Transcript Exp 15 Relative Strength of Acids and Bases

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
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Hydrochloric acid (HCl)
Vinegar or acetic acid (CH3COOH)
Orange juice
Pepsi or Coke
Acidic
• 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)
Neutral Basic
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)
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)
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
(acetic acid)
Ac-(aq) + H3O+(aq)
(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
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
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)
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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
Dissolving Metal Ions in Water
The Acidic Behavior of the Hydrated Al3+ ion
Nearby H2O acts as Bronsted-Lowry
Electron density drawn
base: proton acceptor
toward Al3+
H3O+
H2O
Al(H2O)6 acts as Bronsted-Lowry
acid: proton donor
Al(H2O)63+
Al(H2O)5OH2+
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
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+ OH••
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H+
acid
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H O H
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base
H
H
H+
+
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+
N H
H
acid
base
H
N H
H
8
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
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
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HCl
H3PO4
CH3COOH
NaH2PO4
tap water
- AlCl3
- Zn(NO3)2
- NH4NO3
- vinegar
- distilled water
Exp 15 - Relative Strength of Some Acids
Experimental (I)
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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
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2.
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!
3.
Put 4 mL of the solution to be tested in each of the 4 test tubes
Exp 15 - Relative Strength of Some Acids
Experimental (II)
Determine pH using pH paper and pH indicators (cont’d)
1.
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
2.
Test the following 0.1 M solutions and tap water, distilled water
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HCl
- Zn(NO3)2
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H3PO4
- NH4NO3
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HC3H3O2
- vinegar
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NaH2PO4
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Al(NO3)3
Exp 15 - Relative Strength of Some Acids
Experimental (II)
Determine pH using pH paper and pH indicators (cont’d)
1.
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
2.
Indicator Choices: thymol blue, methyl orange, methyl red, and bromothymol blue
3.
Test the following 0.1 M solutions and tap water, distilled water
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HCl
- Zn(NO3)2
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H3PO4
- NH4NO3
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HC2H3O2
- vinegar
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NaH2PO4
- tap water
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5.
Al(NO3)3
-distilled water
Estimate pH of the solutions by comparing the results with figure D.1 or
the next 2 slides
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Estimate the pH as accurate as possible (up to 0.1-0.5 pH units)
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Record your results
Repeat for each of the solutions that you will test
Exp 15 - Relative Strength of Some Acids
Acid-Base Indicators
pH
HInd
H+ + Ind-
Exp 15 - Relative Strength of Some Acids
Acid-Base Indicators
Indicator
Color 1
Color 2
Thymol Blue
pH<1.2: red
pH 2.8 - 8.0:
yellow
Methyl Orange
pH< 3.1: red
pH> 4.5: yellow
Methyl Red
pH<4.2: red
pH>6.3: yellow
Bromothymol Blue pH<6.0: yellow
pH>7.6: blue
Phenolphthalein pH<8.0: colorless pH>: pink/red
Color 3
pH>9.6: blue
pH Indicators
O
O
S
O
OH
O
N
HO
OH
N
N
Methyl Red
Thymol Blue
O
-
O
S
Na+
O
O
O
N
N
N
Methyl Orange
S
O
O
Br
O
HO
OH
Br
Bromothymol Blue
HO
OH
Phenolphthalein
Exp 15 - Relative Strength of Some Acids
Experimental (III)
Measuring pH with a pH meter
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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
This Thursday
Exp 15:
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Post-lab
Exp 16A: Equilibria with Weak Acids and Weak Bases
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Prelab preparations
Prelab exercises