Grade 11

Grade 11BalanceAcid and Base Theory


Arrhenius concept


The Arrhenius concept of acids and bases is one of the fundamental theories in chemistry that describes how acids and bases behave in aqueous solution. This theory was proposed by Swedish chemist Svante Arrhenius in 1887. His work laid the groundwork for understanding the nature of acids and bases. According to Arrhenius, there are special definitions for the structure of acids and bases, based on how they interact with water.

Understanding the Arrhenius definition of acids and bases

In the Arrhenius concept, an acid is defined as a substance that increases the concentration of hydrogen ions (H +) in an aqueous solution. When an acid dissolves in water, it donates H + ions to the solution.

A common example of an Arrhenius acid is hydrochloric acid (HCl). When HCl is dissolved in water, it dissociates to form hydrogen ions:

HCl (aq) → H + (aq) + Cl - (aq)

In contrast, an Arrhenius base is defined as a substance that increases the concentration of hydroxide ions (OH -) in an aqueous solution. When a base dissolves in water, it donates OH - ions to the solution.

A typical example of an Arrhenius base is sodium hydroxide (NaOH). When NaOH is dissolved in water, it decomposes as follows:

NaOH (aq) → Na + (aq) + OH - (aq)

The role of water

In the Arrhenius concept, water plays an important role as a solvent, providing a medium for acids and bases to dissociate and form ions. The presence of water allows acids to dissociate into H + ions and bases into OH- ions, which characterizes their acidic or basic nature.

Limitations of the Arrhenius concept

While the Arrhenius concept provides a basic understanding of acid and base behavior, it comes with some limitations. One major limitation is that it only applies to aqueous solutions. This means that acids and bases that do not dissociate or form ions in water cannot be effectively described by this theory.

Another limitation is that this concept does not take into account acid-base reactions that do not involve the formation of water. For example, the reaction between ammonia (NH 3) and hydrogen chloride gas (HCl) results in the formation of ammonium chloride (NH 4Cl), but this reaction occurs without water as a solvent:

NH 3 (g) + HCl (g) → NH 4Cl (s)

Concept of Arrhenius

To better understand the Arrhenius concept, we can imagine the dissociation of an acid such as hydrochloric acid in water:

HCl H + CL - Separation

Example: Comparison of different acids and bases

Now, let's look at some examples and understand how different acids and bases behave according to the Arrhenius definition:

Acid

  • Sulfuric acid (H 2SO 4): When sulfuric acid is dissolved in water, it dissociates to release two hydrogen ions:
    H 2SO 4 (aq) → 2H + (aq) + SO 4 2- (aq)
  • Nitric acid (HNO 3): When nitric acid dissolves in water, it dissociates to yield hydrogen ions and nitrate ions:
    HNO 3 (aq) → H + (aq) + NO 3 - (aq)
  • Acetic acid (CH 3COOH): Acetic acid, when dissolved in water, partially dissociates to release hydrogen ions:
    CH 3COOH (aq) ⇌ H + (aq) + CH 3COO - (aq)

Bases

  • Potassium hydroxide (KOH): Potassium hydroxide dissociates in water to release hydroxide ions:
    KOH (aq) → K + (aq) + OH - (aq)
  • Calcium hydroxide (Ca(OH) 2): Calcium hydroxide dissociates in water to form two hydroxide ions:
    Ca(OH) 2 (aq) → Ca 2+ (aq) + 2OH - (aq)
  • Ammonium hydroxide (NH 4OH): Ammonium hydroxide is a weak base that dissociates slightly in water:
    NH 4OH (aq) ⇌ NH 4 + (aq) + OH - (aq)

Conclusion

The Arrhenius concept provides a fundamental understanding of how acids and bases form by emphasizing their behavior in water. While this theory is limited to aqueous solutions and does not explain all acid-base reactions, it is an important stepping stone to the more comprehensive theories used by chemists today. This concept provides an accessible introduction to the dynamic world of acid-base chemistry, setting the stage for further discovery and study.


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