what are acids &bases? - chem.latech.eduupali/chem481/slides/hw%235... · arrhenius definitions...
TRANSCRIPT
3/28/2013
1
Instructor: Dr. Upali Siriwardane
e-mail: [email protected] Office: CTH 311 Phone 257-4941
Office Hours:
8:00-9:00 a.m. & 11:00-12:00 a.m. M, W;
8:00-10:00 a.m. Tu, Th, F.
April 2 , 2013: Test 1 (Chapters 1, 2, 3,)
April 25, 2013: Test 2 (Chapters 5, 6 & 7)
May 14, 2013: Test 3 (Chapters. 19 & 20)
May 15, Make Up: Comprehensive covering all Chapters
Chemistry 481(01) Spring 2013
1
What are Acids &Bases?
Definition?
a) Arrhenius
b) Bronsted-Lowry
c) Lewis
Chapter 4-2
Acid Anything that produces hydrogen ions in a water solution.
HCl (aq) H+ + Cl-
Base Anything that produces hydroxide ions in a water solution.
NaOH (aq) Na+ + OH-
Arrhenius definitions are limited to aqueous solutions.
Acid base reactions: HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
Arrhenius definitions
Chapter 4-3
Expands the Arrhenius definitions
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
Chapter 4-4
3/28/2013
2
Proton in water
Chapter 4-5
Dissociation Equilibrium
HCl(aq) + H2O(l) H3+O(aq) + Cl-(aq)
H2SO4(aq) + H2O(l) H3+O(aq) + HSO4
-(aq)
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)
Chapter 4-6
Bronsted conjugate acid/base pairs in 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
Chapter 4-7
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- SO42-
Brønsted-Lowry definitions
Chapter 4-8
3/28/2013
3
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
Chapter 4-9
Types of Acids and Bases
Binary acids
Oxyacid
Organic acids
Acidic oxides
Basic oxides
Amine
Polyprotic acids
Chapter 4-10
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
Chapter 4-11
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
Chapter 4-12
3/28/2013
4
Aqua Acids
Acidic proton is on a water molecule coordinated
to a central metal ion
[Fe(OH2)6]3+,Al(OH2)6
3+, Si(OH)4
Acidity increase with charge
Acidity increase as metal become smaller
Chapter 4-13
Anhydrous oxides The Lux/Flood Definition
Covers things which would become acids or bases if dissolved in water.
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 water
Chapter 4-14
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
Chapter 4-15
Protic Acids
Monoprotic Acids: The form protic
refers to acidity or 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.
Chapter 4-16
3/28/2013
5
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 or Lewis
acid/base definitions.
Chapter 4-17
What acid base concepts (Arrhenius/Bronsted/Lewis) would best
describe the following reactions:
a) HCl(aq) + NaOH(aq) ---> NaCl(aq) + H2O(l)
b)HCl(g) + NH3(g) ---> NH4Cl(s)
c)BF3(g) + NH3(g) ---> F3B:NH3(s)
d)Zn(OH)2(s) + 2OH-(aq) ---> [Zn(OH)4]2- (aq)
Chapter 4-18
Common acids and bases
Acids Formula Molarity* nitric HNO3 16 hydrochloric HCl 12 sulfuric H2SO4 18 acetic HC2H3O2 18 Bases ammonia NH3(aq) 15 sodium hydroxide NaOH solid
*undiluted.
Chapter 4-19
Acids and bases
Acidic Basic
– Citrus fruits Baking soda
– Aspirin Detergents
– Coca Cola Ammonia cleaners
– Vinegar Tums and Rolaids
– Vitamin C Soap
Chapter 4-20
3/28/2013
6
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]
Chapter 4-21
What is Ka
HCl(aq) + H2O(l) <===> H3+O(aq) + Cl-(aq)
Chapter 4-22
E.g. Ka HCl(aq) + H2O(l) H3
+O(aq) + Cl-(aq)
[H3+O][Cl-]
Ka= ----------------- [HCl] [H+][Cl-] Ka= ----------------- [HCl]
Chapter 4-23
What is Ka1 and Ka2?
H2SO4(aq) + H2O(l) H3+O(aq) + HSO4
-(aq)
HSO4-(aq) + H2O(l) H3
+O(aq) + SO42-(aq)
Chapter 4-24
3/28/2013
7
What is Kb
NH3 (aq) + H2O(l) NH4+ + OH-(aq)
Chapter 4-25
H2SO4 Dissociation
E.g. 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][SO42-]
H2SO4 ; Ka2 = ------------------- [HSO4
-]
Chapter 4-26
Ka and Kb
E.g. 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]
Chapter 4-27
Acidity/Basicity of HA and F-
Chapter 4-28
3/28/2013
8
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.
Chapter 4-29
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.
Chapter 4-30
Autoionization When water molecules react with one another to form ions.
H2O(l) + H2O(l) H3O+(aq) + OH-
(aq)
(10-7M) (10-7M)
Kw = [ H3O+ ] [ OH- ]
= 1.0 x 10-14 at 25oC
Note: [H2O] is constant and is included in Kw.
ion product
of water
Autoionization of water
Chapter 4-31
What is Kw?
H2O(l) + H2O(l) H3+O(aq) + OH-(aq)
This dissociation is called autoionization of water.
Autoionization of water:
Kw = [H3+O][OH-]
Kw is called ionic product of water
Kw = 1 x 10-14
Chapter 4-32
3/28/2013
9
Why is water important for acid/base equilibria?
Water is the medium/solvent for acids and bases.
Acids and bases alter the dissociation equilibrium of water based on Le Chaterlier’s principle
H2O(l) + H2O(l) H3+O(aq) + OH-(aq)
Chapter 4-33
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.
Chapter 4-34
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
pH and other “p” scales
Chapter 4-35
A logarithmic scale used to keep track of the large changes in [H+].
14 7 0
10-14 M 10-7 M 1 M
Very Neutral Very
Basic Acidic
When you add an acid, the pH gets smaller.
When you add a base, the pH gets larger.
pH scale
Chapter 4-36
3/28/2013
10
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 of some common materials
Chapter 4-37
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.
Chapter 4-38
pH, pKw and pOH
The 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
Chapter 4-39
Acid and Base Strength • 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.
Chapter 4-40
3/28/2013
11
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
Chapter 4-41
Titration curves
pH
Equivalence
Point
% titration or ml titrant
Buffer region
Overtitration
Indicator
Transition
Chapter 4-42
Indicators Acid-base indicators are highly colored weak
acids or bases.
HIn In- + H+
color 1 color 2
They may have more than one color transition.
Example. Thymol blue
Red - Yellow - Blue
One of the forms may be colorless - phenolphthalein (colorless to pink)
Chapter 4-43
Selection of an indicator for a titration
a) strong acid/strong base
b) weak acid/strong base
c) strong acid/weak base
d) weak acid/weak base
Calculate the pH of the solution at he equivalence point or end point
Chapter 4-44
3/28/2013
12
Common Ion Effect
Weak acid and salt solutions
E.g. HC2H3O2 and NaC2H3O2
Weak base and salt solutions
E.g. NH3 and NH4Cl.
H2O + C2H3O2- <==> OH- + HC2H3O2
(common ion)
H2O + NH4+ <==> H3
+O + NH3
(common ion)
Chapter 4-45
Solutions that resist pH change when small amounts of acid or base are added.
Two types
weak acid and its salt
weak base and its salt
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Add OH- Add H3O+
shift to right shift to left
Based on the common ion effect.
Buffers
Chapter 4-46
The pH of a buffer does not depend on the absolute amount of the conjugate acid-base pair. It is based on the ratio of the two.
Henderson-Hasselbalch equation.
Easily derived from the Ka or Kb expression.
Starting with an acid
pH = pKa + log
Starting with a base
pH = 14 - ( pKb + log ) [HA]
[A-]
[A-]
[HA]
Buffers
Chapter 4-47
• Control of blood pH
Oxygen is transported primarily by hemoglobin in the red blood cells.
CO2 is transported both in plasma and the red blood cells.
CO2 (aq) + H2O H2CO3 (aq)
H+(aq) + HCO3
-(aq)
The bicarbonate
buffer is essential
for controlling
blood pH
Buffers and blood
Chapter 4-48
3/28/2013
13
Main Group Acid/Bases
Chapter 4-49
Amphoteric Oxides
Chapter 4-50
Strength of oxo-acids by Paulings Rules
For OpE(OH)q, pKa ~ 8 - 5p
The successive pKa values of polyprotic acids (i.e. q >1) increase by 5 units for each successive proton transfer.
Chapter 4-51
pKa Values of Oxy Acids
Chapter 4-52
3/28/2013
14
Lewis Definition
• 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)
Chapter 4-53
Lewis Acids/Bases
Chapter 4-54
Hard and soft acids and bases
Chapter 4-55
Solvent leveling
• If the solvent contains ionizable protons it is said to be protonic, and if it is protonic, it will engage in acid-base reactions.
• All acids/bases which are stronger than the H3O+(aq) or OH-(aq) ion will react to produce hydronium/hydroxide ion, and so their strength will be leveled to that of the H3O+(aq) or OH-(aq) ion.
• In aqueous solution, the strongest acid/base which can exist is the H3O+(aq) or OH-(aq)
Chapter 4-56
3/28/2013
15
Acid-Base Discrimination Windows
Chapter 4-57
Levelling effect in other protic liquid
all acids are levelled to the strength of the
ammonium ion, NH4+, and all bases are levelled
to the strength of the amide ion, NH2-.
2 NH3 NH4+ + NH2
-.
2HNO3 H2NO3+ + NO2
-
3HF H2F+ + HF2-
2H2SO4 H3SO4+ + HSO4
-
Chapter 4-58
Polycation Formation
NaAl13O4(OH)24(H2O)12(SO4)4 .x H2O
Chapter 4-59
Polyanion Formation:Zeolites
The general method of zeolite production
involves dissolving an aluminium source
(metal or oxide) into an aqueous solution
of sodium or potassium hydroxide. Once
this solution has cooled the silica source
(and organic template, if required) is added
in The form of an aqueous slurry and the
resulting gel stirred until homogenous.
Na12[(AlO2)12(SiO2)12.27H2O
Chapter 4-60