Acid Strength: STRONG ACIDS

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Acid Strength STRONG ACIDS

• REVIEW Strong acids dissociate completely in
  water (strong electrolytes)
• HA(aq) H2O(l) lt-gt H3O(aq) A-(aq)
• Acid Base Conjugate A. C.B.
• Equilibrium lies far to the RIGHT (almost all of
  the HA dissociates at equilibrium)
• A STRONG acid has a WEAK conjugate base (a much
  weaker base than water)
• Water wins the competition for H ions

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ACID STRENGTH WEAK ACIDS

• HA(aq) H2O(l) lt-gt H3O(aq) A-(aq)
• Acid Base Conjugate A. C.B.
• Very little dissociation of HA
• Equilibrium lies far to the LEFT
• Conjugate base is a much stronger base than water
• Water loses the competition for H ions

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Common Strong Acids MEMORIZE!!

• Sulfuric Acid H2SO4(aq)
• Hydrochloric Acid HCl(aq)
• Nitric Acid HNO3(aq)
• Perchloric Acid HClO4(aq)
• Diprotic acids have two acidic protons (H2SO4)
• H2SO4(aq) -gt H(aq) HSO4-(aq) strong
• HSO4-(aq) lt-gt H(aq) SO42-(aq) weak
• Monoprotic acids have one acidic proton

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Acids

• Oxyacids acidic proton is attached to an oxygen
  atom (weak and strong)
• Organic Acids acids with a carbon atom backbone,
  commonly contain the carboxyl group (usually weak
  acids)

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Ka

• For monoprotic acids, Ka can be used to determine
  strength
• Larger Ka stronger acid equilibrium lies
  farther RIGHT
• BASE STRENGTH opposite of acid strengthlarger
  Ka stronger acid weaker conjugate base
• Water acts stronger than weak, but weaker than
  strong )

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Monoprotic Ka Values
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Water

• Amphoteric substances can act as either an acid
  or a base (ex water)
• H2O(l) H2O(l) -gt H3O OH-
• Equilibrium Kw H3OOH-
• Kw ion-product constant (dissociation constant
  for water)
• At 25C in pure water H3O OH- 1.0 X 10-7
  M, so Kw 1.0 X 10-14

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About Kw

• In any aqueous solution at 25C, no matter what
  it contains, HOH- must always equal 1.0 X
  10-14
• Neutral solutions, H OH-
• Acidic solutions, H gt OH-
• Basic solutions, OH- gt H
• NO MATTER WHAT, at 25C,
• Kw HOH- 1.0 X 10-14

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Examples

• Calculate H or OH- as required for each of
  the following solutions at 25C, and state
  whether the solution is neutral, acidic, or
  basic.
• 1.0 X 10-5 M OH-
• 1.0 X 10-7 M OH-
• 10.0 M H

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Temperature

• The equilibrium constant Kw varies with
  temperature
• If Kw increases with temperature, energy is a
  reactant (endothermic)
• I will not ask you questions about these

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More Complex Example

• At 60C, the value of Kw is 1 X 10-13
• Using Le Chateliers principle, predict whether
  the following reaction is exothermic or
  endothermic
• 2H2O(l) lt-gt H3O(aq) OH-(aq)
• Calculate H and OH- in a neutral solution at
  60C

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pH Scale

• Ranges from 0-14 and represents the small
  concentrations used in the Kw expression for
  OH- and H
• pH -logH
• pOH -logOH-
• pK -log K
• Log scale is based on 10, so the pH changes by 1
  for every power of 10 change in H
• ACIDS low pHs, BASES high pH, NEUTRAL pH 7

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Example

• Fill in the following table
• Significant figures The of decimal places in
  the log SF in the original

pH pOH H OH- Acid, base, neutral
Soln A 6.88
Soln B 8.4 X 10-14
Soln C 3.11
Soln D 1.0 X 10-7
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Polyprotic Acids

• Acids can break up into more than one proton (ex.
  H2SO4 diprotic or H3PO4 triprotic)
• Ka describes the first proton, Ka2 describes the
  second, etc.
• For a typical weak polyprotic acid, Ka1 gt Ka2 gt
  Ka3the acid will get successfully weaker
• For a typical polyprotic acid in water, only the
  first dissociation step is important in pH
  calculation

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Example

• Using table 14.4 in your book, calculate the pH
  of a 1.40 M H2C2O4 (oxalic acid) solution and the
  equilibrium concentrations of H2C2O4, HC2O4-,
  C2O42-, and OH-.
• ICE table necessary

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Example 2

• Using data from Table 14.4 in your textbook,
  calculate the pH, PO43-, and OH- in a 6.0 M
  phosphoric acid (H3PO4) solution.