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Edward Wen, PhD
Acids & Bases
2
Learning Outcomes• Properties of acids and bases and definitions• pH scale and calculation of pH• Completing and balancing Neutralization
reactions• Titration calculations for neutralization
reactions• Defining Weak vs. Strong electrolytes (using
the concept of equilibrium)• Buffers – recognition of a buffer system, how a
buffer works
3
Types of Ionic Compounds• Acids = form H+ ions in water solution
• Bases = combine with H+ ions in water solution increases the OH- concentration
may either directly release OH- or pull H+ off H2O
• Salts = Ionic compounds formed from Acid and Base. all strong electrolytes Cation: except H+ Anion: except OH-
4
Properties of Acids• Sour taste• react with “active” Metals
i.e. Al, Zn, Fe, but NOT w/ Ag, Au
Zn + 2 HCl ZnCl2 + H2
• react with Carbonates, producing CO2
marble, baking soda, limestone
CaCO3 + 2 HCl CaCl2 + CO2 + H2O
• change color of vegetable dyesblue litmus turns red
• react with Bases to form ionic salts
5
Most food contains acids
• Citric acid (HO2CCH(CO2H)COHCO2H):
citrus fruits, tomato
• Malic acid (HO2CCH2CHOHCO2H):
green apple, tomato, grape
• Ascorbic acid (aka Vitamin C)
• Folic acid
6
Common AcidsChemical Name Formula Uses Strength
Nitric Acid HNO3 explosive, fertilizer, dye, glue Strong
Sulfuric Acid H2SO4 explosive, fertilizer, dye, glue,
batteries Strong
Hydrochloric Acid HCl metal cleaning, food prep, ore
refining, stomach acid Strong
Phosphoric Acid H3PO4 fertilizer, plastics & rubber,
food preservation Moderate
Acetic Acid HC2H3O2 plastics & rubber, food preservation, Vinegar
Weak
Hydrofluoric Acid HF metal cleaning, glass etching Weak
Carbonic Acid H2CO3 soda water Weak
Boric Acid H3BO3 eye wash Weak
7
Binary acids
• (HmX): acid hydrogens attached to a nonmetal atomHCl, HF, HBr, HIH2S, H2Se
Hydrofluoric acid
8
Oxyacids
• acid hydrogens (H+) attached to an oxygen atomH2SO4, HNO3, H3PO4
HClO4
9
Carboxylic acids • Many exist in food like vinegar,
tomato, citrus fruit• -COOH group
HC2H3O2, H3C6H5O3
• only the first H in the formula is acidic the H is on the COOH
10
Properties of Bases• also known as alkalis• taste bitter• solutions feel slippery• change color of vegetable dyes
different color than acid red litmus turns blue
• react with acids to form ionic saltsneutralization
11
Common BasesChemical
Name Formula
Common Name
Common Uses Strength
sodium hydroxide
NaOH lye,
caustic soda soap, plastic,
petrol refining Strong
potassium hydroxide
KOH caustic potash soap, cotton, electroplating
Strong
calcium hydroxide
Ca(OH)2 slaked lime cement Strong
Aluminum hydroxide
Al(OH)3 Antacid Weak
magnesium hydroxide
Mg(OH)2 milk of
magnesia antacid Weak
ammonium hydroxide
NH4OH, {NH3(aq)}
ammonia water
detergent, Windex fertilizer,
explosives, fibers Weak
12
Structure of Bases
• most ionic bases contain OH- ionsDrano clog-remover: NaOH, Ca(OH)2
• some contain CO32- ion: it produces OH- with water
Baking soda: CaCO3
Alka-Seltzer: NaHCO3
• molecular bases that react with H+
Windex: Ammonia (NH3)
13
Acid-Base Reactions (Neutralization, Double Displacement Reaction)
• H+ (from the acid) + OH- (from the base) H2O
it is often helpful to think of H2O as H-OH
• Cation (from base) + Anion (from acid) Salt
acid + base → salt + water
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
14
Acid Reactions. I.Reaction with Metals
• Reaction with many metals: Al, Zn, Fe, Mgbut not all!! Not for Cu, Au, Ag, etc.
• Producing a Salt and hydrogen gas H2
3 H2SO4(aq) + 2 Al(s) → Al2(SO4)3(aq) + 3 H2(g)
15
Acid Reactions. IIReaction with Metal Oxides
• when acids react with metal oxides, they produce a salt and water
3 H2SO4 + Al2O3 → Al2(SO4)3 + 3 H2O
16
Acid Reactions. IIIGas-evolving Reaction with Salts• when acids react with metal carbonate,
bicarbonate, sulfide, sulfite, and bisulfite, gas will be produced along with other products
2 HNO3 + FeCO3 → Fe(NO3)2 + CO2 + H2O
HCl + NaHCO3 → NaCl + CO2 + H2O
ZnS + 2 HBr → ZnBr2 + H2S
CaSO3 + 2 HI → CaI2 + SO2 + H2O
H2SO4 + 2 NH4HSO3 → (NH4)2SO4 + 2SO2 + 2H2O
17
Base Reactions• Neutralization of acids
• Reaction with Nonmetal oxides, CO2
2 NaOH + CO2 → Na2CO3 + H2O
• Strong bases will react with Al metal to form sodium aluminate and hydrogen gas
Example: Dissolving recycled aluminum can with NaOH solution
2 NaOH + 2 Al + 6 H2O → 2 NaAl(OH)4 + 3 H2
18
Relative Strength of Acids Acid Conjugate Base
Strong Acid Hydroiodic acid HI I-
Hydrobromic acid HBr Br-
Hydrochloric Acid HCl Cl-
Sulfuric Acid H2SO4 HSO4-
Nitric Acid HNO3 NO3-
Hydronium ion H3O+ H2O
Phosphoric Acid H3PO4 H2PO4-
Hydrofluoric Acid HF F-
Acetic Acid HC2H3O2 C2H3O2-
Carbonic Acid H2CO3 HCO3-
Ammonium ion NH4+ NH3
19
Strong Acids• The stronger the acid, the
more willing it is to donate H+
use water as the standard base
• Strong acids donate practically all their H+
100% ionized in water
• [H3O+] = [strong acid] [ ] = molarity
Stomach acid
HCl H+ + Cl-
20
Strong Acids
Examples:• Binary Acid: HCl, HBr, HI• Oxyacid: HNO3, H2SO4, HClO4, HClO3
• Example:
HNO3 = H+ + NO3-
H2SO4 = 2H+ + SO42-
21
Weak Acids
• Weak acids donate a small fraction of their H+
most of the weak acid molecules do not donate H+ to water
• [H3O+] << [weak acid]
Vinegar
HC2H3O2 H+ + C2H3O2-
22
Weak Acids
Examples:• Binary Acid: HF, H2S, H2Se• Oxyacid: HNO2, H2SO3, H3PO4, HClO• Most carboxylic acids, such as acetic
acid
23
Strong Bases• The stronger the base, the more
willing it is to accept H+
use water as the standard acid
• Strong bases: practically all molecules are dissociated into OH– or accept H+
1 mol NaOH = 1 mol OH-
1 mol Ca(OH)2 = 2 mol OH-
• [OH–] = [strong base] x (# OH)
DranoTM
NaOH Na+ + OH-
24
Weak Bases
• Definition: a small fraction of molecules accept H+
most of the weak base molecules do not take H+ from water
• [HO–] << [weak base]
WindexTM
NH3 + H2O NH4+ + OH-
25
Autoionization of Water• Water: extremely Weak electrolyte
therefore there must be a few ions present
• about 2 out of every 1 billion water molecules form Ions: Autoionization
H2O + H2O H3O+ + OH–
H2O H+ + OH–
• ALL aqueous solutions contain both H+ and OH–
the concentration of H+ and OH– are equal in water@ 25°C: [H+] = [OH–] = 10-7M
26
Ion Product of Water
• [H+] x [OH–] = constant: Ion Product of water, Kw
• At 25°C, [H+] x [OH–] = 1 x 10-14 = Kw
• as [H+] increases, [OH–] must decrease so the product stays constant
][OH
101][H
14
][H
101][OH
14
27
Acidic and Basic Solutions
• Neutral solutions have equal [H+] and [OH–][H+] = [OH–] = 1 x 10-7 M
• Acidic solutions : [H+] > [OH–][H+] > 1 x 10-7 M [OH–] < 1 x 10-7 M
• Basic solutions: [OH–] > [H+][H+] < 1 x 10-7 M [OH–] > 1 x 10-7 M
28
Practice - Determine the [H+] concentration and whether the solution is acidic, basic or
neutral for the following
All [H+] compared to 1 x 10-7 M
•[OH–] = 3.50 x 10-8 M
•[NaOH] = 0.000250 M
•[HCl] = 0.50 M
Practice - Determine the [H+] concentration and whether the solution is acidic, basic or
neutral for the following
• [OH–] = 3.50 x 10-8 M
• NaOH = 0.000250 M
• [HCl] = 0.50 M
[H+] = 1 x 10-14 3.50 x 10-8
= 2.86 x 10-7 M [H+] >[OH-], therefore acidic
[H+] = 1 x 10-14 0.000250
= 4.00 x 10-11 M [H+] < [OH-], therefore basic
[H+] > 1.0 x 10-7 M therefore acidic
[H+] = 0.50 M
30
Acidic/Basic: [H+] vs. [OH-]
OH-H+ H+ H+ H+ H+
OH-OH-OH-OH-
[OH-]10-14 10-13 10-11 10-9 10-7 10-5 10-3 10-1 100
[H+] 100 10-1 10-3 10-5 10-7 10-9 10-11 10-13 10-14
even though it may look like it, neither H+ of OH- will ever be 0the sizes of the H+ and OH- are not to scale
because the divisions are powers of 10 rather than units
Acid Base
31
pH• The measure of the acidity/basicity of a solution
• pH = -log[H+], [H+] = 10-pH
exponent on 10 with a positive signpHwater = -log[10-7] = 7
need to know the [H+] concentration to find pH
• pH < 7 : Acidic; pH > 7 : Basic• pH = 7 : Neutral
32
pH scale• pH↓, Acidity↑ • pH↑, basicity↑
1 pH unit corresponds to a factor of 10 difference in acidity
• normal range 0 to 14
pH 0 is [H+] = 1 M, pH 14 is [OH–] = 1 M
33
pH measurement
pH can be measured by pH meter:
• The change in [H+] affects the voltage of a standard cell
34
pH of Common SubstancesSubstance pH
1.0 M HCl 0.0
0.1 M HCl 1.0
stomach acid 1.0 to 3.0
lemons 2.2 to 2.4
soft drinks 2.0 to 4.0
plums 2.8 to 3.0
apples 2.9 to 3.3
cherries 3.2 to 4.0
unpolluted rainwater 5.6
human blood 7.3 to 7.4
egg whites 7.6 to 8.0
milk of magnesia (sat’d Mg(OH)2) 10.5
household ammonia 10.5 to 11.5
1.0 M NaOH 14
35
Example - Calculate the pH of the following strong acid or base solutions
• 0.0020 M HCl
• 0.010 M NaOH
36
Example - Calculate the pH of the following strong acid or base solutions
• 0.0020 M HClHCl as strong acid, so [H+] = 0.0020 M
pH = - log (2.0 x 10-3) = 2.7
pH = - log (1.0 x 10-12) = 12
[H+] = 1 x 10-14 1 x 10-2
= 1 x 10-12 M
NaOH as strong base, so [OH-] = 0.010 M
• 0.010 M NaOH
37
pH in everyday life
OH-H+ H+ H+ H+ H+
OH-OH-OH-OH-
[OH-]10-14 10-13 10-11 10-9 10-7 10-5 10-3 10-1 100
[H+] 100 10-1 10-3 10-5 10-7 10-9 10-11 10-13 10-14
pH 0 1 3 5 7 9 11 13 14Acid Base
Stomach acid Vinegar Pure water Windex Drano
38
Example - Calculate the concentration of [H+] for a solution with pH 3.7
[H+] = 10-pH
[H+] = 10-3.7
= 2 x 10-4 M = 0.0002 M
39
Find concentration of Acid or Base? Titration
• Purpose: using Reaction Stoichiometry to determine the Concentration of an unknown solution
• Titrant (solution of known concentration) added from a Buret
• Indicators: chemicals added to help determine when a reaction is complete
• the Endpoint of the titration occurs when the reaction is complete
40
Titration: Color change w/ Indicator
41
TitrationStart: The base solution as titrant in the buret.
Titrating: As the Base is added to the Acid, H+ + OH– HOH. But still excess Acid present so the color does not change.
Endpoint: just enough Base to neutralize all the acid. The indicator changes color.
Calculations in Titration• At the Endpoint of the titration, acid base
neutralization reaction is complete. The mole ratio between acid and base in the reaction mixture is the same as in the balanced equation.
• Given the concentration of titrant, the mole of titrant can be calculated as: mole = Molarity x Volume (L)
• Then the mole of the other reactant can be calculated from the mole of titrant and the mole ratio in the equation (review stoichiometry: mole-to-mole).
• Finally, the molarity of other reactant can be determined.
Example: Acid-Base Titration
44
Example:• The titration of 10.00 mL of HCl solution of unknown
concentration requires 12.51 mL of 0.100 M Ba(OH)2 solution to reach the endpoint. What is the concentration of the unknown HCl solution?
45
• First, write balanced equation:
2 HCl(aq) + Ba(OH)2(aq) → BaCl2 (aq) + 2H2O(l)
2 mole HCl = 1 mole Ba(OH)2
0.100 M Ba(OH)2 0.100 mol Ba(OH)2 1 L sol’n
Information
Given: 10.00 mL HCl
12.54 mL 0.100 M Ba(OH)2
Find: M HCl
Example:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.54 mL of 0.100 M Ba(OH)2 solution to reach the end point. What is the concentration of the unknown HCl solution?
solution liters
solute molesM olarity
46
• Write a Solution Map:
mLBa(OH)2
LBa(OH)2
molBa(OH)2
2
2
Ba O0.100 mol
1 L
H
Ba OHmL 1
L 0010.
molHCl
2
2 mol HCl
1 mol Ba(OH)
Information
Given: 10.00 mL HCl
12.51 mL Ba(OH)2
Find: M HCl
CF: 2 mol HCl = 1 mol Ba(OH)2
0.100 mol Ba(OH)2 = 1 L
M = mol/L
Example:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.51 mL of 0.100 M Ba(OH)2 solution to reach the end point. What is the concentration of the unknown HCl solution?
mLHCl
LHCl
mL 1
L 0010. HC l liters
HC l molesM olarity
47
= 2.50 x 10-3 mol HCl
InformationGiven: 10.00 mL HCl
12.51 mL Ba(OH)2
Find: M HCl
CF: 2 mol HCl = 1 mol Ba(OH)2
0.100 mol Ba(OH)2 = 1 L M = mol/L
SM: mL Ba(OH)2 → L Ba(OH)2 →
mol Ba(OH)2 → mol HCl; mL HCl → L HCl & mol M
Example:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.51 mL of 0.100 M Ba(OH)2 solution to reach the end point. What is the concentration of the unknown HCl solution?
48
InformationGiven: 10.00 mL HCl
12.51 mL Ba(OH)2
Find: M HCl
CF: 2 mol HCl = 1 mol Ba(OH)2
0.100 mol Ba(OH)2 = 1 L M = mol/L
SM: mL Ba(OH)2 → L Ba(OH)2 →
mol Ba(OH)2 → mol HCl; mL HCl → L HCl & mol M
Example:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.51 mL of 0.100 M Ba(OH)2 solution to reach the end point. What is the concentration of the unknown HCl solution?
-32.50 x 10 moles HClMolarity 0.250 M
0.01000 L HCl
H Cl L 01000.0mL 1
L 0 .001H Cl mL 0 .001
49
How does pH change?
Initial pH pH after adding 1 mL 1 M HCl
pH after adding 1 mL 1 M NaOH
1 L Pure water 7.00
4.00 10.00
1 L 0.14 M K2HPO4 + 0.10 M KH2PO4
7.00
6.99 7.01
50
Buffers• Definition: solutions that resist changing pH when
small amounts of acid or base are added• The mixture of 0.14 M K2HPO4 + 0.10 M KH2PO4
solution has much smaller pH change when strong acid or base is added, thus is called Buffer.
• Ingredient: mixing together a weak acid and its conjugate baseor weak base and it conjugate acid
Online demo: https://www.youtube.com/watch?v=P-R-Cqvb5yo
• Human body fluid as buffer: H2CO3/HCO3-
51
BufferComposition:• a weak acid + its salt;
example: HC2H3O2 / NaC2H3O2, HF/KFWhen acid is added:
C2H3O2- + H+ HC2H3O2
When base is added:
OH- + HC2H3O2 C2H3O2- + H2O
• OR, a weak base + its salt
example: NH3 / NH4Cl
52
Acetic Acid/Acetate Buffer
53
Treasure Hunt: Which two can combine into a
Buffer?HCl
NH4+
C2H3O2-
Cl-
HCO3-
CO32-
HC2H3O2
NH3
H2CO3