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In chemistry, Neutralization is a chemical reaction (also called a water forming reaction since a water molecule is formed during the process) in which an acid and a base or alkali (soluble base) react to produce salt and water (H₂O). During the process, hydrogen ions H⁺ (a bare proton) from the acid (proton donor) or a hydronium ion H₃O⁺ and hydroxide ions OH⁻ or oxide ions O²⁻ from the base (proton acceptor) react together to form a water molecule H₂O. In the process, a salt is also formed when the anion from acid and the cation from base react together.

Neutralization reactions are generally classified as exothermic since heat is released into the surroundings. However, an example of endothermic neutralization is the reaction between baking soda (sodium bicarbonate) and vinegar (acetic acid) (or any other weak acid).

Common Reactions

Most generally, the following occurs:

acid + base → salt + water

For example, the reaction between Hydrochloric acid and sodium hydroxide solutions:

hydrochloric acid + sodium hydroxide → sodium chloride + water

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Since the HCl and NaOH dissociate into ions in solution, the ionic equation is:

H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l)

And since the sodium and chloride ions are just spectator ions, the net equation becomes:

H⁺(aq) + OH⁻(aq) → H₂O(l) : Δ_rH = −55.90 kJ mol⁻¹

Generalizing:

• acid + alkali = metal salt + water
• acid + metal = metal salt + hydrogen
• acid + metal oxide = metal salt + water
• acid + metal hydroxide = metal salt + water
• acid + ammonia = ammonia salt + water
• acid + metal carbonate = metal salt + water + carbon dioxide
• acid + metal hydrogencarbonate = metal salt + water + carbon dioxide

The last two reactions are not considered as neutralization in some contexts because carbon dioxide is formed in addition to salt and water.

Applications

• Chemical titration methods are used for analyzing acids or bases to determine the unknown concentration. Either a pH meter or a pH indicator which shows the point of neutralization by a distinct color change can be employed. Simple stoichiometric calculations with the known volume of the unknown and the known volume and molarity of the added chemical gives the molarity of the unknown.
• Excess gastric acid in the stomach (acid indigestion) is typically neutralized by the ingestion of sodium bicarbonate (NaHCO₃) or another neutralizing agent such as an antacid.
• Neutralization can also be used to reduce the pain of insect and plant stings. Bee stings can be neutralized with alkalis and wasps with acids. Nettle stings can be neutralized with alkalis like the one found in dock leaves.
• In neutron wastewater treatment, chemical neutralization methods are often applied to reduce the damage that an effluent may cause upon release to the environment. For pH control, popular chemicals include Calcium Carbonate, Calcium Oxide, magnesium hydroxide, and sodium bicarbonate. The selection of an appropriate neutralization chemical depends on the particular application.

Calculations

Equal numbers of moles of acid and base are needed for neutralization reactions. Hence, the formula becomes

a × [A] × V_a = b × [B] × V_b

where a is the number of acidic hydrogens and b is the constant that tells you how many H₃O⁺ ions the base can accept. [A] denotes the concentration of acid and [B], the concentration of base. V_a is the volume of acid and V_b is the volume of base.

References

Metcalf & Eddy. Wastewater Engineering, Treatment and Reuse. 4th ed. New York: McGraw-Hill, 2003. 526-532.