ACID-BASE TITRATION: Principle, Theory, Limitations and MCQ for GPAT, GATE and CSIR NET JRF

Basic term :-

• Standard solution:- A reagent of a known concentration which used in the titrimetric analysis
• Titration:- The process of adding a standard solution from burette or other liquid-dispensing device to an analyte (in the form of solution) the reaction is believe to be completed
• Equivalence point:- The point in titration process when the amount of titrant is chemically equivalent is the amount of analyte in a sample.
• Back-titration:- The process of adding excess amount of standard titrant, and the excess is determined by back titration using second standard The equivalence point corresponds to the amount of initial titrant is chemically equivalent to the amount of analyte plus the amount of back titrant.
•  End point:- Physical change occurs which related to the chemical change.

ACID BASE TITRATION :-

•  Acid-base titration is a process for calculating the concentration of a known volume of acid or base.
• Acid -base titration is used for analysing the unkonwn concentration of acid and base quantitatively.

PRINCIPLE :-

The principle of acid & base titration is based on neutralization reaction which occur between acid and base.

ACID + BASE ⇒ SALT + WATER 

Phenolphthalein is the most commonly used indicator for acid base titration.

THEORIES OF ACID AND BASES:-

1. Arrhenius theory
2. Bronsted lowry
3. Lewis concept

1. Arrhenius concept of acid and base 

According to arrhenius an acid is any substance which is capable of providing hydrogen ions in aqueous solutions and base is defined as a substance containing hydroxy group and/ or capable of providing hydroxide ions in aqueous solutions.

HA    + BOH   →     B⁺ A^–+ H₂O

(ACID) (BASE)   (SALT) (WATER)

In Arrhenius acid base reaction a salt and water is formed from the reaction between an acid and a base. In other words this is a neutralization reaction.

H₂SO₄ + NaOH    →   Na₂SO₄ + 2H₂O

Limitations :-

1. The definition of acid and base are only in term of aqueous solutions and not in terms of  substance.
2. The theory is not able to explain acidic and basic properties of substance in non aqueous solvent eg,NH4NO3 in liquid NH3 acts as an acid though it dose not give H⁺ions.
3. The neutralization of acid and base in absence of solvent could not be explained.
4. The basic substances which do not contain hydroxide ion could not be explained by the theory. Many organic substance and NH3 which do not have OH^– ions at all are actually known to shows basic character.
5. It cannot explained the acidic character of certain salts such as ALCL3 in aqueous solutions.

Bronsted-Lowry concept

This concept is based upon the idea of protonation of  bases through deprotonation of acids. According to this concept acid and base can be defined as :-

ACIDS :- Is any substance capable of donating a proton (H⁺) in a chemical reaction.

BASE :- Is any substance capable of accepting a proton (H⁺) in a chemical reaction.

Limitations :-

1. It is based on proton transfer. Although it is true that most common acids are protonic in nature yet there are many which are not.
2. A large number of acid-base reaction are known in which no proton tranfer can take place.

SO₂ + SO_{2    ⇒}  SO⁺²  + SO₃^-2

Thus the protonic definition cannot be used to explained the reaction occuring in non-protonic solvents such as COCl2,SO2, etc.

Lewis concept 

Any substance which can accept a share in a pair of electrons in a reaction is called an acid and a base is any substance which can donate a pair of electrons to share with an acid in a reaction.

LIMITATIONS 

The lewis concept have some limitations these are :-

1. Since the strength of lewis acids and bases is found to depends on type of reaction, it is not possible to arrange them in any order of their strength.
2. As lewis acid base reaction involves electrons they are expected to be very fast reaction. However, there are many lewis acid base reaction which are slow.

Law of mass action :- The rate of a chemical reaction is proportional to the active masses of the reacting substance.

A+B ⇔ C+D

K = [C][D]/[A][B]

Where K = Equilibrium constant of the reaction.

MCQ 

1.What is a Equivalence point ?

a.  Amount of titrant  = Amount of analyte in a sample.

b.Amount of titrant  > Amount of analyte in a sample.

c.Amount of titrant  < Amount of analyte in a sample.

d. None of the above

2. According to arrhenius concept which sentence is true ?

a. Proton acceptor is base.

b.Electron pair acceptor is acid.

c. Electron pair donor is base.

d. Hydroxide donor is base.

3. What is law of mass action ?

a.The rate of a chemical reaction is proportional to the active masses of the reacting substance.

b.The rate of a chemical reaction is inversely proportional to the active masses of the reacting substance.

c.a and b

d. None of the above

4.what is titrant ?

a. A solutions of one reactant of accurately known concentration.

b. A solutions of one reactant of unknown concentration.

c. a and b

d. none of the above

5. According to lewi’s concept which sentence is true ?

a.Proton donor is acid.

b. Proton acceptor is base.

c.Hydroxide donor is base.

d. Electron pair acceptor is acid.

6.What is principle of acid base titration ?

a. Neturalization

b. Alkalimetry

c. a and b

d.None of the above

7. Which is not limitation of arrhenius concept ?

a. The theory is not able to explain acidic and basic properties of substance in non aqueous solvent eg,NH4NO3 in liquid NH3 acts as an acid though it dose not give H⁺ions.

b.It cannot explained the acidic character of certain salts such as ALCL3 in aqueous solutions.

c.The definition of acid and base are only in term of aqueous solutions and not in terms of  substance.

d. Proton donor is acid.

8.What is the end point ?

a.Physical change occurs which related to the chemical change.

b.A reagent of a known concentration which used in the titrimetric analysis.

c.A solutions of one reactant of accurately known concentration.

d. Amount of titrant  = Amount of analyte in a sample.

ANSWER KEY 

1.A

2.D

3.A

4.A

5.D

6.A

7.D

8.A

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REFERENCE :-

VOGEL′S TEXT BOOK OF QUANTITATIVE CHEMICAL ANALYSIS FIFTH EDITION BY G.H.JEFERY, J.BASSETT (PG.NO. 262-268)