Acids, Bases and Neutralization Reactions

The human stomach is a muscular organ containing gastric juices, including hydrochloric acid, which help digest food. Certain kinds of foods raise the acidity levels of our stomach, causing acid reflux.

Consuming alkaline food, or a base like an antacid, neutralizes the excess acid, relieving the burning sensation. This is an example of an acid-base reaction.

The simplest acids are substances that contain dissociable hydrogen atoms. In aqueous solutions, they release positively charged hydrogen ions, or protons, along with the corresponding anions. The protons react with water molecules to form hydronium ions.

For example, an aqueous solution of nitric acid contains hydronium ions and nitrate ions. Other commonly known acids are hydrochloric acid, sulfuric acid, and phosphoric acid.

Acids are monoprotic or polyprotic depending on the number of dissociable hydrogen atoms per molecule.

Hydrochloric acid and nitric acid, with one dissociable hydrogen each, are both examples of monoprotic acids, while sulfuric acid and phosphoric acid are polyprotic. Sulfuric acid is diprotic and releases its two protons sequentially, whereas phosphoric acid is triprotic and dissociates sequentially in three steps.

The simplest bases are compounds that yield negatively charged hydroxide ions in aqueous solutions. Easily identifiable bases contain hydroxyl groups; for example, sodium hydroxide, potassium hydroxide, and calcium hydroxide.

Such bases, when added to water, dissociate into hydroxide ions and the corresponding metal cations.

Non-hydroxide bases like ammonia react with water by accepting protons from water molecules to produce ammonium ions and hydroxide ions.

In general, an acid and a base react to form water and a salt. Acid–base reactions are also called neutralization reactions when the moles of protons supplied by the acid and the moles of hydroxide ions supplied by the base are equal.

For example, when equal amounts of aqueous nitric acid and aqueous potassium hydroxide are mixed, the amounts of hydronium and hydroxide ions supplied to the solution are also equal.

The hydronium and hydroxide ions, therefore, completely neutralize each other by combining to form water, leaving no hydronium or hydroxide ions behind. Additionally, the nitrate anions and potassium cations pair up to form a salt — potassium nitrate.

Certain neutralization reactions are gas-evolution reactions. For example, hydrochloric acid and potassium sulfite react to form potassium chloride, water, and sulfur dioxide gas through the sulfite–bisulfite equilibrium system.