The Arrhenius theory of acids and bases was proposed by Swedish chemist Svante Arrhenius in 1884. According to this theory, an acid is a substance that dissociates in water to produce hydrogen ions (H⁺), and a base is a substance that dissociates in water to produce hydroxide ions (OH⁻). This theory is specifically applicable to aqueous solutions.
An acid is a substance that ionizes in water to release hydrogen ions (H⁺). The higher the concentration of H⁺ ions, the stronger the acid.
HA (acid) ⇌ H⁺ (aq) + A⁻ (aq)
Examples: HCl, H₂SO₄, CH₃COOH.
A base is a substance that ionizes in water to release hydroxide ions (OH⁻). The higher the concentration of OH⁻ ions, the stronger the base.
BOH (base) ⇌ B⁺ (aq) + OH⁻ (aq)
Examples: NaOH, KOH, Ca(OH)₂.
When an acid reacts with a base, they undergo a neutralization reaction, resulting in the formation of water and a salt.
HCl (aq) + NaOH (aq) → H₂O (l) + NaCl (aq)
The Bronsted-Lowry theory of acids and bases, proposed by Danish chemist Johannes Brønsted and English chemist Thomas. It defines acids and bases based on their ability to donate or accept protons (H⁺ ions) in chemical reactions. According to this theory:
A Bronsted acid is a substance that can donate a proton (H⁺ ion) to another species in a chemical reaction. OR acid donates its H+ ion to its conjugate base
Example reaction of a Brønsted acid donating a proton:
HCl (acid) + H₂O (base) → Cl⁻ (conjugate base) + H₃O⁺ (conjugate acid)
In this reaction, HCl donates a proton to H₂O, forming the chloride ion (Cl⁻) as its conjugate base and a hydronium ion (H₃O⁺) as its conjugate acid.
A Bronsted base is a substance that can accept a proton (H⁺ ion) from another species in a chemical reaction.
Example reaction of a Brønsted base accepting a proton:
NH₃ (base) + H₂O (acid) ⇌ NH₄⁺ (conjugate acid) + OH⁻ (conjugate base)
In this reaction, NH₃ accepts a proton from H₂O, forming the ammonium ion (NH₄⁺) as its conjugate acid and a hydroxide ion (OH⁻) as its conjugate base.
In any Bronsted acid-base reaction, there will be the formation of a conjugate acid and a conjugate base.
Example of a conjugate acid-base pair:
CH₃COOH (acid) + H₂O (base) ⇌ H₃O⁺ (conjugate acid) + CH₃COO⁻ (conjugate base)
The Lewis theory of acids and bases was proposed by American chemist Gilbert N. Lewis in 1923. The Lewis theory is based on electron pair donation and acceptance.
Key points of the Lewis theory of acids and bases:
Formation of a coordination complex between boron trifluoride (BF₃) and ammonia (NH₃):
BF₃ (Lewis acid) + :NH₃ (Lewis base) → BF₃:NH₃ (Lewis acid-base adduct)
In this reaction, ammonia donates a lone pair of electrons to boron trifluoride, forming a new bond between them.
Formation of a coordination complex between fluorine anion (F–) and boron trifluoride (BF3):
F– (Lewis base) + BF3 (Lewis acid) → BF4– (Lewis acid-base adduct)
Formation of a coordination complex between aluminum chloride (AlCl₃) and chloride ion (Cl⁻):
AlCl₃ (Lewis acid) + Cl⁻ (Lewis base) → AlCl₄⁻ (Lewis acid-base adduct)
In this example, the chloride ion donates a lone pair of electrons to aluminum chloride, resulting in the formation of the AlCl₄⁻ ion.
Neutralization reactions occur when an acid reacts with a base in an aqueous solution, resulting in the production of water and a salt. To represent this reaction, a balanced chemical equation is used, ensuring an equal number of atoms and overall charges on both sides. Balancing a neutralization equation involves considering the acidity of the base and the basicity of the acid. For instance, when sodium hydroxide (NaOH) reacts with hydrochloric acid (HCl), the balanced equation is:
NaOH + HCl → NaCl + H₂O
This balanced equation reflects the neutralization process, where one mole of NaOH releases one mole of OH⁻, and one mole of HCl releases one mole of H⁺. Similarly, other bases like KOH and acids such as HBr, HI, and HNO₃ react in a 1:1 ratio, releasing one mole of H⁺ for every mole of base involved.