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
mol H+
mol acetate ions
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
Nearby H2O acts as Bronsted-Lowry base: proton acceptor
Electron density drawn toward Al3+
Al(H2O)63+
Al(H2O)5OH2+
H3O+
H2O
Al(H2O)6 acts as Bronsted-Lowry acid: proton donor

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
+ OH- •
H O H • H+
acid
base
N H • H
N H H + H+ +
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 = 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 (cont’d)
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 (cont’d)
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 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 IndicatorspH
HInd H+ + Ind-

15. Exp 15 - Relative Strength of Some Acids Acid-Base Indicators
Color 1
Color 2
Color 3
Thymol Blue
pH<1.2: red
pH : yellow
pH>9.6: blue
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

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