Arrhenius Theory

Acid and base were first defined by Svante Arrhenius in 1884, using the ionisation theory. An acid is a hydrogen-containing chemical that releases H+ ions or protons when dissociated in water; a base is a hydroxide compound that releases OH- ions when dissociated in water. Aqueous solutions are the only ones where this principle is valid. Many common acids and bases are covered, as are their chemical reactions, although the Arrhenius principle does not apply to all molecules that have acidic or basic properties.

Arrhenius Theory of Acid and Base

Hydrogen (H+) is one of the ions formed when an acid dissociates in water, according to the 1887 theory proposed by Arrhenius, while in bases , OH ions are formed when a base ionises in water, according to the theory. The hydronium ion (H3O+) is an ion formed when a hydrogen ion and a water molecule join to form a water molecule in solution. Hydronium ions are still commonly referred to as hydrogen ions in practice.

One way to understand the basic properties of common hydroxides and acids is to look at their ability to produce hydrogen in the presence of other hydroxides ( for example, sodium, potassium, and calcium), as well as the ability to produce hydrogen ions from the presence of other hydroxides (for example. sulfuric and hydrochloric acids). It is also possible to classify acids and bases as “strong” or “weak” based on how much hydrogen or hydroxide ion is created in solution. When an acid and a base react, a salt is formed, and water is formed when a hydrogen and a hydroxide ion combine.

Arrhenius Acids

Based on the type of ions that formed when the molecule was added to water, he proposed designating certain substances as acids or bases.

A chemical compound that raises the concentration of H+ ions, or protons, in an aqueous solution is an Arrhenius acid. We might think of hydrochloric acid (HCl) in water as an example of this:

HCl(aq)→ H+(aq)+ Cl-(aq)

H+ and Cl- ions are formed when hydrochloric acid (HCl) is dissociated in water to form an aqueous solution. As a result, hydrochloric acid is classified as an Arrhenious acid because of this.

Let’s imagine that we prepared a 2M aqueous solution of hydrobromic acid, HBr, an Arrhenius acid. We have 2M H+ ions in our solution, right?

Truthfully, no. Water molecules react with positively charged protons to generate hydronium ions, H3O+, in actuality. 

In this case, H+(aq) + H2O(l) → H3O+(aq)

There are no free H+ ions in an aqueous solution, despite the fact that we describe acid dissociation reactions demonstrating the creation of H+(aq). When an acid dissociates in water, H3O+ ions are formed, which are the majority of the ions in the solution. With the use of chemical models, we can see how H2O and hydrogen ions can combine to produce hydronium:

The concentration of H+ and the concentration of H3O+ are frequently used interchangeably by chemists. The dissociation of hydrobromic acid can be written to explicitly show the creation of hydronium, rather than protons: 

HBr(aq) → H+(aq) + Br-(aq)

Arrhenius Bases

If an Arrhenius base raises the concentration of OH- ions in an aqueous solution, then it qualifies as an Arrhenius base. A prime illustration of an Arrhenius base can be found in sodium hydroxide, NaOH. This is how water reacts with sodium hydroxide:

To put it another way, NaOH(aq) is Na+(aq) + Na+(aq) + OH- (aq)

After dissolving into OH- and Na+ in water, sodium hydroxide is found to be more concentrated than previously thought. As a result, NaOH is classified as an Arrhenius compound. In addition to LiOH and Ba(OH)2, various hydroxides from Groups 1 and 2 are frequently used as Arrhenius bases.

Non-hydroxide-containing bases may or may not be categorised as Arrhenius bases. An Arrhenius base can be defined as any chemical compound with at least one unit of the anion hydroxide (OH-) in the formula. While this does not alter the classification of Group 1 and Group 2 hydroxides, molecules like methylamine and CH3NH2 can become quite perplexing.

In the presence of methylamine, this process takes place as follows:

Chemically, CH3NH2(aq) + H2O(l) yields CH3NH3+(aq),OH-(aq)

Since the concentration of OH- ions in the solution rises with increasing concentration of methylamine, it would be an Arrhenius base under our initial definition. The second definition, on the other hand, excludes it as an Arrhenius base due to the absence of hydroxide in the chemical formula.

Conclusion

Any species of Arrhenius acid that raises the concentration of H+ in an aqueous solution is considered an Arrhenius acid. Any species that raises the concentration of OH- in an aqueous solution is referred to as an Arrhenius base. Hydronium ions and H3O+ are formed when H+ ions combine with water molecules in an aqueous solution. An Arrhenius acid and base react to generate water and a salt in an acid-base or neutralisation process.