Acid Base Equilibria

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Acid Base Equilibria
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Properties of Acids(aq)

• Tastes sour
• Electrolytes
• Cause certain dyes (indicators) to change color
• pH lt 7
• Liberates H2(g) in rxns w/ certain metals forms
  a salt
• Arrhenius acid produces H in water

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Properties of Bases(aq)

• Tastes bitter
• Electrolyte
• Cause indicators to change color
• pH gt 7
• feels slippery
• Arrhenius base produces OH- in water

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BrØnsted-Lowry Acids Bases

• Acid-Base rxns involve the transfer of H
• H bonds w/ H2O to form H3O
• H and H3O are used interchangeably
• H is a proton
• Proton Transfer Rxns
• Acid a sub th can donate a proton to another
  sub
• Base a sub th can accept a proton to another
  sub must have a nonbonding e- pair
• Applies to rxns th do not occur in aqueous solns
• A B always work together
• Amphoteric a sub th can act as an A or B

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Conjugate Acid- Base Pairs

• Acid base equilibria involve proton transfer in
  both the forward reverse rxns
• HX(aq) H2O(l) ltgt X-(aq) H3O(aq)
• Forward rxn acid HX base H2O
• Reverse rxn acid H3O base
• Conjugate acid-base pair An A B th differ
  only in the presence or absence of a proton
• 2 sets per A-B rxn

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Relative Strengths of Acids Bases

• The stronger the acid, the weaker its conjugate
  base and vice versa.
• Strong acid- completely transfers protons to
  water 100 dissociation
• Weak acid- partially dissociates mixture of
  protons undissociated constituent ions
• Negligible acidity- a sub th contains H, but does
  not show acidic properties

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Autoionization of Water

• In pure water, to a very small degree, water can
  donate a proton to another water
• H2O H2O ltgt H3O OH-
• not long lasting ionization rapid rxns
• - Most remains as water molecules

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Ion Product of Water

• Kw H3OOH- 1.0 x 10-14
• Applicable to any aqueous soln
• Neutral H3O OH-
• Acidic H3O gt OH-
• Basic H3O lt OH-

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The pH Scale

• pH -logH
• pH 7 , neutral
• pH lt 7, acidic
• pH gt 7, basic
• Measuring pH
• pH Meters- electrodes capable of reading
  millivolts
• indicators- dyes th change color

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Strong Acids Bases

• Strong Acids
• Strong electrolytes 100 dissociated
• HCl, HBr, HI, HNO3, HClO3, HClO4, H2SO4
• Strong Bases
• hydroxides of alkali metals heavier alkaline
  earth metals NaOH, KOH, Ca(OH)2

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Weak Acids

• Only partially ionized
• HA(aq) H2O(l) ltgt H3O(aq) A-(aq) OR
• HA (aq) H2O (l) ltgt H (aq) A- (aq)
• Keq H3OA- OR HA-
• HA HA
• Ka - acid dissociation constant
• Magnitude indicates the tendency of the acid to
  ionize in water
• The larger the Ka value, the stronger the acid

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Calculating Ka from pH

• Use equilibrium concs to calc
• A sml Ka allows for simplification of the calc

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Using Ka to Calculate pH

• Use equilibrium concs to calc H then pH

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Polyprotic Acids

• Acids w/ more than 1 ionizable H atom
• H2SO3(aq) ltgt H(aq) HSO3- (aq)
• Ka1 1.7 x 10-2
• HSO3-(aq) ltgt H (aq) SO3 2-(aq)
• Ka2 6.4 x 10-8
• Easier to remove the 1st proton than the 2nd
• Ka usu differs by a factor of 103 can then
  determine the pH fr the Ka1 value only

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Weak Bases

• React w/ water, abstracting the H so form a
  conjugate acid of the base OH-
• B(aq) H2O(l) ltgt HB(aq) OH-(aq)
• NH3(aq) H2O(l) ltgt NH4(aq) OH- (aq)
• Kb NH4OH-
• NH3
• Kb always refers to a base th reacts w/ H2O to
  form a conjugate acid OH-
• Theses bases must contain 1 or more lone pairs
  to bond w/ H

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Relationship Between Ka Kb

• Ka x Kb Kw
• As the strength of an acid increases, the
  strength of the conjugate base must decrease

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Acid-Base Properties of Salt Solns

• Salt solns can be acidic or basic
• Assume 100 dissociation A B props are due to
  cations anions
• Hydrolysis when ions react w/ water generate
  H or OH-

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Acid-Base Properties of Salt Solns-cont

• An Anions Ability to React w/ Water
• An anion, X- , is a conjugate base of an acid
• fr a strong acid, it will be weak not affect
  pH
• Fr a weak acid, such as HC2H3O2, it will react
  to a sml extent w/ water, producing OH- raising
  the pH
• C2H3O2-(aq) H2O(l) ltgt HC2H3O2(aq) OH-(aq)

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Acid-Base Properties of Salt Solns-cont

• A Cations Ability to React w/ Water
• Polyatomic ions w/ 1 or more protons are the
  conjugate acids of weak bases, such as NH4(aq)
• NH4(aq) H2O(l) ltgt NH3 (aq) H3O(aq)
• This lowers the pH

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The Common Ion Effect

• The dissociation of a weak acid will decrease
  when a strong electrolyte of the same ion is
  added to it according to Le Chatliers principle
• HC2H3O2(aq) ltgt H(aq) C2H3O2- (aq)
• Adding NaC2H3O2 will shift the eq to the left

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Buffered Solutions

• Contain a weak conjugate acid-base pair
• Can resist drastic changes in pH upon the
  addition of strong acids or bases
• Human blood- buffered around pH 7.4
• Sea water- 8.1 to 8.3
• Contain both an acidic species to neutralize OH-
  a basic species to neutralize H
• Cant consume ea other
• Made of a weak acid or base and a salt of th acid
  or base

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Buffer Capacity pH

• Buffer capacity- the amt of A or B the buffer can
  neutralize before the pH begins to change
• Depends upon the amount of A B th the buffer is
  made of
• pH depends upon the Ka for the acid
• Henderson- Hasselbach Equation
• pH pKa log base
• acid

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Addition of Strong Acids or Bases to Buffers

• Assume complete consumption as long as buffering
  capacity is not exceeded
• To calc effect on pH
• Consider the neutralization rxn its effect on
  HX and X-.
• Use Ka the new concs of HX and X- fr step
  1to calc H use Henderson Hasselbach Eq

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Titration

• A soln containing a known conc of base is slowly
  added to an acid (or acid to base).
• Indicators signal the equivalence pt- where
  stoichiometrically equivalent quantities have
  been brought together
• pH titration curve -graph of the pH vs the vol of
  titrant added

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Strong Acid-Strong Base Titration

• Curve has 4 regions
• 1. Initial pH before any base is added.
• 2. Between initial equivalence pt as the
  base is added the pH slowly rises then rapidly
  rises as it nears the equivalence pt.
• 3. Equivalence pt equal moles of acid base
• 4. After equivalence pt pH is determined by
  excess base added

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Strong Acid-Strong Base Titration -cont

• Indicators should change color at the equivalence
  pt, but anywhere along the rapid-rise portion of
  the curve will be sufficient
• End pt the pt where the color change occurs
  (should be close to the equivalence pt)

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Weak Acid- Strong Base Titrations

• Similar shaped curve w/ 4 regions
• Initial pH
• Between initial equivalence pt to calc the pH,
  consider the neutralization rxn AND, the pH of
  the buffer pair produced fr the neutralization
• Equivalence pt equal moles of A B, the anion
  of the acid will be a weak base increase the pH
• After Eq Pt pH determined by excess base added

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Titrations of Polyprotic Acids

• Weak acids w/ more than 1 H, H3PO4, react in
  stages
• H3PO4 OH- -gt H2PO3- H2O
• H2PO3- H2O -gt HPO22- H2O
• Titration curve shows multiple equivalence pts