Limitations of Arrhenius Theory
The Arrhenius theory provides a current description and classification of acids and bases based on their solubility in aqueous solutions.
There are numerous classification theories and notions for acids and bases. Historically, substances that tasted sour (such as citric acid, acetic acid, and oxalic acid) were termed acids, and those that tasted bitter were considered bases. Later, when scientists were unable to explain the concept of acids and bases in a solvent other than water or aqueous solution, this proved to be false. Since then, however, numerous theories have been developed to explain the acidic behaviour of acids and the fundamental properties of hydroxides in terms of their ability to yield hydrogen and hydroxide ions in solutions. These include the Arrhenius theory, the Lewis Acid-Base theory, the Lowry-Bronsted Acid-Base theory, Pearson’s concept, and many others.
Arrhenius Hypothesis
Svante Arrhenius, a Swedish scientist, proposed the Arrhenius theory in 1884. Svante August Arrhenius was well-known for his contributions to the field of electrolytic dissociation and for the Arrhenius equation. In 1903, he was also awarded the Nobel Prize. On the basis of the type of ions they dissociate when dissolved in water, this hypothesis was used to classify acids and bases. If a chemical releases H+ upon dissociation, it is classified as an acid, and if it releases OH–, it is classified as a base.
Arrhenius Acid Definition
In accordance with the Arrhenius theory, an acid is a chemical that, when dissolved in water, dissociates into hydrogen ions. For instance: When hydrogen chloride gas dissolves in water, hydrogen ions are produced.
The release of hydrogen ions raises the concentration of hydrogen ions in solution and acidifies it.
Arrhenius Base Definition
According to Arrhenius, a base is a material that, when dissolved in water, produces hydroxide ions.
When sodium hydroxide dissolves in water, for instance, it releases hydroxide ion.
The release of hydroxide ion increases the solution’s concentration and renders it basic.
Benefits of the Arrhenius Theory
The multiple benefits of Arrhenius theory include the provision of knowledge about:
- Characteristics of acids and bases
- The relative acidity and basicity of acids and bases
- Neutralisation and hydrolysis as a concept
Neutralisation
When equal amounts of acid and base react, a salt is formed that is neither acidic nor basic.
HCl + NaOH -> NaCl + H2OThe Arrhenius theory, proposed in 1887 by the Swedish scientist Svante Arrhenius, states that acids are substances that dissociate in water to produce electrically charged atoms or molecules, called ions, one of which is a hydrogen ion (H+), and that bases ionise in water to produce hydroxide ions (OH).
Limitations of the Arrhenius Theory
- Acids and bases are explained by the Arrhenius hypothesis in terms of their presence in aqueous solution, not as substances. Therefore, the theory is applicable solely to the study of acids and bases in aqueous solution and is inapplicable to gaseous and non-aqueous solutions.
- The Arrhenius theory only applies to acids of the formula HA. For instance, HCl and HBr. It does not apply to SO2, CO2, or AlCl3.
- The Arrhenius theory applies solely to bases with the formula BOH. For instance, NaOH and KOH. This does not apply to Na2CO3, NH3, or pyridine. In fact, the requirement to demonstrate the relationship between base and OH ion led to the suggestion of NH4OH as the base of ammonia in water, whereas the exact formula is NH3.
- According to this idea, solvents play no role in determining the acidic or basic constitution of substances. For instance, hydrochloric acid is weak when dissolved in benzene but strong when dissolved in water.
- This theory fails to explain the characteristics of acids and bases in non-water-based solvents (such as benzene or acetone) or gases.
- According to Arrhenius, all salts should generate neither acidic nor basic solutions. Exceptions exist, however, and the Arrhenius theory fails to explain this. For instance:
Equal proportions of HCL and ammonia produce a mildly acidic solution.
Equal quantities of acetic acid and sodium hydroxide produce a mildly basic solution.
- According to this idea, hydrogen ions are released into the aqueous solution and remain there, hence raising the hydrogen ion concentration in the solution. Hydrogen cannot exist freely as hydrogen ions, hence this statement is false. Hydronium exists as the ion H3O+.
How is Hydrogen present in Water?
Hydrogen does not exist freely as hydrogen ions (H+) in water. It exists as the hydronium ion (H3O+), an aqueous cation.
Conclusion
The Arrhenius theory only applies to substances dissolved in water. Only chemicals that release hydrogen ions in aqueous solutions are considered acids, and only substances that release hydroxide ions in aqueous solutions are considered bases. The idea fails to explain the neutralisation exception. The Arrhenius concept was the very first inference to be made, and because it was conceived so long ago when no other chemistry-related ideas existed, it contained a number of errors. Since nothing was known at the time, Arrhenius based his theories on water, or rather protons, while ignoring electrons, unlike Lewis in subsequent years.
As the world around us evolves, so do natural phenomena and their underlying explanations, or science. Ideas, cognitive processes, and judgments change and evolve with time, and we develop new theories because the old ones failed to adequately describe some parts of the phenomenon. In reality, Arrhenius was awarded the Nobel Prize despite the fact that his hypothesis was deemed too particular in 1923, when the Bronsted Lowry and Lewis theory was deemed more plausible.
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Frequently Asked Questions -FAQs
What is Arrhenius theory of ionization?
The Arrhenius theory, proposed in 1887 by the Swedish scientist Svante Arrhenius, states that acids are substances that dissociate in water to produce electrically charged atoms or molecules, called ions, one of which is a hydrogen ion (H+), and that bases ionise in water to produce hydroxide ions (OH).
What is the importance of Arrhenius theory?
Even though it contained flaws, it was the first theory to give precise criteria for acids and bases. It was also the first theory on acids and bases that acknowledged the significance of solvents in neutralisation and water formation during the reaction.
What are the limitation of Arrhenius theory of electrolytic dissociation?
The Arrhenius theory applies only to aqueous solutions and not to non-aqueous or gaseous solutions, as electrolyte is defined in terms of an aqueous solution and not as a material. The nature of the strength of an electrolyte is not determined by the solvent’s function.
What is meant by degree of ionization?
The degree of dissociation (also known as the degree of ionisation) is a measure of the acid’s strength. It is the ratio between the number of ionised molecules and the number of dissolved molecules in water.
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