Design of laboratory activities to enhance learning of acid base equilibria

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Design of laboratory activities to enhance
learning of acid base equilibria

• Russell Kohnken
• Evanston Township High School, Evanston, IL
  60204.
• Email kohnkenr_at_eths.k12.il.us
• Documents may be downloaded from
  http//facweb.eths.k12.il.us/kohnkenr/index.htm

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Abstract

• Acid base equilibrium is one of the more
  challenging units conceptually for AP students.
  I utilize a series of laboratory activities that
  are designed to assist both learning of
  laboratory skills common to acid base chemistry
  and a deeper understanding of the reactions and
  mathematics of acid base equilibrium problems.
  An initial minilab involves learning to calibrate
  and use pH meters. A second minilab examines the
  effects of pH on the appearance of four
  indicators. Recognition of their application
  will prove useful in a later activity. In a full
  laboratory experiment, students prepare a buffer
  solution of a pH of their choosing. In this lab,
  they both mathematically predict and
  experimentally determine the effect on pH of acid
  and base addition to the buffer. Another minilab
  involves a challenge to mix HCl and NaOH in equal
  amounts to attain a pH 7, which introduces the
  dramatic change of pH around the equivalence
  point in a titration. Finally, students perform
  a titration experiment to determine the purity of
  a known acid. Development of possible lab
  practicals is also shown. The progression of
  these labs allows students to manipulate
  reactions to explore all the types of acid base
  equilibria both mathematically and physically.

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Care, use and calibration of pH meters

• Students calibrate pH meters, both digital
  hand-held and analog, using pH 4, 7, and 10
  standard solutions
• Students measure pH of 3 out of 6 total prepared
  buffer solutions
• Students measure pH of distilled water

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Class data for pH measurementspH for water
5.91 /- 1.00
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Indicator solutions

• Students use 24 well plates to observe appearance
  of indicator solutions at different pH
• Use bromphenol blue, phenolphthalein, phenol red,
  thymol blue
• Use pH 2, 4, 6, 8, 10, and 12
• Students learn value and appearance of indicator
  solutions

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Indicator solution data
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pH challenge

• Students mix 50 ml 1M HCl and 50 ml 1M NaOH and
  measure pH
• Any volume measuring device can be used
• Introduces the idea that pH can change
  dramatically, particularly around the equivalence
  point of a titration
• Teacher must titrate these two solutions as
  accurately as possible the day before

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pH challenge data
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Students prepare a buffer

• Given many choices, students prepare a buffer,
  using an appropriate acid/base pair, to the pH
  that they choose (discuss issues of pH extremes
  and polyprotic acids)
• Students add amounts of NaOH and HCl that they
  choose to aliquots of their buffer to observe
  buffer action
• Students compare lab results with calculated
  outcomes
• Same format can be used for a lab practical
  except teacher chooses pH of buffer and amounts
  of NaOH and HCl to add

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Acids available for buffer preparation

• Name Formula of acid Ka
• Phosphoric acid H3PO4 7.1 x 10-3
• H2PO4- 1.5 x 10-7
• HPO42- 4.2 x 10-13
• Carbonic acid H2CO3 4.5 x 10-7
• HCO3- 4.7 x 10-11
• Formic acid HCOOH 1.8 x 10-4
• Acetic acid CH3COOH 1.8 x 10-5
• Oxalic acid (COOH)2 5.4
  x 10-2
• HOOCCOO- 5.4 x 10-5
• Iron (III) chloride Fe(H2O)63 5.5 x 10-3
• Hydrogen sulfate HSO4- 1.3 x 10-2
• Ammonium chloride NH4 5.6 x 10-10
• Citric acid C6H8O7 7.4 x 10-4
• C6H7O7- 1.7 x 10-5
• C6H6O72- 4.0 x 10-7
• Succinic acid C4H6O4 6.2 x 10-5
• C4H5O4- 2.3 x 10-6
• Tartaric acid C4H6O6 1.0 x 10-3

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Titration of glycolic acid

• Students first calibrate NaOH using potassium
  hydrogen phthalate standard
• Students titrate glycolic acid to determine
  precise concentration
• Students use both pH meter and indicator solution
  to monitor titration
• Students determine Ka from titration curve and
  compare to known value

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Ka determination lab practical

• Using an unknown indicator solution, students
  are to determine its Ka
• Challenges them to use techniques learned
• However, the low concentration of the indicator
  introduces some difficulties
• Multiple possible procedures with varying degrees
  of efficacy
• Thus, this practical is more about process than
  correct outcome

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Summary

• Students experience and practice acid/base
  laboratory skills
• Students apply calculations to the design of lab
  procedures and the determination of acid/base
  equilibria
• Students get to do in the lab what they learn in
  the classroom