Acids and Bases

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Acids and Bases

• Chapter 14

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Classifying Acids

• Organic acids contain a carboxyl group or
  -COOH -- HC2H3O2 citric acid.
• Inorganic acids -- HCl, H2SO4, HNO3.
• Oxyacids -- acid proton attached to oxygen
• H3PO4. more oxygen the stronger the acid
• Monoprotic -- HCl HC2H3O2
• Diprotic -- H2SO4 Triprotic
  -- H3PO4

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Models of Acids and Bases

• Arrhenius Concept Acids produce H in solution,
  bases produce OH? ion.
• Brønsted-Lowry Acids are H donors, bases are
  proton acceptors.
• HCl H2O ? Cl? H3O
• acid base

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Bronsted-Lowry Model

• The Bronsted-Lowry Model is not limited to
  aqueous solutions like the Arrhenius Model.
• NH3(g) HCl(g) ----gt NH4Cl(s)
• This is an acid-base reaction according to
  Bronsted-Lowry, but not according to Arrhenius!

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Hydronium Ion

• Hydronium ion is a hydrated proton -- H.H2O.
• The H ion is simply a proton. It has a very
  high charge density, so it strongly is attracted
  to the very electronegative oxygen of the polar
  water molecule.

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Conjugate Acid/Base Pairs

• HA(aq) H2O(l) ? H3O(aq) A?(aq)
• conj conj conj
  conj
• acid 1 base 2 acid 2
  base 1
• conjugate base everything that remains of the
  acid molecule after a proton is lost.
• conjugate acid formed when the proton is
  transferred to the base.
• Which is the stronger base--H2O or A-?

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• HF(aq) OH-(aq) -gt F-(aq) H2O(l)
• HCl OH- ? Cl- H2O
• HCl NH3 ? Cl- NH4

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The relationship of acid strength and conjugate
base strength for acid-base reactions.
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Acid Strength
Strong Acid

• Its equilibrium position lies far to the right.
  (HNO3)
• Yields a weak conjugate base. (NO3?)

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Acid Strength(continued)
Weak Acid

• Its equilibrium lies far to the left. (CH3COOH)
• Yields a much stronger (water is relatively
  strong) conjugate base than water. (CH3COO?)

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Acid Dissociation Constant (Ka)

• HA(aq) H2O(l) ? H3O(aq) A?(aq)
• Ka values for common acids are found in Table
  14.2 on page 663.

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• HCOOH(aq) H20 (l) ? COOH- (aq) H30 (aq)
• HClO4(aq) H20 (l) ? ClO4- (aq) H3O(aq)

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A strong acid is nearly 100 ionized, while a
weak acid is only slightly ionized.
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Diagram a represents a strong acid, while b
represents a weak acid which remains mostly in
the molecular form.
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Water as an Acid and a Base

• Water is amphoteric (it can behave either as an
  acid or a base).
• H2O H2O ? H3O OH?
• 
  conj conj
• acid 1 base 2 acid 2 base 1

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Ion product Constant, Kw

• Kw is called the ion-product constant or
  dissociation constant.
• Kw 1 ? 10?14 M2 at 25C
• neutral solution H OH- 1.0 x 10 -7 M
• acidic solution H gt OH- H gt 1.0 x
  10-7 M
• basic solution H lt OH- OH- gt 1.0 x
  10-7 M
• No matter what the concentration of H or OH- in
  an aqueous solution, the product, Kw, will remain
  the same for that Temp.

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H OH- Calculations

• Calculate the H for a 1.0 x 10-5 M OH-.
• Kw HOH-
• H Kw/OH-
• H 1.0 x 10-14 M2/1.0 x 10-5 M
• H 1.0 x 10-9 M

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H OH- CalculationsContinued

• Calculate the OH- for a 10.0 M H.
• Kw HOH-
• OH- Kw/H
• OH- 1.0 x 10-14 M2/10.0 M
• OH- 1.0 x 10-15 M

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The pH Scale

• pH ?logH
• pH in water usually ranges from 0 to 14.
• Kw 1.00 ? 10?14 H OH?
• pKw 14.00 pH pOH
• As pH rises, pOH falls (sum 14.00).

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pH scale and pH values for common substances. A
pH of 1 is 100 times more acidic than a pH of 3.
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Logarithms

• -log 1.00 x 10-7 7.000
• 7.000
• characteristic mantissa
• The number of significant digits in 1.00 x 10-7
  is three, therefore, the log has three decimal
  places. The mantissa represents the log of 1.00
  and the characteristic represents the exponent 7.

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pH Significant Figures

• decimal places pH -------gt Significant
  Figure in H
• Significant Figures H -------gt
  decimal places pH
• pH - log H H 10(-pH)
• H 1.0 x 10-5 M pH 5.00

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pH Calculations

• What is the pOH, H, OH- for human blood
  with a pH of 7.41?
• pH pOH 14.00
• pOH 14.00 - pH
• pOH 14.00 - 7.41
• pOH 6.59

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pH CalculationsContinued

• What is the pOH, H, OH- for human blood
  with a pH of 7.41?
• pH - log H
• H antilog (-pH)
• H antilog (-7.41)
• H 3.9 x 10-8 M

Note The number of significant figures in the
antilog is equal to the number of decimal places
in the pH.
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pH CalculationsContinued

• What is the pOH, H, OH- for human blood
  with a pH of 7.41?
• pOH - log OH-
• OH- antilog (-pOH)
• OH- antilog (-6.59)
• OH- 2.6 x 10-7 M

Note The number of significant figures in the
antilog is equal to the number of decimal places
in the pOH.
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• Review Book 338-340
• 1-22

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pH of Strong Acid Solutions

• Calculate the pH of a 0.10 M HNO3 solution.
• Major species are H, NO3-,
• Sources of H are from HNO3 and H2O -- amount
  from water is insignificant.
• ?H 0.10 M pH - log H
• pH - log
  0.10
• pH 1.00

Note The number of significant figures in the
H is the same as the decimal places in the pH.
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• A solution is prepared by adding 15.8g of HCl to
  enough water to make a total volume of 400.ml
  What is the pH of the solution ?

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Solving Weak Acid Equilibrium Problems

• Write equilibrium expression for dominant
  equilibrium.
• Use an ice table .

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pH of Weak Acid Solutions

• Calculate the pH of a 0.100 M HOCl solution.
• Ka HOCl 3.5 x 10-8
• Major species HOCl and HOH
• ? HOCl will be only significant source of H.
• Ka 3.5 x 10-8 HOCl-/HOCl

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pH of Weak Acid SolutionsContinued

• ICE
• HOCl OCl-
  H
• Initial (mol/L) 0.100 0
  0
• Change (mol/L) - x x
  x
• Equil. (mol/L) 0.100 - x 0 x 0 x

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pH of Weak Acid SolutionsContinued

• Ka 3.5 x 10-8 HOCl-/HOCl
• 3.5 x 10-8 xx/0.100 - x
• Ka is more than 100 x smaller than concentration,
  x can be neglected in the denominator.
• Ka 3.5 x 10-8 xx/0.100
• x2 3.5 x 10-9
• x 5.9 x 10-5 M

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pH of Weak Acid SolutionsContinued

• Approximation check
• dissociation (x/HOClo) (100)
• dissociation (5.9 x 10-5/0.100)(100)
• dissociation 0.059
• This is much less than 5 and therefore the
  approximation was valid.

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Percent Dissociation (Ionization)
The percent dissociation calculation is exactly
the same as the one to check the 5
approximation.
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pH of a weak acid practice

• Calculate the Ph of a .500M aqueous solution of
  formic acid,HCOOH (Ka1.77x10-4)
• Rb324

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pH of a weak acid practice II

• Calculate the pH of a .200 M HCA Solution with a
  Ka7.45x10-4
• Rb324-325

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Bases

• Bases are often called alkalis because they often
  contain alkali or alkaline earth metals.
• Strong and weak are used in the same sense
  for bases as for acids.
• strong complete dissociation (hydroxide ion
  supplied to solution)
• NaOH(s) ? Na(aq) OH?(aq)

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Bases(continued)

• weak very little dissociation (or reaction with
  water)
• H3CNH2(aq) H2O(l) ? H3CNH3(aq) OH?(aq)
• Kb calculations are identical to Ka calculations.

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• Calculate the pH of a solution made by adding
  4.63 g of LiOH into water for a total volume of
  400 ml
• Calculate the pH of a .350M solution of CH3NH2
  (Kb4.38x10-4)

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• What is the pH of a 0.100 M solution of ammonia
  (NH3) (Kb 1.8x10-5)?

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Kw Ka x Kb
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• Calculate the pH of a .500M KF solution at 25dc
• Kb for 1.4.0x10-11
• text689

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• pH 8.31

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pH of a Salt

• Calculate the pH of a .500M NaNO2 solution at
  25dc
• (Ka for HNO2 4.0x10-4
• Rb334

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• 8.55

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Ph of a salt practice acid

• Calculate the pH of a .010M AlCl3 solution .
• Ka Value for Al(H2O6)3 is 1.4x10-5

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• Calculate the pH of a .010M NH4Cl solution .
• Kb Value for NH3 is 5.6x10-10

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• pH5.13

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Polyprotic Acids

• . . . can furnish more than one proton (H) to
  the solution.

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• Calculate the pH of a 5.0M H3PO4 solution and the
  equilibrium concentrations of H3PO4, H2PO4-,
  HPO4-2, PO3-3
• Ka1 7.5 x10-3
• Ka2 6.2 x10-8
• Ka3 4.8 x10-13 Rb 331-332