Chapter 11 Chemical Equilibrium

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Chapter 11 Chemical Equilibrium
2

• Homework Assignment, Ch 8 (buffers)
• Problems 5,6,9,11,12,13,18,19,20
• Due Fri, Nov 1

3

• Exam 2 on Wed, Nov 6
• Covers Chapters 5, 6, 7, 8 plus
• Gravimetric and Volumetric Chloride
• Determinations

4

• Homework Assignment, Ch 12 (EDTA)
• Problems 1,8,10,12,17,23
• Due Fri, Nov 8

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Chapter 11 Chemical Equilibrium

• Our understanding of the phenomena shown on the
  proceeding slide is that the inert salt increases
  the ionic atmosphere (environment), allowing each
  cation or anion to be surrounded by species of
  the opposite charge, but farther separated from
  the counter ion which caused its original
  precipitation.

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Chapter 11 Chemical Equilibrium

• This effect of the ionic environment within the
  solution is known as the ionic strength and may
  be represented as ? (your author) or I (other
  authors).
• ? I ½ (c1z12 c2z22 ) ½ ? cizi2
• The sum of terms includes all of the ions in
  solution. An example of this calculation is
  shown in Problem 1

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What is the ionic strength of a solution that is
0.0100 M in KNO3 and 0.0100 M Na2SO4?

• ½ ? cizi2
• ½ 0.01(1)2 0.01(-1)2 0.02(1)2
  0.01(-2)2
• ½ 0.08 0.04M

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Activity Coefficients

• The actual or effective concentration of an
  ionic species in solution is known as the
  activity your author uses the symbol A (more
  commonly used is simply a lower case a)
• I will use the later symbol, so that his
  equation 11-2 is written as
• aC C ?C

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Activity Coefficients

• The exact equilibrium constant K is then
  expressed in terms of the activities of the
  species involved instead of the more commonly
  concentrations.
• For the reaction aA bB lt gt cC dD
• K (ac)c(aD)d / (aA)a(aB)b
• or K (C ?C)c(D ?D)d / (A ?A)a(B ?B)b

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Activity Coefficients

• The individual values for the activity
  coefficients ? of each of the species is a
  function of the ionic strength as shown by the
  extended Debye-Huckel equation
• log 10 ? -0.51z2? ? / 1 (??? /(305))
  (Eq11-5)
• where ? is the size of the ion in pm
  (picometers). Examples of ? for the F- and I-
  ions are shown in the next slide.

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Activity Coefficients

• A more general form of the extended Debye-Huckel
  equation is
• log 10 ? -0.509z2? ? / (1 ? ?)
• Although this equation is less exact, one does
  not need all of the parameters of the previous
  form.
• From Hargis, Analytical Chemistry Principles
  and Techniques, p 19, Prentice-Hall, 1988.

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Activity Coefficients

• The effects of ionic strength on activity for
  various charges of ions are shown in the next
  slide.

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Figure 11-4 ? as a function of ? for different
values of z
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What is the activity coefficent of Mg2 in a 3.3
mM solution of Mg(NO3)2, using both the authors
equation 11-5 and the given simplified forms?

• In both, we first need the ionic strength ?
• ? ½ (3.3mM)(2)2 (6.6mM)(-1)2
• ? ½ 13.2 6.6 ½19.8 9.9 mM
• ? 0.0099 M

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What is the activity coefficent of Mg2 in a 3.3
mM solution of Mg(NO3)2, using both the authors
equation 11-5 and the given simplified forms?

• Using the authors eqn 11-5,
• log ? (-0.51)(2)2(0.0099)1/2 / 1
  (800)(0.0099)1/2/305
• log ? - 0.203/1.261 -0.161
• ? 10 -0.161 0.690

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What is the activity coefficent of Mg2 in a 3.3
mM solution of Mg(NO3)2, using both the authors
equation 11-5 and the given simplified forms?

• Using the simplified equation
• log ? (-0.509)(2)2(0.0099)1/2 / 1
  (0.0099)1/2
• log ? - 0.203/1.099 -0.185
• ? 10-0.185 0.654

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What is the activity coefficent of Mg2 in a 3.3
mM solution of Mg(NO3)2, using both the authors
equation 11-5 and the given simplified forms?

• In summary, the more exact equation 11-5 gives ?
  0.690 while the the simplified equation gives a
  value of ? 0.654. In most cases the difference
  between the 2 values would not be important.

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Activity Coefficients

• - Ignore activity coefficients for nonionic
  compounds.

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Activity Coefficients

• Whenever the ionic strength is high (? gt 1 M)
  the activity coefficient values we could
  calculate are not very meaningful because of the
  lower concentration of the solvent and the
  concentrations of the solutes increase.

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Figure 11-5 ? of H in 0.010 M HClO4 as a
function of NaClO4
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pH

• The response of the glass electrode for
  measurement of pH is dependent on the ionic
  strength of the solution. The technical
  definition of pH is
• pH - log 10 (aH) log 10 ?H H
• Also, be aware that most glass electrodes show
  significant errors at pH gt 12 because of the high
  concentrations of the counter ions such as Na
  and K.