Post on 08-Oct-2019
Pharmaceutical Analytical Chemistry (PHCM223-SS16) Lecture 1
ACID- BASE EQUILIBRIUM-I “pH of acids and bases”
Dr. Rasha Hanafi
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 1
LEARNING OUTCOMES
By the end of this session the student should be able to:
1. Get acquainted with the course description.
2. Identify acids and bases.
3. Determine Acid strength.
4. Apply Kw Calculations.
5. Define the pH scale.
6. Determine the pH of weak/strong acids and bases.
7. Determine the pH of Polyprotic acids.
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 2
COURSE DESCRIPTION
• COURSE INSTRUCTORS: 1. Lecturers: Dr. Rasha Hanafi (Lectures# 1-6), B5-101. Dr. Nesrine Elgohary (Lectures# 7-12), B7-207. 2. Teaching assistants: Noura Essam (B7-205), Dina Elsaeed (B1-225), Engy Saad (B7-207), Nadine George (B7-207), Magy Maged (B7-207), Maha Farag (B7-207), Dalia Khalil (B1-207), Salma Mokbel. •TEXT BOOKS 1. Chemistry, 10th ed., Raymond Chang, ISBN 978-0-07-017264-7, McGraw Hill.
2. D. A. Skoog, D.A. West, F.J. Holler, S.R. Crouch, Analytical Chemistry, an introduction, 7th Edition, ISBN 0-03-020293-0.
• INTERNET LINK FOR COURSE MATERIAL, GENERAL INFO AND ANNOUNCEMENTS http://pbt.guc.edu.eg/Courses/Undergrad.aspx
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 3
Assessment methods Weight
Contribution in tutorials 10%
Quizzes (Best 2 quizzes out of 3, 10% each) 20%
Midterm exam 30%
Final term exam 40%
All Quizzes will take place during tutorial and will cover up to the previous tutorial session. Each Practical exam will have a theoretical quiz covering its content.
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 4
Name Room number Office hours (slots)
Dr. Rasha Hanafi B5-101 Sunday 3rd and Monday 3rd
Dr. Nesrine Elgohary B7-207 Tuesday 1st and Thursday 3rd
Salma Mokbel B7-207 Sunday 5th and Tuesday 1st
Engy Saad B7-207 Thursday 3rd and 4th
Dalia Elhelw B1-205 Monday 5th and Wednesday 3rd
Magy Maged B7-207 Monday 3rd and 4th
Nadine George B7-207 Monday 2nd and 4th
Noura Essam B7-205 Tuesday 3rd and Wednesday 3rd
Dina Atef B1-225 Monday 3rd and 4th
Maha Magdy B7-207 Monday 4th and Wednesday 3rd
COURSE INSTRUCTORS
THEORETICAL COURSE CALENDAR
Lecture 1, Dr. Rasha Hanafi.
Lecture no. [instructor]
Week Title of lecture Tutorials Practical sessions Compensations
Lecture 1 [RH] 13/2- 18/2 Acid- Base Equilibrium (ABE)
pH of Acids/bases --
Lecture 2 [RH] 20/2- 25/2 ABE
pH of Salts pH of Acids/bases
Introduction and Acid base titration
Lecture 3 [RH] 27/2- 3/3 ABE
pH of Buffers pH of Salts Acid base titration
Lecture 4 [RH] 5/3- 10/3 ABE
Titration Curves pH of Buffers Acid base titration
Lecture 5 [RH] 12/3- 17/3 ABE
Polyprotic acids+ Double indicators Titration Curves Exam 1
Lecture 6 [RH] 19/3- 24/3 ABE
Precipitimertry Polyprotic acids+ Double
indicators Precipitimertry
No teaching 26/3- 31/3
Revision week
No teaching 2/4- 7/4 Midterm exams
Lecture7 [NE] 9/4- 14/4 Complexometry 1 Precipitimertry Complexometry
Lecture8 [NE] 16/4- 21/4 Complexometry 2 Complexometry 1 Complexometry
Lecture 9 [NE] 23/4 - 28/4 Redox 1 Complexometry 2 Exam 2 for 24th 25th and 28th of
April
Lecture 10 [NE] 30/4 - 5/5 Redox 2 Redox 1 Exam 2, cont.
Lecture 11[NE] 7/5 - 12/5 Gravimetry Redox 2 Redox for 1st and 2nd of May
Lecture 12 [NE] 14/5 - 19/5 Applications on different types of equilibria
Gravimetry Exam 3
No teaching Revision week
PHCM223,SS16 5
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 6
pH in living systems Compartment pH
Gastric Acid 1
Human Skin 5.5
Water 7
Blood 7.35-7.45
Mouth pH 5-7
Small intestine
pH 6-7.5
Large intestine
pH 5-7
Stomach
pH 1-3
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 7
Acid-Base Chemistry
Reversible reaction
Equilibrium
Proton Exchange
Donates protons
Acids
Accepts protons
Bases
strong
weak
Neutralization reaction
Titration
]reactants
products[k
AMPHOTERIC NATURE OF WATER
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 8
Aqueous acid-base reactions occur in water. Water can behave as an acid or as a base, by donating a proton (H+) or accepting a proton, respectively.
CH3COOH + H2O H3O+ (aq) + CH3COO-
(aq) acid
acid base
base
conjugate conjugate
EQUILIBRIUM CONSTANT OF ACIDS, Ka
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 9
HA + H2O H3O+ (aq) + A-
(aq)
acid Conjugate acid base Conjugate base
][
]][[
]][[
]][[constant mequilibriu The
2
3
HA
AH
OHHA
AOHka
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 10
ACIDS
Classified by the equilibrium position of its dissociation reaction
STRONG
equilibrium lies far to the right, i.e. almost all HA is
dissociated (ionized).
yields a weak conjugate base (that has low affinity
to H+).
Ex: HCl, H2SO4, HNO3, HClO4
WEAK
equilibrium lies far to the left. HA dissociates only to
a small extent (<1%).
Yields a strong conjugate base (that has high affinity to
H+).
H3PO4, CH3COOH, C6H5COOH
AUTOIONIZATION OF WATER
The equilibrium expression of water is
Kw = [H+][OH-]= 1.0x10-14 at 25°C
Kw is “the ion-product constant” or “dissociation constant for water”.
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 11
-log Kw= -log [H+] – log[OH-]
Kw depends on T: At 0C, Kw = 0.114x10-14 At 50C, Kw= 5.47x10-14 At 100C, Kw = 49x10-14
In aqueous solutions, if [H+] = [OH-] Neutral soln. [H+] > [OH-] Acidic soln. [H+] < [OH-] Basic soln.
The term –log is commonly referred to “p”, so
pKw = pH + pOH Since Kw = 1.0x10-14
pH + pOH = 14.00
H2O + H2O OH- + H3O+
Test yourself
Since [OH-] and [H+] are formed only from the dissociation of water (there is no acid nor base in the solution), hence, the concentrations of [OH-] and [H+] must be equal: [H+]=[OH-]=
wK
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 12
Calculate the hydrogen and hydroxide ion concentrations of pure water at 25C and 100C.
At 25C, Kw= [OH-][H+] = 1.00x10-14
So, [OH-]=[H+] = (1.00x10-14)1/2 = 1.00x10-7M
THE pH SCALE
pH = - log[H+] Thus for solution of [H+] = 1.0x10-7, pH = -log(10-7) = 7.00 (2nd decimal is a significant figure). N.B.: A solution of pH 3 has a [H+]concentration 10 times more than that of a solution of pH 4 and 100 times more than that of a solution of pH 5.
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 13
The pH scale is a compact way to represent acidity of solutions.
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 14
Test yourself
Calculate [H+] or [OH-] as required for each of the following solutions at 25°C and state whether the solution is neutral, acidic or basic.
1.0x10-5 M OH-
[H+] =1.0x10-14/ 1.0x10-5 = 1.0x10-9M
[OH-] > [H+]
The soln. is basic.
1.0x10-7 M OH-
[H+] = 1.0x10-14/ 1.0x10-7 = 1.0x10-7M
[H+] = [OH-]
The soln. is neutral
10.0 M H+
[OH-] = 1.0x10-14/ 10.0 = 1.0x10-15M
[H+] > [OH-]
The solution is acidic.
Kw = [H+][OH-]= 1.0x10-14
pH OF WEAK ACIDIC SOLUTIONS
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 15
STRONG ACIDS WEAK ACIDS
Fully ionize in water Less than 1% ionization
HCl H+ + Cl-
nM n M n M
CH3COOH CH3COO- + H+
(n – x) M x M x M
pH = -log[H+]= -log[n] pH = -log[H+]= - log[x]
It is easy to get [H+] because it is the same concentration of the acid
To get [H+], not only the concentration of the acid needs to be known, but
how much exactly dissociates from the acid, i.e. the dissociation constant (ka)
needs also to be known
][
]][[
3
3
COOHCH
COOCHHka
x2 = (7.2x10-4)(1.00)
x= 2.7x10-2 = [H+]
pH = -log(2.7x10-2) = 1.57
pH of weak acid = -log
] [ acidweakKa
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 16
pH OF WEAK ACIDIC SOLUTIONS
00.1
.
][
]][[ xx
HF
FHka
Example: to calculate the pH of 1.00M soln. of the weak acid HF, the dissociation constant needs to be known. It is Ka = 7.2x10-4.
HF H+ + F-
(1-x) M x M x M
(1-x) ≈1 because it is < 1% of x, thus
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 17
BASES
Classified by the equilibrium position of its dissociation reaction
STRONG
equilibrium lies far to the right, i.e. almost all the base is dissociated
(ionized).
yields a weak conjugate acid (that has low ability to donate H+).
All hydroxides of group 1A (LiOH, NaOH, KOH, …etc) and 2A alkaline earth
hydroxides (Ca(OH)2, Mg(OH)2) are strong bases.
NaOH and KOH are common lab reagents.
WEAK
equilibrium lies far to the left. The base dissociates only to a small
extent (<1%).
Yields a strong conjugate acid
(that has ability to donate H+).
Ammonia (NH3), pyridine
Calculate the pH for a 15.0 M NH3 (Kb = 1.8x10-5)
NH3(aq) + H2O(l) NH4+ + OH-
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 18
pH OF WEAK BASIC SOLUTIONS
5-
3
x108.100.15
.
][
]][[4
xx
NH
OHNHkb
x = [OH-] = 1.6x10-2 M [H+] = Kw/[OH-] = 1.0x10-14/1.6x10-2 = 6.3x10-13 M pH = -log(6.3x10-13) = 12.20 N.B.: pOH can be calculated directly from [OH-] and then subtract it from 14.
Polyprotic acids (ex: H2SO4, H3PO4…etc) dissociate in a stepwise manner, yielding one proton at a time. The successive Ka values for the dissociation equilibria are designated Ka1 and Ka2.
Carbonic acid (important in maintaining blood pH at 7.35)
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 19
pH OF POLYPROTIC ACIDIC SOLUTIONS
The conjugate base HCO3- of the first dissociation equilibrium
becomes the acid in the second step.
STEPWISE DISSOCIATION CONSTANTS FOR SOME COMMON POLYPROTIC ACIDS
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 20
For weak polyprotic acids, Ka1>Ka2>Ka3 and the relative acid strengths in case of H3PO4 for example would be H3PO4» H2PO4
-»HPO42-.
This means that in a solution prepared by dissolving H3PO4 in water, only the first dissociation step makes an important contribution to [H+] and this greatly simplifies the pH calculations for polyprotic acids in general.
pH= -log ][acidKa
REFERENCES
1. S.S. Zumdahl, S.A. Zumdahl, Chemistry 6th Ed., Houghton Mifflin Company, ISBN 0-618-22156-5, (Chapters 14, 15)
2. D. A. Skoog, D.A. West, F.J. Holler, S.R. Crouch, Analytical Chemistry, an introduction, 7th Edition, ISBN 0-03-020293-0. (Chapters 4, 8, 10, 11, 12, 13, 14, 15, 18)
3. Lecture 1, PHCM223, by Prof. Rasha Elnashar, GUC, SS 2014.
4. Introduction to acid-base chemistry (http://www.chem1.com/acad/pdf/c1xacid1.pdf)
PHCM223,SS16 Lecture 1, Dr. Rasha Hanafi. 21