Important topics for JEE mains : Acids and bases

important topics for JEE mains

Lets start wit Acids and bases (Bronsted and Lewis concepts); Hydrolysis of salts. It is one of Important topics for JEE mains.

Acids and Bases

 

In 1923 the Danish chemist Johannes Brønsted and, in parallel, the British chemist Thomas Lowry developed a theory. This theory states that acids are substances capable of yielding protons (hydrogen ions H +) and the bases substances capable of accepting them. The presence of hydrogen in the acid is still contemplated, as in Arrhenius theory, but an aqueous medium is no longer needed.

The concept of acid and base of Brønsted and Lowry helps to understand why a strong acid displaces another weak acid from its compounds (the same occurs between a strong base and a weak one). Acid-base reactions are seen as a competition for protons. In the form of a chemical equation, the following reaction of Acid (1) with Base (2):

Acid (1) + Base (2) ⇋ Acid (2) + Base (1)

important topics for JEE mains

It is produced by transferring a proton the Acid (1) to the Base (2). When losing the proton, the Acid (1) becomes its conjugate base, Base (1). By winning the proton, the Base (2) becomes its conjugate acid, Acid (2). The equation described constitutes a balance that can be moved to the right or to the left. HCl is a strong acid in water because it easily transfers a proton to water forming a hydronium ion (H3O +):

HCl + H2O → H3O + + Cl-

Acid (1) Base (2) Acid (2) Base (1)

strong weak strong weak

We see that, the stronger the acid is compared to another chemical species, the weaker its conjugate base.

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The Brønsted-Lowry acid-base concept allows establishing a table of ordered acid / base conjugate pairs.

Acid

HCl

HNO3

H2 SO4

HSO 4–

H 2 CO 3

CH 3 COOH

H 2 O

OH –

H 3 O +

H 2 S

HS –

NH 3

NH 4 +

Base

Cl –

NO 3 –

HSO 4–

SO 42–

HCO 3 –

CH 3 COO –

OH –

O 2–

H 2 O

HS –

S 2–

NH 2 –

NH 3

The Brønsted definition does not make specific mention of the solvent and can be applied to solvents

not watery like ammonia. For the importance of water as a solvent in inorganic reactions, a

Then the Brønsted acid-base processes in water are discussed.

Processes: Acids and bases in water solution

The basic concepts of the Brønsted acid-base concept on the previous definition of Arrhenius are:

• highlights the fundamental role played by the solvent in acid-base reactions

• Simplifies the study of acid-base reactions in water by unifying your treatment. While the

kinds of reactions that we are going to see were different in the Arrhenius concept, in Brønsted’s, all of them can be considered as reactions of the type:

acid 1 + base 2 –> conjugate base of 1 + conjugate acid of 2

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The hydrogen ion in water

The nature of the hydrogen ion in water is complex. Due to its high power of polarization, some of the water molecules that surround it are linked to it and not only by forces of Van de Waals but by a covalent bond. The hydrogen ion in water is usually represented as H + (aq), ignoring the real nature of the species existing in solution, or as H 3 O + (aq), representation that is not more accurate than the previous one (the H + is actually linked to more than one

water molecule), has the advantage of highlighting the participation of water in processes

acid-base:

HCl (aq) –> H+ (aq) + Cl (aq)

HCl(aq) + H 2 O(l) –> H 3 O + (aq) + Cl (aq)

Important topics for JEE mains : Water autoionization

The ionization of water is nothing more than an acid-base transfer process of a proton from one water molecule to another:

H 2 O (l) + H 2 O (l) ⇋H 3 O + (aq) + OH (aq)

At equilibrium, [H 3 O + ] [ OH ] = K w, where K w is a constant called ionization constant or

ionic product of water, and whose value at 25 ° C is 1.0 10 -14 mol 2 l -2.

Pure water and aqueous solutions that hold [H +] = [OH ], are said to be neutral. Aqueous solution is acidic when [H +]> [OH ]. An aqueous solution is basic when [H +] <[OH ].

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Ionization of an acid or a base

An acid can give its proton to a molecule of water, ionizing itself. The ionization of a base can be produced by capturing a proton from a molecule of Water. The ionization equilibria of an acid or a base are characterized by a constant equilibrium called acid or basic ionization constant (K a or K b):

HA(aq)+ H 2 O(l) ⇋ H 3 O + (aq) + A (aq)

K a = [A ][H + 3 O] / [HA]

B(aq) + H 2 O(l) ⇋ BH + (aq) + OH (aq)

K b = [BH + ][OH ] / [B]

Acids and bases can be classified as strong and weak. They are strong acids and bases those for which, in ordinary concentrations, practically all their molecules are dissociated.

Weak acids and bases have small ionization constants, so when they dissolve with ordinary concentrations in water, a large part of its molecules remain undissociated. Some acids can give more than one proton. These acids are called polyprotic acids. Each dissociation of each of the protons has its own dissociation constant, observing that its value decreases as protons are lost: K a 1> K a 2> K a 3

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Important topics for JEE mains : Hydrolysis of salts

The anion from the ionization of an acid is a base (conjugate base). The reaction of an anion with water is called basic hydrolysis of the anion.

HCN (aq) + H 2 O (l) ⇋ H 3 O + (aq) + CN (aq)

CN (aq) + H 2 O (l) ⇋ HCN (aq) + OH (aq)

The cation from the ionization of a base is an acid (conjugate acid). The reaction of the cation with water it is called acid hydrolysis of the cation.

NH 3 (aq) + H 2 O (l) ⇋NH 4+ (aq) + OH (aq)

NH 4+ (aq) + H 2 O (l) ⇋NH 3 (aq) + H 3 O + (aq)

It is easy to understand that “the stronger an acid is, the weaker its conjugate base“. The bases or conjugated acids of strong acids or bases do not undergo hydrolysis:

Cl (aq) + H 2 O (l) ← HCl (aq) + OH (aq)

HCl (aq) + H 2 O (l) → H 3 O + (aq) + Cl (aq)

Na + (aq) + 2H 2 O (l) ← NaOH (aq) + H 3 O + (aq)

NaOH (aq) → Na + (aq) + OH (aq)

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Salts containing a cation or conjugated anion of a weak acid, will give non-neutral solutions when dissolved in water:

NaCl (s) → Na + (aq) + Cl (aq)

There is no hydrolysis. Neutral dissolution.

 

NH 4 Cl (s)→ NH 4+ (aq) + Cl (aq)

 

NH 4+ (aq) + H 2 O (l) ⇋ NH 3 (aq) + H 3 O + (aq)

 

 

Acid hydrolysis. Acid dissolution.

 

NaAc (s) →Na+ (aq) + Ac (aq)

 

Ac (aq) + H 2 O (l) ⇋ HAc (aq) + OH (aq)

 

 

Basic hydrolysis. Basic dissolution.

 

NH 4 Ac (s) →NH 4 + (aq) + Ac (aq)

 

NH 4+ (aq) + H 2 O (l) ⇋ NH 3 (aq) + H 3 O + (aq)

 

 

Acid hydrolysis.

 

Ac(aq) + H 2 O (l) ⇋ HAc (aq) + OH (aq)

 

 

Basic hydrolysis.

There is enough theory in Blog post. If you have still doubts please refer to this book.

Best of luck for IIT JEE preparation. Keep reading the blog !

Cheers 🙂

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