Acids taste sour

1
Exp 15 - Relative Strength of Some Acids Acids
and Bases
Acid-Base Indicator

• Acids taste sour
• They change litmus from blue to red
• Examples of acids
• Hydrochloric acid (HCl)
• Vinegar or acetic acid (CH3COOH)
• Orange juice
• Pepsi or Coke
• Bases feel slippery on the fingers
• They turn red litmus blue
• Examples of bases
• Sodium hydroxide, NaOH
• Ammonia (NH3, or NH4OH)
• Bleach (solution of sodium hypochlorite, NaClO)

Acidic Neutral Basic
2
Exp 15 - Relative Strength of Some Acids

• Arrhenius definition of acids and bases
• Acids contain hydrogen
• HCl, H2SO4, CH3COOH
• An acid is a species that produces H (protons)
  in water
• HCl(aq) H2O ? Cl-(aq) H3O(aq)
• Bases contain OH- in their formula
• KOH, NaOH
• A base is a species that produces OH- (hydroxyl-)
  ions when dissolved in water
• NaOH(s) H2O ? Na(aq) OH-(aq)

3
Exp 15 - Relative Strength of Some Acids

• Arrhenius definition of acids and bases
• Aqueous solutions of acids have an increased
  amount of hydronium ions (H3O) by donating
  protons (H) to H2O
• HCl(g) H2O(l) ? Cl-(aq) H3O(aq)
• H2SO4(l) H2O(l) ? HSO4-(aq) H3O(aq)
• HSO4-(aq) H2O(l) ? SO42-(aq) H3O(aq)
• Aqueous solutions of bases contain an increased
  amount of hydroxyl ions (OH-)
• NaOH(s) H2O ? Na(aq) OH-(aq)
• NH3(g) H2O(l) ? NH4(aq) OH-(aq)

4
Exp 15 - Relative Strength of Some Acids

• Strong acids (HCl, H2SO4)
• dissociate completely in water
• donate all their H to H2O to form H3O
• HCl(aq) H2O ? Cl-(aq) H3O(aq)
• They are completely ionized
• Weak acids (acetic acid, vinegar (CH3COOH))
• dissociate partly in water
• donate a small portion of H to H2O to form H3O
• HAc(aq) H2O Ac-(aq) H3O(aq)
• (acetic acid) (acetate ion)
• They are partially ionized
• Solution of 0.1 mol acetic acid in 1 liter of
  water
• 0.0004 mol H
• 0.0004 mol acetate ions
• 0.0996 mol acetic acid

5
Exp 15 - Relative Strength of Some Acids

• Strong bases (NaOH, KOH)
• dissociate completely in water
• release all their OH- in solution
• NaOH(s) ? Na(aq) OH-(aq)
• They are completely ionized
• Weak bases (NH3)
• dissociate partly in water
• remove a proton (H) from water, resulting in
  increase in OH- in aqueous solution
• NH3(aq) H2O NH4(aq) OH-(aq)
• They are partially ionized
• Solution of 0.1 mol ammonia in 1 liter of water
• 0.001 mol NH4
• 0.001 mol OH-
• 0.099 mol NH3

6
Exp 15 - Relative Strength of Acids and Bases

• Note
• Metal ions create acidic solutions
• Al3 and Zn2 are Bronsted/Lowry acids
• Al(H2O)63(aq) H2O(l) Al(H2O)5OH2(aq)
  H3O(aq)
• The hydrated Al3 ion donates a proton to water
  (Bronsted-Lowry)
• Al3 accepts an electron pair from O in H2O
  (Lewis acid)
• solution become slightly acidic
• AlCl3 is the salt of a weak base and a strong
  acid
• NH4 is a Bronsted/Lowry acid
• NH4(aq) H2O(l) NH3 H3O(aq)
• NH4 donates a proton to water
• solution becomes slightly acidic
• NH4NO3 is the salt of a weak base and a strong
  acid

7
Dissolving Metal Ions in Water The Acidic
Behavior of the Hydrated Al3 ion
8
Definition of An Acid
Arrhenius acid is a substance that produces H
(H3O) in water
A Brønsted acid is a proton donor
A Lewis acid is a substance that can accept a
pair of electrons
A Lewis base is a substance that can donate a
pair of electrons
H
acid
base
acid
base
9
Exp 15 - Relative Strength of Some Acids pH and
Acidity

• pH -log H3O
• In pure water at 25oC
• H3O 10-7 mol/L
• OH- 10-7 mol/L
• pH -log H3O
• pH 7? H3O OH-
• pH high ? H3O low, OH- high
• pH low ? H3O high, OH- low
• Stronger acid has lower pH
• Weaker acid has higher pH

10
Exp 15 - Relative Strength of Some Acids

• Purpose
• Determine the pH of a series of isomolar (same
  molarity) solutions, using pH paper, acid-base
  indicators and a pH meter
• Determine the relative strength of the acids
  based on the estimated pH
• Test the following 0.1 M solutions as described
• HCl - AlCl3
• H3PO4 - Zn(NO3)2
• CH3COOH - NH4NO3
• NaH2PO4 - vinegar
• tap water - distilled water

11
Exp 15 - Relative Strength of Some Acids
Experimental (I)

• Get 4 large test tubes
• Use a volumetric pipet to put 4.0 mL dH2O water
  in each tube
• Mark the meniscus with a Sharpie
• Pour out H2O and dry tube
• Determine pH using pH paper and pH indicators
• Get pH paper (only one kind available) and put
  them on a piece of paper towel (Use clean dry
  watch glass instead!!)
• Get 4 clean and dry test tubes for each solution
  to be tested!
• Put 4 mL of the solution to be tested in each of
  the 4 test tubes

12
Exp 15 - Relative Strength of Some Acids
Experimental (II)

• Determine pH using pH paper and pH indicators
  (contd)
• Place 1 drop of each of the solutions on a strip
  of the wide-range pH paper and record the
  approximate pH, as compared to the color
  indicators on the pH paper container
• Test the following 0.1 M solutions and tap water,
  distilled water
• HCl - Zn(NO3)2
• H3PO4 - NH4NO3
• HC3H3O2 - vinegar
• NaH2PO4
• Al(NO3)3

13
Exp 15 - Relative Strength of Some Acids
Experimental (II)

• Determine pH using pH paper and pH indicators
  (contd)
• Add 1-2 drops of a different indicator (closest
  to the pH paper reading) to each of the 10 test
  tubes that contains one of the following
  solutions
• Indicator Choices thymol blue, methyl orange,
  methyl red, and bromothymol blue
• Test the following 0.1 M solutions and tap water,
  distilled water
• HCl - Zn(NO3)2
• H3PO4 - NH4NO3
• HC2H3O2 - vinegar
• NaH2PO4 - tap water
• Al(NO3)3 -distilled water
• Estimate pH of the solutions by comparing the
  results with figure D.1 or the next 2 slides
• Estimate the pH as accurate as possible (up to
  0.1-0.5 pH units)
• Record your results
• Repeat for each of the solutions that you will
  test

14
Exp 15 - Relative Strength of Some Acids
Acid-Base Indicators
HInd H Ind-
15
Exp 15 - Relative Strength of Some Acids
Acid-Base Indicators
Indicator Color 1 Color 2 Color 3
Thymol Blue pHlt1.2 red pH 2.8 - 8.0 yellow pHgt9.6 blue
Methyl Orange pHlt 3.1 red pHgt 4.5 yellow
Methyl Red pHlt4.2 red pHgt6.3 yellow
Bromothymol Blue pHlt6.0 yellow pHgt7.6 blue
Phenolphthalein pHlt8.0 colorless pHgt pink/red
16
pH Indicators
Methyl Red
Thymol Blue
Methyl Orange
Bromothymol Blue
Phenolphthalein
17
Exp 15 - Relative Strength of Some Acids
Experimental (III)

• Measuring pH with a pH meter
• Calibrate pH meter with solutions of known pH
  according to the instructions (will be given by
  instructor)
• Measure and record the pH of every 0.1 M solution
  to the nearest 0.01 pH unit
• Record the data and write the data on the board

18
This Thursday

• Exp 15
• Post-lab
• Exp 16A Equilibria with Weak Acids and Weak
  Bases
• Prelab preparations
• Prelab exercises