOH

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Strengths of Acids and Bases

• Weak acid/base acids/bases that dissociate only
  partly in water
• There is an inverse relationship between the
  strengths of conjugate acid-base pairs
• The stronger the acid, the weaker the conjugate
  base and vise versa
• The stronger the base, the weaker the conjugate
  acid and vise versa

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Dissociation Constants

• Acid dissociation constant (Ka) the
  equilibrium constant for the rxn of an aqueous
  weak acid and water
• Base dissociation constant (Kb) the
  equilibrium constant for the rxn of an aqueous
  weak base w/ water
• Both are derived from the ratio of the
  concentration of the products and reactants at
  equilibrium

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Acid Dissociation Constant

• Ka H3O A-
• HA
• Ka is a measure of the strength of an acid
• Ka values for weak acids are always less than one
• Used mostly w/ weak acids because the Ka values
  for strong acids approach infinity

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Ka Example

• Complete the reaction and write the equilibrium
  constant expression for the following reactions
• HCl (aq) H2O (l)
• HCO3- (aq) H2O (l)

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Ka Example

• A monoprotic weak acid has a concentration of
  0.092M. At equilibrium, the concentration of
  hydronium is 0.0024M. What is the Ka for this
  acid?

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Example

• Assume that enough lactic acid is dissolved in
  sour milk to give a solution concentration of
  0.100 M lactic acid. A pH meter shows that the
  pH of the sour milk is 2.43. Calculate Ka for
  the lactic acid equilibrium system.

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Base Dissociation Constant

• Kb HB OH-
• B
• Kb is a measure of the strength of a base
• Kb values for weak bases are always less than 1
• Kb values for strong bases approach infinity

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Kb Example

• Complete the reaction and write the equilibrium
  constant expression for the following reactions
• CH3NH2 (aq) H2O (l)
• NH4 (aq) H2O (l)

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Kb Example

• 0.23mol of a weak base is mixed with 1.5L of
  water. At equilibrium, the concentration of OH-
  is 0.0040M. What is the Kb for the base?

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Salt Hydrolysis Reactions

• Reactions of ions from salts to form H3O or OH-
  ions
• Can predict whether a salt hydrolysis rxn
  produces an acidic soln or basic soln
• Consider the acid and base the salt is formed
  from see next slide

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Salt Hydrolysis Reactions

• 4 possibilities
• strong acid strong base neutral salt
• strong acid weak base acidic salt
• weak acid strong base basic salt
• weak acid weak base acidic, basic, or neutral
  salt, depending on the relative strengths of the
  acids and bases which the salt is formed

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Identifying Acids

• Acids
• Acidic hydrogen hydrogens that can be donated
• Not every hydrogen is acidic - HC2H3O2
• Acidic hydrogens - have a slight positive charge
  while it is still a part of a molecule
• the acidic hydrogen is on the positive end of a
  polar covalent bond

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3 Categories of Acids

• binary acids contains hydrogen and 1 other
  element
• oxy acids contains hydrogen, oxygen, and 1
  other element (polyatomic ions)
• carboxylic acids organic acids acids that
  contain carbon atoms

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Identifying Bases

• Bases
• Bronsted-Lowry base always contains an unshared
  pair of electrons
• 2 categories of bases
• anions includes monatomic and polyatomic anions
• amines contains nitrogen atoms that has an
  unshared pair of electrons ammonia derivative

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• Ex. What is the concentration of hydroxide ions
  in blood, if the hydronium ion concentration is
  4.5 x 10-8? Is blood acidic, basic, or neutral?

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Water

• Water can dissociate into its component ions, H
  and OH-
• 2H2O (l) ? H3O (aq) OH- (aq)
• One water molecule acts as a weak acid, and the
  other acts as a weak base
• The ions are present in such small amounts they
  cant be detected by a conductivity apparatus
• In pure water, H3O 1.0 x 10 7 M and OH-
  1.0 x 10-7 M

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Dissociation Constant for Water

• It is defined as Kw the ion product constant for
  water
• Kw H3O OH-
• Kw (1.0 x 10-7)(1.0 x 10-7)
• Kw 1.0 x 10-14
• The value of Kw can always be used to find the
  concentration of either H3O or OH- given the
  concentration of the other

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pH and H3O

• pH number that is derived from the
  concentration of hydronium ions (H3O) in soln
• pH -log H3O
• As pH increases, H3O decreases
• Scale ranges from 0 14
• pH 7 is neutral
• pH lt 7 is acidic
• pH gt 7 is basic

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Measuring pH

• 2 ways to measure pH
• indicators
• pH meters
• both detect the presence of H3O ions
• indicators change color based on the H3O ions
• common indicators litmus paper, thymol blue,
  methyl orange, methyl red, bromthymol blue,
  phenolphthalein
•  

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pOH

• pOH - log OH-
• pH pOH 14.00
• Calculating ion concentrations from pH
• H antilog (-pH)
• OH- antilog (-pOH)

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Examples

• What is the pH of a 0.001 M soln of HCl, a
  strong acid?

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Examples

• What is the pH of a soln if H3O 3.4 x 10-5
  M?

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Examples

• The pH of a soln is measured with a pH meter and
  determined to be 9.00. What is the H3O?

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Examples

• The pH o f a soln is measured with a pH meter
  and determined to be 7.52. What is H3O?

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Buffers

• A mixture that is able to release or absorb H
  ion, keeping a solutions pH constant
• Can control the pH within very narrow limits
• Most common buffers are mixtures of weak acids
  and their conjugate bases

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Buffer Capacity

• The amount of acids or base that a buffer can
  neutralize
• All buffers have a limited capacity to neutralize
  H3O ions or OH- ions
• If you add H3O ions or OH- ions beyond the
  buffer capacity, the ions will remain in the
  solution, changing the pH
• The greater concentration of buffer in the
  solution, the greater the buffer capacity

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Titrations

• An analytical procedure used to determine the
  concentration of a sample by reacting it with a
  standard soln
• In a titration, an indicator is used to determine
  the end point
• Standard soln a soln of precisely known
  concentration
• Indicator any substance in soln that changes
  color as it reacts with either an acid or a base

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Titrations

• Each indicator changes its color over a
  particular range of pH values (transition
  interval)
• An unknown acid soln will be titrated with a
  standard soln that is a strong base
• An unknown base soln will be titrated with a
  standard soln that is a strong acid

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Titrations

• Equivalence point point at which the
  concentration of H3O ions is the same as the
  concentration of OH- ions H3O OH-
• Endpoint the point at which the indicator
  changes color
• Titration curve graph that shows how pH changes
  in a titration

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Titrations

• The equivalence point is at the center of the
  steep, vertical region of the titration curve
• At the equivalence point, pH increases greatly w/
  only a few drops

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Example Problem 1

• What is the molarity of a CsOH solution if 20.0
  mL of the solution is neutralized by 26.4 mL of
  0.250M HBr solution?
• HBr CsOH ? H2O CsBr

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Example Problem 2

• What is the molarity of a nitric acid solution if
  43.33 mL 0.200M KOH solution is needed to
  neutralize 20.00 mL of unknown solution?

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Example Problem 3

• What is the concentration of a household ammonia
  cleaning solution (NH4OH) if 49.90 mL of 0.5900M
  H2SO4 is required to neutralize 25.00 mL
  solution?