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 where acid–alkali reactions produce water and a metal salt.

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

Neutralization reactions do not necessarily imply a resultant pH of 7. The resultant pH will vary based on on the strength of the acid and base reactants.

Arrhenius acids and bases

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

Acid-alkali

An acid–alkali reaction is an neutralization reaction which is considered a special case of an acid–base reaction, where the base used is also an alkali. When an acid reacts with an alkali it forms a metal salt and water.

In general, acid–alkali reactions can be simplified to

OH⁻
(aq) + H⁺
(aq) → H
₂O

by omitting spectator ions.

Acids are in general pure substances that contain hydrogen ions (H⁺
) or cause them to be produced in solutions. Hydrochloric acid (HCl) and sulfuric acid (H
₂SO
₄) are common examples. In water, these break apart into ions:

HCl → H⁺
(aq) + Cl⁻
(aq)

H
₂SO
₄ → H⁺
(aq) + HSO⁻
₄(aq)

An alkali is a base that contains a metal from column 1 or 2 of the periodic table (the alkali metals or the alkaline earth metals). Alkalis may be defined as soluble bases, which means they must be able to dissolve in water. In general, bases are defined as substances that contain hydroxide ion (OH⁻
) or produce it in solution. Therefore, one may also speak of hydroxide bases that dissolve in water, and thus these would also be alkalis. Some examples, then, of alkalis would be sodium hydroxide (NaOH), potassium hydroxide (KOH), magnesium hydroxide (Mg(OH)₂), and calcium hydroxide (Ca(OH)₂). Note that only hydroxides with an alkali metal — column 1 — are very soluble in water; hydroxides with an alkaline earth metal — column 2 — are not as soluble. Some sources will even say the alkaline earth metal hydroxides are insoluble.

To produce hydroxide ions in water, the alkali breaks apart into ions as below:

NaOH → Na⁺
(aq) + OH⁻
(aq)

However, alkalis may also have a broader definition that includes carbonates (CO²⁻
₃) bonded to a column 1 metal, an ammonium ion (NH⁺
₄), or an amine (NH_x radical) as the positive ion. Examples of alkalis would then also include Li
₂CO
₃, Na
₂CO
₃, and (NH₄)₂CO₃.

Non-Aqueous Reactions

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

Resultant pH

Neutralization reactions do not necessarily imply a resultant pH of 7. In the case that a strong acid and strong base participate in a neutralization 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 neutralization 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 neutralization 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 neutralization 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 neutralization reactions between a weak acid and a weak base.

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.

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.

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.

There are many uses of neutralization reactions that are acid-alkali reactions. A very common use is antacid tablets. These are designed to neutralize excess gastric acid in the stomach (HCl) that may be causing discomfort in the stomach or lower esophagus. This can also be rememdied by the ingestion of sodium bicarbonate (NaHCO₃).

Also in the digestive tract, neutralization reactions are used when food is moved from the stomach to the intestines. In order for the nutrients to be absorbed through the intestinal wall, an alkaline environment is needed, so the pancreas produce an antacid bicarbonate to cause this transformation to occur.

Another common use, though perhaps not as widely known, is in fertilizers and control of soil pH. Slaked lime (calcium hydroxide) or limestone (calcium carbonate) may be worked into soil that is too acidic for plant growth. Fertilizers that improve plant growth are made by neutralizing sulfuric acid (H₂SO₄) or nitric acid (HNO₃) with ammonia gas (NH₃), making ammonium sulfate or ammonium nitrate. These are salts utilized in the fertilizer.

Industrially, a by-product of the burning of coal, sulfur dioxide gas may combine with water vapor in the air to eventually produce sulfuric acid, which falls as acid rain. To prevent the sulfur dioxide from being released, a device known as a scrubber gleans the gas from smoke stacks. This device first blows calcium carbonate into the combustion chamber where it decomposes into calcium oxide (lime) and carbon dioxide. This lime then reacts with the sulfur dioxide produced forming calcium sulfite. A suspension of lime is then injected into the mixture to produce a slurry, which removes the calcium sulfite and any remaining unreacted sulfur dioxide.

References

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