15-1 CHEM 102 Summer 2015, LA TECH Instructor: Dr. Upali Siriwardane e-mail: [email protected]...
-
Upload
malcolm-barrett -
Category
Documents
-
view
219 -
download
1
Transcript of 15-1 CHEM 102 Summer 2015, LA TECH Instructor: Dr. Upali Siriwardane e-mail: [email protected]...
15-1CHEM 102 Summer 2015, LA TECH
Instructor: Dr. Upali Siriwardane
e-mail: [email protected]
Office: CTH 311
Phone 257-4941
Office Hours: M,Tu, W,Th,F 9:00-11:00 am or by appointment..;
Test Dates:
Chemistry 102(060) Summer 2015
July 20, 2015 (Test 1): Chapter 13
July 27, 2015 (Test 2): Chapter 14
August 4, 2015 (Test 3): Chapter 15
August 12, 2015 (Test 4): Chapter 17
August 13, 2015 (Make-up test) comprehensive:
Chapters 13-17
15-2CHEM 102 Summer 2015, LA TECH
Chapter 15. Acids and Bases
15.1 Heartburn 659
15.2 The Nature of Acids and Bases 660
15.3 Definitions of Acids and Bases 662
15.4 Acid Strength and the Acid Dissociation Constant (Ka) 665
15.5 Autoionization of Water and pH 668
15.6 Finding the and pH of Strong and Weak Acid Solutions 673
15.7 Base Solutions 682
15.8 The Acid–Base Properties of Ions and Salts 685
15.9 Polyprotic Acids 693
15.1 0 Acid Strength and Molecular Structure 698
15.1 1 Lewis Acids and Bases 700
15.1 2 Acid Rain 701
15-3CHEM 102 Summer 2015, LA TECH
Types of Reactionsa) Precipitation Reactions. Reactions of ionic compounds or
saltsb) Acid/base Reactions. Reactions of acids and basesc) Redox Reactions. reactions of oxidizing & reducing
agents
15-4CHEM 102 Summer 2015, LA TECH
What are Acids &Bases?
Definition?
a) Arrhenius
b) Bronsted-Lowry
c) Lewis
15-5CHEM 102 Summer 2015, LA TECH
Arrhenius, Svante August (1859-1927), Swedish chemist, 1903 Nobel Prize in chemistry
•Acid Anything that produces hydrogen ions in a water solution.
HCl (aq) H+
( aq) + Cl-
( aq)
•Base Anything that producs hydroxide ions in a water solution.
NaOH (aq) Na+
( aq) + OH
- ( aq)
•Arrhenius definitions are limited proton acids and hydroxide bases
to aqueous solutions.
Arrhenius Definitions
15-6CHEM 102 Summer 2015, LA TECH
Expands the Arrhenius definitions to include many bases other than hydroxides and gas phase reactions
Acid Proton donor
Base Proton acceptor
This definition explains how substances like ammonia can act as bases.
Eg. HCl(g) + NH3(g) ------> NH4Cl(s)
HCl (acid), NH3 (base).
NH3(g) + H2O(l) NH4+
+ OH-
Brønsted-Lowry definitions
15-7CHEM 102 Summer 2015, LA TECH
Lewis DefinitionG.N. Lewis was successful in including acid and bases
without proton or hydroxyl ions.
Lewis Acid: A substance that accepts an electron pair.
Lewis base: A substance that donates an electron pair.
E.g. BF3(g) + :NH3(g) F3B:NH3(s)
the base donates a pair of electrons to the acid forming a
coordinate covalent bond common to coordination
compounds. Lewis acids/bases will be discussed later in
detail
15-8CHEM 102 Summer 2015, LA TECH
1) Acids and bases can be defined in several ways. Which definitions of the bases that fits the description below?
a) a compound that produces more OH- ions in water:
b) a proton acceptor:
c) an electron pair donor:
15-9CHEM 102 Summer 2015, LA TECH
Types of Acids and Bases
Binary acids
Oxyacid
Organic acids
Acidic oxides
Basic oxides
Amine
Polyprotic acids
15-10CHEM 102 Summer 2015, LA TECH
Types of Acids and BasesBinary acids: HCl, HBr, HI, H2S
More than two elements: HCN
Oxyacid: HNO3, H2SO4, H3PO4
Polyprotic acids: H2SO4, H3PO4
Organic acids: R-COOH, R= CH3-, CH3CH2-
Acidic oxides: SO3, NO2, CO2,
Basic oxides: Na2O, CaO
Amine: NH3. R-NH2, R= CH3-, CH3CH2- : primary
R2-NH : secondary, R3-N: tertiary
Lewis acids & bases: BF3 and NH3
15-11CHEM 102 Summer 2015, LA TECH
Acidic Oxides
These are usually oxides of non-metallic elements such as P, S and N.
E.g. NO2, SO2, SO3, CO2
They produce oxyacids when dissolved in waterSO3 + H2O ---> H2SO4
CO2 + H2O ---> H2CO3
NO2 + H2O ---> HNO3
15-12CHEM 102 Summer 2015, LA TECH
Basic Oxides
Oxides oxides of metallic elements such as Na, K, Ca. They produce hydroxyl bases when dissolved in water.
e.g. CaO + H2O ---> Ca(OH)2
Na2O + H2O ---> 2 NaOH
15-13CHEM 102 Summer 2015, LA TECH
Protic Acids
Monoprotic Acids: The form protic refers to acidity due to protons. Monoprotic acids have only one acidic proton. e.g. HCl.
Polyprotic Acids: They have more than one acidic proton.
e.g. H2SO4 - diprotic acid
H3PO4 - triprotic acid.
15-14CHEM 102 Summer 2015, LA TECH
Polyprotic Acidsacids where more than one hydrogen per molecule
is released
15-15CHEM 102 Summer 2015, LA TECH
Amines
Class of organic bases derived from ammonia NH3 by replacing hydrogen by organic groups. They are defined as bases similar to NH3 by Bronsted-Lowery or Lewis acid/base definitions.
15-17CHEM 102 Summer 2015, LA TECH
2) Identify types of acids/bases as: binary acids, oxy acids, organic acids, acidic oxides, basic oxides, amine and polyprotic acids.
a) HF b) HBr c) H3PO4
d) H2SO4 e) HNO3 f) R-COOH
g) NO2 h) SO3 j)CaO
k) R-NH2
15-18CHEM 102 Summer 2015, LA TECH
Influence of Molecular Structure on Acid Strength
Binary Hydrides• hydrogen & one other element
Bond Strengths• weaker the bond, the stronger the acid
Stability of Anion• higher the electronegativity, stronger the acid
15-19CHEM 102 Summer 2015, LA TECH
Binary Acids
Compounds containing acidic protons bonded to a more electronegative atom.
e.g. HF, HCl, HBr, HI, H2S
The acidity of the haloacid (HX; X = Cl, Br, I, F)Series increase in the following order: HF < HCl < HBr < HI
15-20CHEM 102 Summer 2015, LA TECH
Oxyacids
Compounds containing acidic - OH groups in the molecule.
Acidity of H2SO4 is greater than H2SO3 because of the extra O (oxygens)
The order of acidity of oxyacids from the a halogen (Cl, Br, or I) shows a similar trend.
HClO4 > HClO3 > HClO2 > HClO
perchloric chloric chlorus hyphochlorus
15-21CHEM 102 Summer 2015, LA TECH
Influence of Molecular Structure on Acid Strength
Oxyacids• hydrogen, oxygen, & one other element
H-O-E• higher the electronegativity on E, stronger the acid as
this weakens the bond between the O and H
15-23CHEM 102 Summer 2015, LA TECH
3) Which of the following is stronger acid or base:
a) H2SO4 or H2SO3:
b) HCl or HI:
c) HClO or HClO3:
d) H2S or HF:
e) CF3COOH or CH3COOH:
f) CH3COOH or CH3CH2COOH
15-24CHEM 102 Summer 2015, LA TECH
3) Which of the following is stronger acid or base:
a) H2SO4 or H2SO3: Ka H2SO4>> 1 ; H2SO3= 1.4 x 10-2
b) HCl or HI:
c) HClO or HClO3:
d) H2S or HF: H2S: Ka:H2S=6.3 x 10-8 ; HF= 6.3 x 10-4
e) CF3COOH or CH3COOH: 2.5 x 10-3 and 1.8 x 10-5
f) CH3COOH or CH3CH2COOH: 1.8 x 10-5 and 1.2 x 10-5
15-25CHEM 102 Summer 2015, LA TECH
DissociationStrong Acids:
HCl(aq) + H2O(l) H3+O(aq) + Cl-(aq)
H2SO4(aq) + H2O(l) H3+O(aq) + HSO4
-(aq)
Dissociation Equilibrium Weak Acid/base:
H2O(l) + H2O(l) H3+O(aq) + OH-(aq)
This dissociation is called autoionization of water.
HC2H3O2(aq) + H2O(l) H3+O(aq) + C2H3O2
-(aq)
NH3 (aq) + H2O(l) NH4+ + OH-(aq)
Equilibrium constants: Ka, Kb and Kw
15-26CHEM 102 Summer 2015, LA TECH
4) Write equations for the dissociation equilibrium reactions for the following acids and bases in water. Which of these are acid or dissociations?
a) HCl:
b) H2SO4 :
15-27CHEM 102 Summer 2015, LA TECH
4) Write equations for the dissociation equilibrium reactions for the following acids and bases in water. Which of these are acid or dissociations?
c) H2O (autoionization): What is auto ionization?
d) HC2H3O(acetic acid):
e) NH3:
15-28CHEM 102 Summer 2015, LA TECH
Bronsted acid/conjugate base and base/conjugate acid pairs inacid/base equilibria
HCl(aq) + H2O(l) H3+O(aq) + Cl-(aq)
HCl(aq): acid
H2O(l): base
H3+O(aq): conjugate acid
Cl-(aq): conjugate base
H2O/ H3+O: base/conjugate acid pair
HCl/Cl-: acid/conjugate base pair
15-29CHEM 102 Summer 2015, LA TECH
Conjugate acid-base pairs.
Acids and bases that are related by loss or gain of H+ as H3O+
and H2O.
Examples. Acid Base
H3O +
H2O
HC2H3O2 C2H3O2-
NH4 +
NH3
H2SO4 HSO4-
HSO4- SO4
2-
Brønsted-Lowry Definitions
15-30CHEM 102 Summer 2015, LA TECH
Select acid, base, acid/conjugate base pair,base/conjugate acid pair
H2SO4(aq) + H2O(l) H 3+O(aq) + HSO4
-(aq)acid
base
conjugate acid
conjugate base
base/conjugate acid pair
acid/conjugate base pair
15-31CHEM 102 Summer 2015, LA TECH
5) For HOCl write:
a) Dissociation equilibrium reaction for the HOCl:
b) Identify the acid/conjugate base pair:
c) Identify the base/conjugate acid pair:
d) The equilibrium constant expression:
15-32CHEM 102 Summer 2015, LA TECH
Strong Acid vs. Weak AcidsStrong acidcompletely ionizedHydrioidic HI Ka ~ 1011 pKa = -11Hydrobromic HBr Ka ~ 109 pKa = -9Perchloric HClO4 Ka ~ 107 pKa = -7Hyrdrochloric HCl Ka ~ 107 pKa = -7Chloric HClO3 Ka ~ 103 pKa = -3Sulfuric H2SO4 Ka ~ 102 pKa = -2Nitric HNO3 Ka ~ 20 pKa = -1.3Weak acidpartially ionizedHydrofluoric acid HF Ka = 6.6x10-4 pKa = 3.18Formic acid HCOOH Ka = 1.77x10-4 pKa = 3.75Acetic acid CH3COOH Ka = 1.76x10-5 pKa = 4.75Nitrous acid HNO2 Ka = 4.6x10-4 pKa = 3.34Acetyl Salicylic acid C9H8O4 Ka = 3x10-4 pKa = 3.52Hydrocyanic acid HCN Ka = 6.17x10-10 pKa = 9.21
15-33CHEM 102 Summer 2015, LA TECH
Strong Base vs. Weak BaseStrong Basecompletely ionizedLithium hydroxide LiOHSodium hydroxide NaOH
Potassium hydroxide KOH Kb~ 102-103
Rubidium hydroxide RbOHCesium hydroxide CsOHBoarder-line Bases
Magnesium hydroxide Mg(OH)2
Calcium hydroxide Ca(OH)2
Strotium hydroxide Sr(OH)2 Kb~ 0.01 to0.1
Barium hydroxide Ba(OH)2
Weak Basepartially ionized
Ammonia NH3 Kb=1.79x10-5 pKb = 4.74
Ethyl amine CH3CH2NH2 Kb=5.6x10-4 pKb = 3.25
15-34CHEM 102 Summer 2015, LA TECH
•Strong acids Ionize completely in water. HCl, HBr, HI,
HClO3,
HNO3, HClO4, H2SO4.
•Weak acids Partially ionize in water.
Most acids are weak.
•Strong bases Ionize completely in water.
Strong bases are metal hydroxides - NaOH,
KOH
•Weak bases Partially ionize in water.
Acid and Base Strength
15-35CHEM 102 Summer 2015, LA TECH
Common Acids and BasesAcids Formula Molarity*
nitric HNO3 16
hydrochloric HCl 12
sulfuric H2SO4 18
acetic HC2H3O2 18
Bases
ammonia NH3(aq) 15
sodium hydroxide NaOH solid
*undiluted.
15-37CHEM 102 Summer 2015, LA TECH
Autoionization When water molecules react with one another to form ions.
Acids and bases alter the dissociation equilibrium of water based on Le Chaterlier’s principle
Kw = [ H3O+
] [ OH- ]
= 1.0 x 10-14
at 25oC
Note: [H2O] is constant and is included in Kw.
ion product
of water
ion product
of water
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
(10-7
M) (10-7M)
Autoionization of Water
15-38CHEM 102 Summer 2015, LA TECH
What is pH?
Kw = [H3+O][OH-] = 1 x 10-14
[H3+O][OH-] = 10-7 x 10-7
Extreme cases:
Basic medium
[H3+O][OH-] = 10-14 x 100
Acidic medium
[H3+O][OH-] = 100 x 10-14
pH value is -log[H+]
spans only 0-14 in water.
15-39CHEM 102 Summer 2015, LA TECH
We need to measure and use acids and bases over a very large
concentration range.
pH and pOH are systems to keep track of these very large ranges.
pH = -log[H3O+
]
pOH = -log[OH-]
pH + pOH = 14
Substance pH
1 M HCl 0.0
Gastric juices 1.0 - 3.0
Lemon juice 2.2 - 2.4
Classic Coke 2.5
Coffee 5.0
Pure Water 7.0
Blood 7.35 - 7.45
Milk of Magnesia 10.5
Household ammonia 12.0
1M NaOH 14.0
Substance pH
1 M HCl 0.0
Gastric juices 1.0 - 3.0
Lemon juice 2.2 - 2.4
Classic Coke 2.5
Coffee 5.0
Pure Water 7.0
Blood 7.35 - 7.45
Milk of Magnesia 10.5
Household ammonia 12.0
1M NaOH 14.0
pH and other “p” scales
15-40CHEM 102 Summer 2015, LA TECH
A logarithmic scale used to keep track of the large changes in [H+].
0 7 14
10 0
M 10-7
M 10-14
M
Very Neutral Very
acidic Basic
When you add an acid to, the pH gets smaller.
When you add a base to, the pH gets larger.
pH scale
15-42CHEM 102 Summer 2015, LA TECH
pH, pKw and pOHThe relation of pH, Kw and pOH Kw = [H+][OH-] log Kw = log [H+] + log [OH-] -log Kw= -log [H+] -log [OH-] ; previous equation multiplied by -1 pKw = pH + pOH; pKw = 14 since Kw =1 x 10-14
14 = pH + pOH
pH = 14 - pOH
pOH = 14 - pH
14 = pH + pOH
pH = 14 - pOH
pOH = 14 - pH
15-43CHEM 102 Summer 2015, LA TECH
Measuring pH
Arnold Beckman
inventor of the pH meter
father of electronic instrumentation
15-44CHEM 102 Summer 2015, LA TECH
7. Identify the following as acidic/basic/neutral and estimate/calculate pH.
Solution Acidic/basic/neutral pH of the solution
a) [ H+] > [OH-] and [H+] > 1.0 x 10-7 M : :
b) [H+] < [OH-] and [H+] < 1.0 x 10-7 M : :
c) [H+] = [OH-] = 1.0 x 10-7 M : :
d) [H+] > [OH-] = 1.0 × 10-14 M : :
e) [H+]< [OH-] = 1.0 x 10+7 M : :
15-45CHEM 102 Summer 2015, LA TECH
pH and pOH calculations of acid and base solutions
a) Strong acids/bases
dissociation is complete for strong acid such as HNO3 or base NaOH
[H+] is calculated from molarity (M) of the solution
b) weak acids/bases
needs Ka , Kb or percent(%)dissociation
15-46CHEM 102 Summer 2015, LA TECH
pH of 0.5 M H2SO4 Solution
H2SO4(aq) + H2O(l) H3+O(aq) + HSO4
-(aq)
HSO4-(aq) + H2O(l) H3
+O(aq) + SO42-(aq)
[H3+O][HSO4
-]
H2SO4 ; Ka1 = -------------------
[H2SO4]
[H3+O][SO4
2-]
H2SO4 ; Ka2 = ------------------- ; Ka2 ignored
[HSO4-]
15-47CHEM 102 Summer 2015, LA TECH
H2SO4(aq) + H2O(l) H3+
O(aq) + HSO4-(aq)
the moles of H+
ions in the solution is equal to moles of H2SO4 at the beginning.
[H2SO4] = [H+
] = 0.5 mole/L
pH = -log [H+
]
pH = -log(0.5)
pH = 0.30
pH of 0.5 M H2SO4 Solution
15-48CHEM 102 Summer 2015, LA TECH
1.5 x 10-2 M NaOH.1.5 x 10-2 M NaOH.
NaOH is also a strong base dissociates completely in water.
[NaOH] = [HO- ] = 1.5 x 10-2 mole/L
pOH = -log[HO-]= -log(1.5 x 10-2)
pOH = 1.82
As defined and derived previously:
pKw= pH + pOH; pKw= 14
pH = pKw + pOH
pH = 14 - pOH
pH = 14 - 1.82 ; pH = 12.18
15-49CHEM 102 Summer 2015, LA TECH
8) For a 0.10 M solution of Ba(OH)2.Is it a strong base?
a) How many OH- are in the compound?
b) Calculate the [OH-] and [H+]:
c) pH of the solution:
15-50CHEM 102 Summer 2015, LA TECH
9) Calculate the pH of the strong acid 0.2 M H2SO4.
a) Is it a strong acid?
b) Is it polyprotic acid?
c) Dissociation equilibria:
d) Why second dissociation equilibria is not considered for [H+] concentration?
e) Calculate the [H+]
f) pH of the solution:
15-51CHEM 102 Summer 2015, LA TECH
pH of Mixtures of Strong and Weak Acidsthe presence of the strong acid retards the
dissociation of the weak acid
The pH of the solution is mainly based on the strong acid
Eg. 1.0 M HCl and 1.0 HC2H3O2
HCl(aq) + H2O(l) H3+O(aq) + Cl-(aq)
HC2H3O2(aq) + H2O(l) H3+
O(aq) + C2H3O2-(aq)
15-52CHEM 102 Summer 2015, LA TECH
pH of Mixtures of Acids and BasesThe pH of the solution is mainly based on the
excess acid or base present
Eg. 10 mL of 1.0 M HCl and 20 mL 1.0 NaOH
Moles of excess NaOH ( M x L) = 1.0 x 0.001= 0.001
Mixed together volume = 30 mL = 0.030 L
Molarity of excess NaOH = 0.001/0.030= 0.030
Calculate pOH and then pH
15-53CHEM 102 Summer 2015, LA TECH
Equilibrium, Constant, Ka & Kb
Ka: Acid dissociation constant for a equilibrium reaction.
Kb: Base dissociation constant for a equilibrium reaction.
Acid: HA + H2O H3+O + A-
Base: BOH + H2O B+ + OH-
[H3+O][ A-] [B+ ][OH-]
Ka = --------------- ; Kb = -----------------
[HA] [BOH]
15-54CHEM 102 Summer 2015, LA TECH
HCl(aq) + H2O(l) H3+O(aq) + Cl-(aq)
[H3+O][Cl-]
Ka= ----------------- [HCl]
[H+][Cl-]
Ka= ----------------- [HCl]
Acid Dissociation Constant
15-55CHEM 102 Summer 2015, LA TECH
Base Dissociation Constant
NH3 + H2O NH4+ + OH-
[NH4+
][OH-]
K =
[NH3]
15-56CHEM 102 Summer 2015, LA TECH
Comparing Kw and Ka & Kb
Any compound with a Ka value greater than Kw of water will be a an acid in water.
Any compound with a Kb value greater than Kw of water will be a base in water.
15-58CHEM 102 Summer 2015, LA TECH
WEAKER/STRONGER Acids and Bases & Ka and Kb values
A larger value of Ka or Kb indicates an equilibrium favoring product side.
Acidity and basicity increase with increasing Ka or Kb.
pKa = - log Ka and pKb = - log Kb
Acidity and basicity decrease with increasing pKa or pKb.
15-59CHEM 102 Summer 2015, LA TECH
Which is weaker?a. HNO2 ; Ka= 4.0 x 10-4.
b. HOCl2 ; Ka= 1.2 x 10-2.
c. HOCl ; Ka= 3.5 x 10-8.
d. HCN ; Ka= 4.9 x 10-10.
15-60CHEM 102 Summer 2015, LA TECH
What is Ka1 and Ka2?H2SO4(aq) + H2O(l) H3
+O(aq) + HSO4-(aq)
HSO4-(aq) + H2O(l) H3
+O(aq) + SO42-(aq)
15-61CHEM 102 Summer 2015, LA TECH
H2SO4(aq) + H2O(l) H3+O(aq) + HSO4
-(aq)
HSO4-(aq) + H2O(l) H3
+O(aq) + SO42-(aq)
[H3+O][HSO4
-]
H2SO4 ; Ka1 = -------------------
[H2SO4]
[H3+O][SO4
2-]
H2SO4 ; Ka2 = -------------------
[HSO4-]
Ka Examples
15-62CHEM 102 Summer 2015, LA TECH
HC2H3O2(aq) + H2O(l) H3+O(aq) + C2H3O2
-(aq)
[H+][C2H3O2
-]
H C2H3O2; Ka= ------------------
[H C2H3O2]
NH3 (aq) + H2O(l) NH4+ + OH-(aq)
[NH4+][OH-]
NH3; Kb= --------------
[ NH3]
Ka Examples
15-63CHEM 102 Summer 2015, LA TECH
% Dissociation gives x (amount dissociated) need for pH calculation
Amount dissociated
% Dissoc. = ------------------------- x 100
Initial amount/con.
x
% Dissoc. = --------------------------- x 100
concentration
15-64CHEM 102 Summer 2015, LA TECH
How do you calculate pH of weak acids/bases?
a) From % dissociation
b) From Ka or Kb
What is % dissociation
Amount dissociated
% Dissoc. = ------------------------- x 100
Initial amount
15-65CHEM 102 Summer 2015, LA TECH
How do you calculate % dissociation from Ka or Kb
1.00 M solution of HCN; Ka = 4.9 x 10-10
What is the % dissociation for the acid?
15-66CHEM 102 Summer 2015, LA TECH
1.00 M solution of HCN; Ka = 4.9 x 10-10
First write the dissociation equilibrium equation:
HCN(aq) + H 2O(l) <===> H 3+O(aq) + CN-(aq)
[HCN] [H+ ] [CN- ]Ini. Con. 1.00 M 0.0 M 0.00 M
Cha. Con -x x x
Eq. Con. 1.0 - x x x
[H 3+O ][CN-] x2
Ka = ------------------- = ----------------
[HCN] 1.0 - x
1.00 M solution of HCN; Ka = 4.9 x 10-10
15-67CHEM 102 Summer 2015, LA TECH
1.0 - x ~ 1.00 since x is small
x2
Ka = -----------; Ka = 4.9 x 10-10 = x2
1.0
x = 4.9 x 10-10 = 2.21 x 10 -5
Amount disso. 2.21 x 10 -5
----------------- x 100 =- ------------- x 100 Ini. amount 1.00
% Diss. =2.21 x 10 -5 x 100 = 0.00221 %
1.00 M solution of HCN; Ka = 4.9 x 10-10
15-68CHEM 102 Summer 2015, LA TECH
10) Calculate the % dissociation of 2.00 M solutions of HCN (Ka= 4.9 x 10-10)
a) Dissociation equilibria:
b) ICE setup:
c) Amount dissociated:
d) % dissociation::
15-69CHEM 102 Summer 2015, LA TECH
1 M HF, 2.7% dissociated
Notice the conversion of % dissociation to a fraction (x): 2.7/100=0.027) x=0.027
Calculate the pH of a weak acid from % dissociation
15-70CHEM 102 Summer 2015, LA TECH
HF(aq) + H 2O(l) <===> H 3+O(aq) + F-(aq)
[H+][F-]
Ka = ----------- [HF] [HF] [H+ ] [F- ]Ini. Con. 1.00 M 0.0 M 0.00 MChg. Con -x x xEq.Con. 1.0-0.027 0.027 0.027 pH = -log [H+] pH = -log(0.027) pH = 1.57
Calculate the pH of a weak acid from % dissociation
15-71CHEM 102 Summer 2015, LA TECH
11) Calculate the Ka of if 5.0 M HF, 2.7% dissociated:
a) Dissociation equilibrium:
b) ICE setup:
c) Amount dissociated:
d) Ka :
15-72CHEM 102 Summer 2015, LA TECH
Weak acid Equilibria
Example
Determine the pH of a 0.10 M benzoic acid solution at 25 oC if Ka = 6.28 x 10-5
HBz(aq) + H2O(l) H3O+(aq) + Bz-(aq)
The first step is to write the equilibrium expression
Ka = [H3O+
][Bz-]
[HBz]
15-73CHEM 102 Summer 2015, LA TECH
Weak acid Equilibria
HBz H3O+ Bz-
Initial conc., M 0.10 0.00 0.00
Change, DM -x x x
Eq. Conc., M0.10 - x x x
[H3O+] = [Bz-] = x
We’ll assume that [Bz-] is negligible compared to [HBz]. The contribution of H3O+ from water is also negligible.
15-74CHEM 102 Summer 2015, LA TECH
Weak Acid Equilibria
Solve the equilibrium equation in terms of x
Ka = 6.28 x 10-5 =
x = (6.28 x 10-5 )(0.10)
H3O+ = 0.0025 M
pH = 2.60
x2
0.10
15-75CHEM 102 Summer 2015, LA TECH
pH from Ka or Kb
1.00 M solution of HCN; Ka = 4.9 x 10-10
First write the dissociation equilibrium equation:
HCN(aq) + H 2O(l) H 3+O(aq) + CN-(aq)
[HCN] [H+ ] [CN- ]
Ini. Con. 1.00 M 0.0 M 0.00 M
Chg. Con -x x x
Eq. Con. 1.0 - x x x
15-76CHEM 102 Summer 2015, LA TECH
[H 3+O ][CN-] x2
Ka = --------------- = ----------------
[HCN] 1.0 - x
1.0 - x ~ 1.00 since x is small x2 Ka = -----------; Ka = 4.9 x 10-10 = x2
1.0
x = 4.9 x 10-10 = 2.21 x 10 -5
pH = -log [H+]
pH = -log(2.21 x 10-5)
pH = 4.65
Weak Acid Equilibria
15-77CHEM 102 Summer 2015, LA TECH
The Conjugate Partners of Strong Acids and BasesThe conjugate acid/base of a strong base/acid has
no net effect on the pH of a solution
The conjugate base of a weak acid hydrolyze in water and basic or
pH of a solution > 7.00 E.g. Na+C2H3O2- sodium
acetate
The conjugate acid of a weak base hydrolyze in water and acidic or
pH of a solution < 7.00 E.g NH4Cl
15-78CHEM 102 Summer 2015, LA TECH
12) Calculate the [H+], [OH-] and pH of 0.90 M HC2H3O2; Ka= 1.8 x 10-5.
a) Dissociation equilibria:
b) ICE setup:
c) [H+] and [OH-]:
d) pH:
15-79CHEM 102 Summer 2015, LA TECH
13) Calculate the [H+], [OH-] pOH and pH 5.0 M NH3; Kb = 1.8 x 10-5
a) Dissociation equilibria:
b) ICE setup:
c) [H+] and [OH-]:
d) pOH and pH:
15-80CHEM 102 Summer 2015, LA TECH
14) Calculate the pH of a 0.015 M solution of lactic acid. The Ka for lactic acid is 1.4 x 10-4.
15-81CHEM 102 Summer 2015, LA TECH
15) Calculate the pH of a 0.14 M solution of an acid with Ka = 6.2 x 10-8 (pH = 4.03)
15-83CHEM 102 Summer 2015, LA TECH
Reaction of a basic anion or acidic cation with water is an ordinary Brønsted-Lowry acid-
base reaction.
CH3COO-(aq) + H2O(l) CH3COOH(aq) + OH-(aq)
NH4+
(aq) + H2O(l) NH3 (aq) + H3O+
(aq)
This type of reaction is given a special name.
Hydrolysis
The reaction of an anion with water to produce the conjugate acid and OH-.
The reaction of a cation with water to produce the conjugate base and H3O+
.
Hydrolysis
15-84CHEM 102 Summer 2015, LA TECH
What salt solutions would be acidic, basic and neutral?
1) strong acid + strong base = neutral
2) weak acid + strong base = basic
3) strong acid + weak base = acidic
4) weak acid + weak base = neutral,
basic or an acidic solution depending on the relative strengths of the acid and the base.
15-85CHEM 102 Summer 2015, LA TECH
What pH? Neutral, basic or acidic?
•a)NaCl • neutral•b) NaC2H3O2
• basic•c) NaHSO4 • acidic•d) NH4Cl• acidic
15-86CHEM 102 Summer 2015, LA TECH
1) If the following substance is dissolved in pure water, will the solution be acidic, neutral, or basic?
a) Solid sodium carbonate-(Na2CO3):
b) Sodium chloride- (NaCl):
c) Sodium acetate- (NaC2H3O2):
d) Ammonium sulfate-((NH4)2SO4):
15-87CHEM 102 Summer 2015, LA TECH
How do you calculate pH of a salt solution?Find out the pH, acidic or basic?
If acidic it should be a salt of weak base
If basic it should be a salt of weak acid
if acidic calculate Ka from Ka= Kw/Kb
if basic calculate Kb from Kb= Kw/Ka
Do a calculation similar to pH of a weak acid or base
15-88CHEM 102 Summer 2015, LA TECH
What is the pH of 0.5 M NH4Cl salt solution? (NH 3; Kb = 1.8 x 10-5)
Find out the pH, acidic
if acidic calculate Ka from Ka= Kw/Kb
Ka= Kw/Kb = 1 x 10-14 /1.8 x 10-5)
Ka= 5.56. X 10-10
Do a calculation similar to pH of a weak acid
15-89CHEM 102 Summer 2015, LA TECH
Continued
NH4+ + H2O H 3
+O + NH3
[NH4+] [H3
+O ] [NH3 ]Ini. Con. 0.5 M 0.0 M 0.00 MChange -x x xEq. Con. 0.5 - x x x [H 3+O ] [NH3 ]
Ka(NH4+) = -------------------- =
[NH 4+] x2
---------------- ; appro.:0.5 - x . 0.5 (0.5 - x)