Neutralization (chemistry)

Neutralization reaction between sodium hydroxide and hydrochloric acid. Indicator agent bromothymol blue.

In chemistry, neutralization, or neutralisation (see spelling differences) is a chemical reaction in which an acid and a base react to form a salt. Water is frequently, but not necessarily, produced as well. Neutralizations with Arrhenius acids and bases always produce water:

YOH + HX → XY + H₂O

Y and X represent a monovalent cation and anion respectively. XY would be the salt produced. An example reaction of this form is the reaction between sodium hydroxide and hydrochloric acid, where sodium is Y and chlorine is X:

HCl + NaOH → NaCl + H₂O

Water and common table salt are produced.

The reaction can also be considered as a net ionic equation:

H⁺ + OH^- → H₂O

This representation is inaccurate, however, as the hydrogen ion (H⁺) does not actually occur in solution during a neutralization. In fact, the hydronium ion (H₃O⁺) occurs, produced by the following reaction:

H⁺ + H₂O → H₃O⁺

Considering the hydronium ion, the actual net ionic reaction occurring is: H₃O,⁺ + OH^- → 2H₂O

In non-aqueous reactions, water is less likely to be formed; however, there is always a donation of protons (see Brønsted-Lowry acid-base theory). Since a variety of definitions of acids and bases exist, a variety of reactions may be considered neutralization reactions. All of the following may be considered neutralization reactions under different definitions:

HCl + NaOH → NaCl + H₂O

2HCl + Mg → MgCl₂ + H₂

2HCO₂H + MgO → Mg(HCO₂)₂ + H₂O

HF + NH₃ → NH₄F

Often, neutralization reactions are exothermic (the enthalpy of neutralization). For example, the reaction of sodium hydroxide and hydrochloric acid. An example of an endothermic neutralization is the reaction between sodium bicarbonate (baking soda) and acetic acid (vinegar).

Neutralization means to neutralize, in chemistry the neutral PH of 7 would be counted as neutral.

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 alkalines and wasps with acids. (This does not actually reduce the pain, the acidity of the sting is not what causes itching but other chemicals in the venom. The reason the pain is reduced is merely psychological).
• 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.

Neutralization is American and Neutralisation is Australian/UK In chemistry if anything has a pH of 7 it is considered to be a neutral.

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.