Neutralization

Neutralization is a chemical reaction, also called a water forming reaction, in which an acid and a base or alkali (soluble base) react and produce a salt and water(H₂O). In other words, it can be said that neutralization is the combination of hydrogen ions H⁺ and hydroxide ions OH⁻ (or oxide ions O²⁻) to form water molecule H₂O. In the process, a salt is formed. In the process of neutralization a proton (usually hydrogen) is transferred. An acid is a proton donor, while a base is a proton receiver. H₂O can act as either an acid or a base, depending on whether it comes in contact with an acid or a base (it takes the opposite form) Neutralization is generally exothermic, meaning it produces heat. One example of an endothermic neutralization is that of baking soda and vinegar (or other weak acids)

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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 not involved in the reaction, the net equation becomes:

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

This illustrates why neutralization reactions are also referred to as water forming reactions. Of course the sodium and chloride ions are still in solution so the result is pH neutral salt water.

Therefore we can conclude -

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 context because carbon dioxide is formed in addition to salt and water.

Chemical titration methods are used for analyzing acids or bases to determine the unknown concentration. A pH meter can be used to determine the point of neutralization or a pH indicator such as UI (universal indicator) which shows the point of neutralization by a distinct color change can be used. 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 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.

Equal amounts (numbers of moles) of acid and base are needed for neutralization reactions between strong acids and strong bases. Use the formula:

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.