Acid–base reaction

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An acid–base reaction is a chemical reaction where an acid and a base interact to form new substances. These reactions are important for solving everyday problems, like calculating pH, making solutions buffered, or understanding how stomach acid works.

Three major ideas explain how acids and bases behave, each with a different focus:

- Arrhenius theory (water-based): An acid increases the concentration of hydrogen ions (H+) in water, while a base increases the concentration of hydroxide ions (OH−). In water, H+ is usually shown as H3O+ (the hydronium ion). This theory works well for reactions in aqueous solutions but doesn’t cover non-water solvents.

- Brønsted–Lowry theory (proton transfer): An acid donates a proton (H+) and a base accepts a proton. The reaction creates conjugate acids and conjugate bases (the acid becomes its conjugate base after losing H+, the base becomes its conjugate acid after gaining H+). This view works in any solvent, not just water. A common example is HCl reacting with water: HCl + H2O ⇌ H3O+ + Cl−.

- Lewis theory (electron pairs): An acid accepts an electron pair, and a base donates an electron pair. This broad idea explains many reactions that don’t involve protons. For example, BF3 (a Lewis acid) can accept electrons from F− (a Lewis base) to form a bond.

What happens in a typical acid–base reaction
- In Brønsted–Lowry terms: HA + B → BH+ + A−
- HA is the acid (donates H+).
- B is the base (accepts H+).
- BH+ is the conjugate acid of the base, and A− is the conjugate base of the acid.
- In Arrhenius terms (aqueous solutions): an acid produces H+, and a base produces OH−; they neutralize to form water and a salt (for example, HCl + NaOH → NaCl + H2O).

A common example
- Hydrochloric acid reacting with sodium hydroxide in water: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l). The strong acid and strong base react so that H+ and OH− essentially cancel each other out to form water, with Na+ and Cl− as spectator ions.

Other points to know
- pH measures how acidic or basic a solution is. It reflects the concentration of H3O+ (the hydronium ion) in water.
- The idea of acids and bases can depend on the solvent. A substance might act as an acid in one solvent and as a base in another.
- There are many specialized theories and refinements (for non-aqueous solvents, hard/soft acid–base concepts, and more). They help chemists predict which acids and bases will interact most strongly.

In short, acid–base reactions involve proton or electron transfers that transform reactants into new acids and bases, with neutralization often giving rise to water and a salt in water-based solutions.