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# Arrhenius Theory

All acids characterised by H+ in solution and base by hydroxide ion OH-. The best way to identify the presence of acid and base is to use litmus paper which shows a color change in acidic and basic medium. The Red litmus paper turns blue if exposed to an acid, and blue litmus paper turns red when a base is present. Other natural detectors, besides taste, are red onions, red cabbage and grape juice. In very acidic conditions, the colour is red. In alkaline solutions, it will be gone from blue to green.

Red cabbage juice is a purple color solution that changes to red in acidic medium and yellow in basic medium. There are various acidic substances which we use in our daily life such as citrus fruits like orange, lemon etc. Lactic acid is present in milk and curd whereas vinegar is a dilute solution of acetic acid. Similarly many bases are used in different household items like detergents, soaps, cleaning agents etc. The tablets of antacids are also manufactured by magnesium hydroxide which is a basic compound. Several theories were proposed by different scientists for the complete explanation of acids and base. Let’s discuss the first theory of acid and base that is called as Arrhenius theory.

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## Arrhenius Definition

According to Arrhenius theory, an acid is a substance which has a hydrogen atom and can be given in the form of hydrogen ion in aqueous solution. Such substances are called as Arrhenius acids. For example, when acetic acid (CH3COOH) dissolves in water, it will form acetate ion (CH3COO-) and hydronium ion (H3O+).

In the same way, HCl acts as an Arrhenius acid in water and it converts to Cl- ion by transferring hydrogen ion to water.

## Arrhenius Theory of Acids and Bases

Arrhenius defines acids as hydrogen containing compounds which ionize in water to give protons(H+). Two common examples for Arrhenius acids are HNO3 (Nitric acid) and HCl (Hydrochloric acid).
HNO3(lq) H+(aq) + NO3-(aq)
HCl(g) H+(aq)+ Cl-(aq)
When Arrhenius acids are in the pure state (not in solution) they are covalent compounds, that is, they do not contain H+ ions.

The ions are formed through an interaction between water and the acid when they are mixed. Ionization is the process in which individual positive and negative ions are produced from a molecular compound that is dissolved in solution.

## Arrhenius Base

According to Arrhenius acid-base theory, those substances, which can produce hydroxyl ion in their aqueous solution, are called as Arrhenius base. For example,
NaOH(aq) Na+ + OH- produces OH- in water
Some other examples of Arrhenius bases are KOH, Ca(OH)2 etc.
Those compounds which do not have hydroxyl ion cannot be basic in nature. For example, Ammonia is not a base according to the Arrhenius acid-base theory. Since there is no hydroxyl group present in the ammonia (NH3) molecule, so, it should be basic.

But ammonia shows basic nature. This can be explained by using the concept of aqueous solution. Since ammonia forms ammonium ion and hydroxyl ion in its aqueous solution hence it is ammonia base.
NH3(aq) NH4+ + OH- produces OH- in water

### Strength of Arrhenius acids and base

On the basis of ionization of acid and base, they can be classified into two types:
Strong acid: Those acids, which are completely ionized and give maximum number of protons (H+) in a solution are known as strong acids.

The value of acid dissociation constant for strong acids (Ka) is very high. Hence, the strength of the acid is directly proportional to the acid dissociation constant (Ka).

### Strength of Acid and Acid dissociation constant

Weak acids are partially ionized in solution, like, acetic acid, hydrofluoric acid etc. The acid dissociation constant is less for weak acids compared to strong acids.

 Acid Formula Ka Perchlorate HClO4 Very large Hydrochloric HI Very large Hydrophobic HBr Very large Hydrochloric HCl Very large Nitric HNO3 Very large Sulfuric H2SO4 Very large Hydroponic ion H3O+ 1.0 Ionic HIO3 1.7 x 10-1 Oxalic H2C2O4 5.9 x 10-2 Sulfurous H2SO3 1.5 x 10-2 Hydrogen sulfate ion HSO4- 1.2 x 10-2 Phosphoric H3PO4 7.5 x 10-3 Citric H3C6H5O7 7.1 x 10-4 Nitrous HNO2 4.6 x 10-4 Hydrofluoric HF 3.5 x 10-4 Formic HCOOH 1.8 x 10-4 Cenozoic C6H5COOH 6.5 x 10-5 Acetic CH3COOH 1.8 x 10-5 Carbonic H2CO3 4.3 x 10-7 Hydrogen sulfate ion HSO3- 1.0 x 10-7 Hydrogen sulfide H2S 9.1 x 10-8 Hydrochloric HClO 3.0 x 10-8 Di hydrogen phosphate ion H2PO4- 6.2 x 10-8 Boris H3BO3 7.3 x 10-10 Ammonium ion NH4+ 5.6 x 10-10 Hydroponic HCN 4.9 x 10-10 Phenol C6H5OH 1.3 x 10-10 Hydrogen carbonate ion HCO3- 5.6 x 10-11 Hydrogen peroxide H2O2 2.4 x 10-12 Mono hydrogen phosphate ion HPO42- 2.2 x 10-13 Water H2O 1.0 x 10-14

Arrhenius strong bases show complete ionization in aqueous solution and a high value of base dissociation constant. But weak bases show low values of base dissociation constant.

 Base Formula Kb Ammonia NH3 4.75 Aniline C6H5NH2 9.37 Codeine C18H21O3N 6.05 Diethylamine (C2H5)2NH 4.51 Dimethylamine (CH3)NH 3.23 Ethylamine C2H5NH2 3.36 Hydrazine N2H4 5.77 Hydroxylamine HONH2 9.04 Methylamine CH3NH2 3.38 Morphine C17H19O3N 6.13 Piperidine C5H5N 2.88 Pyridine C5H5N 8.70 Quinoline C9H7N 9.20 Triethanlamine C6H15O3N 6.24 Triethylamine (C2H5)3N 3.28 Trimethylamine (CH3)3N 4.20

### Acidity and Basicity

The number of replaceable hydrogen ions in an acid is termed as the basicity of the respective acids. For example, Sulfuric acid (H2SO4) when ionized in its aqueous solution, gives two hydrogen ions, hence the basicity of sulfuric acid is two.

Some other examples are as follows,
HCl , HNO3 , HClO4 , H3PO2 , H3BO3→ Monobasic acid (basicity=1)
(COOH)2 , H2SO4 , H2SO3 , H3PO3 → Dibasic acid (Basicity =2)
H3PO4 → Tribasic acid (Basicity=3)

The number of replaceable hydroxyl ions in a base is known as the acidity of that base. On the basis of acidity, bases can be monoacidic, diacidic, triacidic and so on.
For example,
NaOH , KOH , LiOH , NH4OH → Monoacidic (Acidity=1)
Ca(OH)2 , Ba(OH)2 , Sr(OH)2 → Diacidic (Acidity=2)
Al(OH)3 → Triacidic (Acidity=3)
Neutralization reaction

Arrhenius successfully explained the neutralization reaction of acids and bases. Acids and bases react together and form salt and water. This reaction is known as neutralization reaction.

For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water.
HCl + NaOH NaCl + H2O
The acidic or basic nature of the product (salt) depends upon the strength of the acid and base involved in the reaction.

Some common salts are as follows:

 Acid Base Salt Nitric acid Sodium hydroxide Sodium nitrate Sulfuric acid Potassium hydroxide Potassium sulphate Hydrochloric acid Ammonium hydroxide Ammonium chloride Acetic acid Ammonium hydroxide Ammonium acetate

## Arrhenius Theory of Ionization

In 1877, Arrhenius put forth his theory of electrolytic dissociation or ionization. He formulated the hypothesis of ionization or electrolytic dissociation to account for the strange behavior of acids, bases and salts.

What are symptoms of Arrhenius theory of dissociation?

The salient features of the Arrhenius theory of dissociation are as follows.
• When electrolyte molecules are dissolved in water, they split into charged particles called ions. The positively charged ion is called cation and the negatively charged ion is called anion.
• Ions present in the solution can reunite to give the parent electrolyte molecules. There is a state of equilibrium between ions and electrolyte.

AB A+ + B-
(Electrolyte) (Cation) (Anion)

• The total charge on cation is equal to the total charge on anion. Thus the electrolytic
solution maintains electrical neutrality.
• During electrolysis, the cation moves towards the cathode and anion moves towards the anode and discharge their charge at the respective electrodes.
• The electrical conductivity depends on the number of ions present in the solution.
• Ions act like molecules in depressing freezing point and exhibit other colligative properties.
• The ratio of concentration of dissociated molecules to that of the total number is termed as degree of dissociation. Degree of dissociation increases with increase in dilution.
• At infinite dilution the electrolyte is totally dissociated into ions and therefore the equivalent conductivity reaches a limiting value.
• The degree of dissociation ($\alpha$) is related to the equivalent conductivity through the expression. $\alpha=\frac{\Lambda_v}{\Lambda_{\infty}} =\frac{equivalent\ conductivity\ of\ solution\ at\ any\ dilution}{equivalent\ conductivity\ of\ solution\ at\ infinite\ dilution}$

## Drawbacks of the Arrhenius Theory

Although Arrhenius theory was successful in introducing a new concept of ions, it was soon discovered that there are many limitations while applying it for strong electrolytes and it left many observations unanswered. Some of the drawbacks are mentioned below.
1. The degree of dissociation of strong electrolytes obtained from conducto-metric measurements was found to be different from the values obtained from colligative properties.
2. Arrhenius theory assumes ionization of electrolytes only in solution phase, but x-ray diffraction studies of crystalline alkali halides unambiguously prove that they exist in ionized forms even in the solid state.
3. The molten salts were found to conduct electricity and undergo electrolysis. Arrhenius theory could provide a satisfactory explanation for this conduction but failed to answer what transpires in water.
4. Arrhenius theory could not explain the variation of ionic conductivity with concentration of the solution.
5. This theory also fails to explain the variation of equivalent conductance with dilution in the case of NaCl, which exists in full ionized form even in the solid state.

## Drawbacks of the Arrhenius Theory

Although Arrhenius theory was successful in introducing a new concept of ions, it was soon discovered that there are many limitations while applying it for strong electrolytes and it left many observations unanswered. Some of the drawbacks are mentioned below.
1. The degree of dissociation of strong electrolytes obtained from conducto-metric measurements was found to be different from the values obtained from colligative properties.
2. Arrhenius theory assumes ionization of electrolytes only in solution phase, but x-ray diffraction studies of crystalline alkali halides unambiguously prove that they exist in ionized forms even in the solid state.
3. The molten salts were found to conduct electricity and undergo electrolysis. Arrhenius theory could provide a satisfactory explanation for this conduction but failed to answer what transpires in water.
4. Arrhenius theory could not explain the variation of ionic conductivity with concentration of the solution.
5. This theory also fails to explain the variation of equivalent conductance with dilution in the case of NaCl, which exists in full ionized form even in the solid state.
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