Neutralization (chemistry)

File:Neutralisation reaction between sodium hydroxide and hydrochloric acid.jpg

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

In chemistry, neutralisation, or neutralization (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. Neutralisations 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 neutralisation. 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 neutralisation reactions. All of the following may be considered neutralisation reactions under different definitions:

HCl + NaOH → NaCl + H₂O

2HCl + Mg → MgCl₂ + H₂

2HCO₂H + MgO → Mg(HCO₂)₂ + H₂O

HF + NH₃ → NH₄F

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

Neutralisation reactions do not necessarily imply a resultant pH of 7. In the case that a strong acid and strong base participate in a neutralisation reaction, the resultant pH will be 7. For example, the strong acid, HCl, and the strong base, NaOH, react to give water and a salt, NaCl:

HCl + NaOH → H₂O + NaCl

Since there is no net change in the concentrations of either H₃O⁺ or OH^-, the end pH is 7.

If a weak acid and a strong base participate in a neutralisation reaction, the resultant pH will be greater than 7. For example, the weak acid, CH₃COOH, and the strong base, NaOH, react to give water, Na⁺, and acetate, CH₃COO^-:

CH₃COOH + NaOH → Na⁺ + H₂O + CH₃COO^-

Na⁺ behaves as a spectator ion. However, acetate is a weak base that hydrolyzes water to give OH^- ions.

CH₃COO^- + H₂O ⇌ CH₃COOH + OH^-

Thus, the resultant solution is basic.

If a weak base and a strong acid participate in a neutralisation reaction, the resultant pH will be less than 7. For example, the weak base, CN^-, and the strong acid, HCl, react to give Cl^- and hydrocyanic acid, HCN:

CN^- + HCl → Cl^- + HCN

Cl^- behaves as a spectator ion. However, hydrocyanic acid is a weak acid that hydrolyzes water to give H₃O⁺ ions.

HCN + H₂O ⇌ CN^- + H₃O⁺

Thus, the resultant solution is acidic.

Finally, if a weak acid and a weak base participate in a neutralisation reaction, the resultant pH will depend on the relative strength of the acid and base reactants. For example, the weak base, CN^-, and the weak acid, CH₃COOH, react to give HCN and CH₃COO^-. Because CH₃COOH (pKa=4.75) is a stronger acid than HCN (pKa=9.2), the equilibrium is driven to the right, assuming equimolar initial concentrations of the weak acid and weak base.

CN^- + CH₃COOH ⇌ HCN + CH₃COO^-

The acetate ions further react with water to give acetic acid and OH^-.

CH₃COO^- + H₂O ⇌ CH₃COOH + OH^-

In this particular example, the resultant solution is basic. However, this is not a general rule for all neutralisation reactions between a weak acid and a weak base.

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 neutralisation 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.
• Neutralisation 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 neutralisation 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 neutralisation chemical depends on the particular application.

Calculations

Equal numbers of moles of acid and base are needed for neutralisation 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.