Di and Polybasic Acids and Bases – Definition, Ionisation, Basicity, Acidity

What are Polybasic Acids?

Acids capable of yielding more than one hydronium ion per molecule are called polybasic acids, the dibasic, tribasic etc indicating the number of replaceable hydrogen.

Taking the example of a few acids, such as sulphuric acid, and phosphoric acid, we can see that they contain more than one ionisable ion per molecule. Such acids are termed polybasic acids.

We use so many acids and bases every day, such as vinegar or acetic acid in the kitchen, boric acid for laundry, baking soda for the purpose of cooking, washing soda for cleaning etc. Many of the acids, that we do not consume in the household are used in the laboratories, which includes acids such as HCl, H2SO4 etc. and bases such as NaOH, KOH etc.

Some of these acids and bases have a single hydronium ion or a hydroxyl ion to shed, but most of them have multiple ions. In this section, we will learn about acids and bases which contain more than one ionisable ion per molecule.

Ionisation of Polybasic Acids

Let us consider the following ionisation reaction of a typical polybasic acid.

H2X (aq) ⇋ H + (aq) + HX (aq)

HX (aq) ⇋ H + (aq) + X 2- (aq)

This is an example of dissociation of a dibasic acid into its constituent ions.

The equilibrium constant for the above reaction can be given as,

H2X (aq) ⇋ H + (aq) + HX (aq)

Ka1 = { [H +] [HX ]} / [H2X]

HX (aq) ⇋ H + (aq) + X 2- (aq)

Ka2 = { [H +] [X 2- ]} / [HX ]Acids capable of yielding more than one hydronium ion per molecule are called polybasic acids, the dibasic, tribasic etc indicating the number of replaceable hydrogen.

So the dissociation constant of polybasic acid is the multiplication of dissociation constants of constituent ions.

Ka = Ka1 × Ka2 = { [H+] [HX] / [H2X]} × { [H+] [X2-] / [HX]}

= [H+]2 [X2-] / [H2X]

Here, Ka1 and Ka2 are the first and the second ionisation constants of the acid H2X. Similarly, for a tribasic acid we have three ionidation constants: Ka1, Ka2, and Ka3, and so on. Examples of some polybasic acids are H2SO4 , H2S, H3PO4 etc.

Example: Ionisation of H2SO4 is

H2SO4 (aq) ⇋ 2 H+ (aq)   + SO42- (aq)  

Ka =  [H+]2 [SO42-] / [H2SO4]

The formation of acid salts may be elucidated by the following considerations. Most of the inorganic acids combine with bases in such a manner that 1 atom of the acid is united with 1 atom of a metallic oxide, so that they may be termed monobasic acids.

Certain acids, however, exist of which an instance has already presented itself in the pyrophosphoric, 1 atom of which possesses the power of combining with 2 atoms of base such acids are hence termed dibasic acid. Numerous acids of this class are found among those obtained from the vegetables and the animal kingdoms tartaric acid and malic acid are examples of this kind.

Poly Acidic Bases

A base is a substance which gives (OH-) ions when dissolved in water. The strength of the base depends on OH ions concentration. The number of ionisable hydroxide ions (OH) present in one molecule of base is called the acidity of bases. On the basis of acidity, bases can be classified as monoacidic base, diacidic base, triacidic base etc.

Ionisation of Poly Acidic Base

Taking the example of a few bases such as calcium hydroxide, aluminium hydroxide, we can see that they contain more than one ionisable ion per molecule of base. Such acids are termed as poly-acidic bases.

Similarly ionisation reaction of a typical poly-acidic base.

M(OH)2 (aq) ⇋ M2+ (aq) + 2 OH (aq)

Acids capable of yielding more than one hydronium ion per molecule are called polybasic acids, the dibasic, tribasic etc indicating the number of replaceable hydrogen. Kb =  {[M2+]  [OH]2} / [M(OH)2]

Examples: Ca(OH)2 , Zn (OH)2 etc.

Ca(OH)2 (aq) ⇋ Ca2+ (aq) + 2 OH (aq)

Acids capable of yielding more than one hydronium ion per molecule are called polybasic acids, the dibasic, tribasic etc indicating the number of replaceable hydrogen.Kb =  {[Ca2+]  [OH]2} / [Ca(OH)2 ]

Frequently Asked Questions- FAQs

Q1

What are Polyprotic acids and bases?

Polyprotic acids are specific acids that can lose more than one proton per molecule in acid-base reactions. While polyprotic bases are bases which can gain more than one proton .

Q2

How does bond strength affect acidity?

In general, the bond strength of an acid depends on the size of the ‘A’ atom: the smaller the ‘A’ atom, the stronger the H-A bond. The atoms get bigger when moving down a path in the Periodic Table and the strength of the bonds becomes weaker, which means that the acids get stronger.

Q3

Is HCl a strong acid?

Hydrochloric acid has a very low pH, highly acidic, higher H+ ion concentration than OH-ions, and high ionisation / dissociation levels of H+ ions when dissolved in water. HCl dissociates entirely. So HCl is a strong acid.

Q4

What is a monobasic acid?

For an acid-base reaction, a monobasic acid is an acid that has just one hydrogen ion to give to a base. So a monobasic molecule only has one replaceable atom of hydrogen. Sources of this include HCl and HNO3.

Q5

What is a weak Diprotic acid?

When we do a mild diprotic acid solution, we get a solution that contains a mixture of acids. Carbonic acid, H2CO3, is a case in point of weak diprotic acid. The ion of bicarbonate can behave as an acid, too. This ionises and produces even smaller concentrations of the hydronium ions and carbonate ions.

Q6

Do acids and bases ionise in water?

Acids and bases dissolve in water, and they inhibit water dissociation as they increase the concentration of one of the water self-ionisation components, either protons or hydroxide ions. Acidic solutions have a lower pH while fundamental solutions have a higher pH.

Q7

What is ionisation of an acid?

Acid Ionisation. The degree of Ionisation refers to an acid or base strength. It is said that a strong acid ionises entirely in water while a weak acid is said to only partly ionise.

Q8

What is Diprotic base?

The acid balance problems addressed so far have focused on a class of compounds known as monoprotic acids. Each of these acids has a single H+ ion, or proton, as it acts as a Bronsted acid, it can give.

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.

CBSE Class 11 Chemistry Syllabus

CBSE Class 11 Chemistry Syllabus is a vast which needs a clear understanding of the concepts and topics. Knowing CBSE Class 11 Chemistry syllabus helps students to understand the course structure of Chemistry.

Unit-wise CBSE Class 11 Syllabus for Chemistry

Below is a list of detailed information on each unit for Class 11 Students.

UNIT I – Some Basic Concepts of Chemistry

General Introduction: Importance and scope of Chemistry.

Nature of matter, laws of chemical combination, Dalton’s atomic theory: concept of elements,
atoms and molecules.

Atomic and molecular masses, mole concept and molar mass, percentage composition, empirical and molecular formula, chemical reactions, stoichiometry and calculations based on stoichiometry.

UNIT II – Structure of Atom

Discovery of Electron, Proton and Neutron, atomic number, isotopes and isobars. Thomson’s model and its limitations. Rutherford’s model and its limitations, Bohr’s model and its limitations, concept of shells and subshells, dual nature of matter and light, de Broglie’s relationship, Heisenberg uncertainty principle, concept of orbitals, quantum numbers, shapes of s, p and d orbitals, rules for filling electrons in orbitals – Aufbau principle, Pauli’s exclusion principle and Hund’s rule, electronic configuration of atoms, stability of half-filled and completely filled orbitals.

UNIT III – Classification of Elements and Periodicity in Properties

Significance of classification, brief history of the development of periodic table, modern periodic law and the present form of periodic table, periodic trends in properties of elements -atomic radii, ionic radii, inert gas radii, Ionization enthalpy, electron gain enthalpy, electronegativity, valency. Nomenclature of elements with atomic number greater than 100.

UNIT IV – Chemical Bonding and Molecular Structure

Valence electrons, ionic bond, covalent bond, bond parameters, Lewis structure, polar character of covalent bond, covalent character of ionic bond, valence bond theory, resonance, geometry of covalent molecules, VSEPR theory, concept of hybridization, involving s, p and d orbitals and shapes of some simple molecules, molecular orbital theory of homonuclear diatomic molecules(qualitative idea only), Hydrogen bond.

UNIT V – Chemical Thermodynamics

Concepts of System and types of systems, surroundings, work, heat, energy, extensive and intensive properties, state functions. First law of thermodynamics – internal energy and enthalpy, measurement of U and H, Hess’s law of constant heat summation, enthalpy of bond dissociation, combustion, formation, atomization, sublimation, phase transition, ionization, solution and dilution. Second law of Thermodynamics (brief introduction)
Introduction of entropy as a state function, Gibb’s energy change for spontaneous and nonspontaneous processes.
Third law of thermodynamics (brief introduction).

UNIT VI – Equilibrium

Equilibrium in physical and chemical processes, dynamic nature of equilibrium, law of mass action, equilibrium constant, factors affecting equilibrium – Le Chatelier’s principle, ionic equilibrium- ionization of acids and bases, strong and weak electrolytes, degree of ionization,
ionization of poly basic acids, acid strength, concept of pH, hydrolysis of salts (elementary idea), buffer solution, Henderson Equation, solubility product, common ion effect (with illustrative examples).

UNIT VII – Redox Reactions

Concept of oxidation and reduction, redox reactions, oxidation number, balancing redox reactions, in terms of loss and gain of electrons and change in oxidation number, applications of redox reactions.

UNIT VIII – Organic Chemistry: Some basic Principles and Techniques

General introduction, classification and IUPAC nomenclature of organic compounds. Electronic displacements in a covalent bond: inductive effect, electromeric effect, resonance and hyper conjugation. Homolytic and heterolytic fission of a covalent bond: free radicals, carbocations, carbanions, electrophiles and nucleophiles, types of organic reactions.

UNIT IX – Hydrocarbons

Classification of Hydrocarbons
Aliphatic Hydrocarbons:
Alkanes – Nomenclature, isomerism, conformation (ethane only), physical properties, chemical reactions.
Alkenes – Nomenclature, structure of double bond (ethene), geometrical isomerism, physical properties, methods of preparation, chemical reactions: addition of hydrogen, halogen, water, hydrogen halides (Markovnikov’s addition and peroxide effect), ozonolysis, oxidation, mechanism of electrophilic addition.
Alkynes – Nomenclature, structure of triple bond (ethyne), physical properties, methods of preparation, chemical reactions: acidic character of alkynes, addition reaction of – hydrogen, halogens, hydrogen halides and water.

Aromatic Hydrocarbons:

Introduction, IUPAC nomenclature, benzene: resonance, aromaticity, chemical properties: mechanism of electrophilic substitution. Nitration, sulphonation, halogenation, Friedel Craft’s alkylation and acylation, directive influence of functional group in monosubstituted benzene. Carcinogenicity and toxicity.

To know the CBSE Syllabus for all the classes from 1 to 12, visit the Syllabus page of CBSE. Meanwhile, to get the Practical Syllabus of Class 11 Chemistry, read on to find out more about the syllabus and related information in this page.

CBSE Class 11 Chemistry Practical Syllabus with Marking Scheme

In Chemistry subject, practical also plays a vital role in improving their academic scores in the subject. The overall weightage of Chemistry practical mentioned in the CBSE Class 11 Chemistry syllabus is 30 marks. So, students must try their best to score well in practicals along with theory. It will help in increasing their overall academic score.

CBSE Class 11 Chemistry Practical Syllabus

The experiments will be conducted under the supervision of subject teacher. CBSE Chemistry Practicals is for 30 marks. This contribute to the overall practical marks for the subject.

The table below consists of evaluation scheme of practical exams.

Evaluation SchemeMarks
Volumetric Analysis08
Salt Analysis08
Content Based Experiment06
Project Work04
Class record and viva04
Total30

CBSE Syllabus for Class 11 Chemistry Practical

Micro-chemical methods are available for several of the practical experiments. Wherever possible such techniques should be used.

A. Basic Laboratory Techniques
1. Cutting glass tube and glass rod
2. Bending a glass tube
3. Drawing out a glass jet
4. Boring a cork

B. Characterization and Purification of Chemical Substances
1. Determination of melting point of an organic compound.
2. Determination of boiling point of an organic compound.
3. Crystallization of impure sample of any one of the following: Alum, Copper Sulphate, Benzoic Acid.

C. Experiments based on pH

1. Any one of the following experiments:

  • Determination of pH of some solutions obtained from fruit juices, solution of known and varied concentrations of acids, bases and salts using pH paper or universal indicator.
  • Comparing the pH of solutions of strong and weak acids of same concentration.
  • Study the pH change in the titration of a strong base using universal indicator.

2. Study the pH change by common-ion in case of weak acids and weak bases.

D. Chemical Equilibrium
One of the following experiments:

1. Study the shift in equilibrium between ferric ions and thiocyanate ions by increasing/decreasing the concentration of either of the ions.
2. Study the shift in equilibrium between [Co(H2O)6] 2+ and chloride ions by changing the concentration of either of the ions.

E. Quantitative Estimation
i. Using a mechanical balance/electronic balance.
ii. Preparation of standard solution of Oxalic acid.
iii. Determination of strength of a given solution of Sodium hydroxide by titrating it against standard solution of Oxalic acid.
iv. Preparation of standard solution of Sodium carbonate.
v. Determination of strength of a given solution of hydrochloric acid by titrating it against standard Sodium Carbonatesolution.

F. Qualitative Analysis
1) Determination of one anion and one cation in a given salt
Cations‐ Pb2+, Cu2+, As3+, Al3+, Fe3+, Mn2+, Ni2+, Zn2+, Co2+, Ca2+, Sr2+, Ba2+, Mg2+, NH4 +
Anions – (CO3)2‐ , S2‐, NO2 , SO32‐, SO2‐ , NO , Cl , Br, I‐, PO43‐ , C2O2‐ ,CH3COO
(Note: Insoluble salts excluded)

2) Detection of ‐ Nitrogen, Sulphur, Chlorine in organic compounds.

G) PROJECTS
Scientific investigations involving laboratory testing and collecting information from other sources.

A few suggested projects are as follows:

  • Checking the bacterial contamination in drinking water by testing sulphide ion
  • Study of the methods of purification of water.
  • Testing the hardness, presence of Iron, Fluoride, Chloride, etc., depending upon the regional
    variation in drinking water and study of causes of presence of these ions above permissible
    limit (if any).
  • Investigation of the foaming capacity of different washing soaps and the effect of addition of
    Sodium carbonate on it.
  • Study the acidity of different samples of tea leaves.
  • Determination of the rate of evaporation of different liquids Study the effect of acids and
    bases on the tensile strength of fibres.
  • Study of acidity of fruit and vegetable juices.

Note: Any other investigatory project, which involves about 10 periods of work, can be chosen with the approval of the teacher.

Practical Examination for Visually Impaired Students of Class 11

Below is a list of practicals for the visually impaired students.

A. List of apparatus for identification for assessment in practicals (All experiments)
Beaker, tripod stand, wire gauze, glass rod, funnel, filter paper, Bunsen burner, test tube, test tube stand,
dropper, test tube holder, ignition tube, china dish, tongs, standard flask, pipette, burette, conical flask, clamp
stand, dropper, wash bottle
• Odour detection in qualitative analysis
• Procedure/Setup of the apparatus

B. List of Experiments A. Characterization and Purification of Chemical Substances
1. Crystallization of an impure sample of any one of the following: copper sulphate, benzoic acid
B. Experiments based on pH
1. Determination of pH of some solutions obtained from fruit juices, solutions of known and varied
concentrations of acids, bases and salts using pH paper
2. Comparing the pH of solutions of strong and weak acids of same concentration.

C. Chemical Equilibrium
1. Study the shift in equilibrium between ferric ions and thiocyanate ions by increasing/decreasing
the concentration of eitherions.
2. Study the shift in equilibrium between [Co(H2O)6]2+ and chloride ions by changing the
concentration of either of the ions.

D. Quantitative estimation
1. Preparation of standard solution of oxalic acid.
2. Determination of molarity of a given solution of sodium hydroxide by titrating it against standard
solution of oxalic acid.

E. Qualitative Analysis
1. Determination of one anion and one cation in a given salt
2. Cations – NH+4
Anions – (CO3)2-, S2-, (SO3)2-, Cl-, CH3COO-
(Note: insoluble salts excluded)
3. Detection of Nitrogen in the given organic compound.
4. Detection of Halogen in the given organic compound.

Note: The above practicals may be carried out in an experiential manner rather than recording observations.

We hope students must have found this information on CBSE Syllabus useful for their studying Chemistry. Learn Maths & Science in interactive and fun loving ways with ANAND CLASSES (A School Of Competitions) App/Tablet.

Frequently Asked Questions on CBSE Class 11 Chemistry Syllabus

Q1

How many units are in the CBSE Class 11 Chemistry Syllabus?

There are 9 units in the CBSE Class 11 Chemistry Syllabus. Students can access various study materials for the chapters mentioned in this article for free at ANAND CLASSES (A School Of Competitions).

Q2

What is the total marks for practicals examination as per the CBSE Class 11 Chemistry Syllabus?

The total marks for the practicals as per the CBSE Class 11 Chemistry Syllabus is 30. It includes volumetric analysis, content-based experiment, salt analysis, class record, project work and viva.

Q3

Which chapter carries more weightage as per the CBSE Syllabus for Class 11 Chemistry?

The organic chemistry chapter carries more weightage as per the CBSE Syllabus for Class 11 Chemistry.