Arrhenius Acid

You must familiar with automobile battery which contains sulphuric acid is a key part of acid rain. Household drain cleaners often contain strong bases which are very basic and reactive chemicals. Acid and base are capable of causing severe “burns” similar to burns produced by heat. No doubt both acid and base play an important role in our lives but we have to use them very carefully as they are corrosive poisons which react locally on tissue cells.

The eyes and lungs are mainly sensitive to corrosive poisons. The eye cornea could also be damaged by acid or base burns. Air pollutants such as sulphur oxides, nitrogen oxides, chlorine, and ammonia all have corrosive effects on the respiratory tract. They mainly affect the lining of the nose, sinuses and lungs. The victim may die off due to suffocation or suffer permanent lung damage.

How can you differentiate between acids and bases? Today we know acids are compounds which can give hydrogen ions and bases are organic compounds which can give hydroxyl ions. There are several theories to explain the concept of acid and base.

Arrhenius Acid Definition

According to the Arrhenius theory, a substance which has hydrogen atom and can easily give hydrogen ion or proton in its aqueous solution is called as Arrhenius acid. For example, when hydrochloric acid is dissolved in water, it forms chloride ion (Cl) and hydronium ion (H3O+). In the same way, acetic acid (CH3COOH) also acts as an Arrhenius acid in its aqueous solution, as it forms acetate ion (CH3COO) and the hydronium ion.

What is an Acid according to Arrhenius?

Those compounds, which can give proton in their aqueous solution, are known as acids. Or it is a substance that when added to water, increases the concentration of protons in the solution.

Some other examples of Arrhenius acids are Hydrofluoric acid (HF) , Nitric acid (HNO3), Hydrobromic acid (HBr) , Sulphuric acid (H2SO4), Sulphurous acid (H2SO3) , Perchloric acid (HClO4), Phosphoric acid (H3PO4) , Hydroiodic acid (HI), Carbonic acid (H2CO3) etc.

Basicity of Arrhenius acids

Basicity of an Arrhenius acid is termed as the number of replaceable hydrogen ions present in an acid. For example, Phosphoric acid (H3PO4) ionizes in its aqueous solution and gives three hydrogen ions, hence the basicity of Phosphoric acid is three.

H3PO4 → H+ + H2PO4

H3PO4 → H+ + HPO42-

HPO42- → H+ + PO43-

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)

Arrhenius Acid as Strong Electrolytes

Arrhenius who was trying to discover why certain solutions could conduct an electric current found that conductivity arose from the presence of ions. He observed that when the substances HCl, HNO3 and H2SO4 were dissolved in water they behave as strong electrolytes. This was the result of ionization reactions in water.

As these substances are strong electrolytes that produce H+ ions, they are called strong acids. This was the result of ionization reactions in water.

Studies show that when HCl, HNO3 and H2SO4 are placed in water, virtually every molecule dissolves to give ions. This means that when 100 molecules of HCl are dissolved in water, 100 H+ ions and 100 Cl ions are produced. Virtually, no HCl molecules exist in aqueous solution because these substances are strong electrolytes that produce H+ ions and are called strong acids.

1. Strong acid

Strong acids get completely ionized in an aqueous solution and increase the number of protons (H+) in the solution.

HA ↔ H+ + A

The equilibrium constant for the dissociation of an acid is called acid dissociation constant (Ka). The magnitude of Ka is very high for strong acids. Hence the strength of an acid is directly proportional to the acid dissociation constant (Ka).

2. Weak Acid

Weak acids due to their partial or incomplete dissociation do not release hydrogen ions and thus exist as an equilibrium mixture of undissociated acid along with the ions released during the partial dissociation. The number of hydrogen ions is very low and they show a high pH value, compared to strong acids. The acid dissociation constant is less for weak acids compared to strong acids.

List of Arrhenius Acids

Arrhenius acid-base theory is the most restrictive theory requiring the compound to have an H+ or OH and to be soluble in water.

The list of Arrhenius acids is given below:

Acid Formula Acid in aqueous solution
Perchloric HClO4 HClO4 + H2O → H3O+ + ClO4
Hydroiodic HI HI + H2O → H3O+ + I
Hydrobromic HBr HBr + H2O → H3O+ + Br
Hydrochloric HCl HCl + H2O → H3O+ + Cl
Nitric HNO3 HNO3 + H2O → H3O+ + NO3
Sulfuric H2SO4 H2SO4 + 2H2O → 2H3O+ + SO42-
Iodic HIO3 HIO4 + H2O → H3O+ + IO3-
Oxalic H2C2O4 H2C2O4 + 2H2O → 2H3O+ + C2O4

When Arrhenius acids are in pure state they are covalent compounds; that is, they do not contain H+ ions. The ion is formed through the interaction between water and the acid when they are mixed.

Ionization is the process in which the individual positive and negative ions are produced from a molecular compound which is dissolved in the solution.

The reactions of these acids are as follows.

HCl (gas) → H+(aq) + Cl(aq)

HNO3 (liq) → H+(aq) + NO3(aq)

H2SO4 (liq) → 2H+(aq) + SO42- (aq)