What Do All Acids and All Bases Have in Common

Acids and Bases are the most important group of chemicals in chemistry. They play an important role not only in our lives but in almost all natural systems from oceans to rocks, acids and bases have a part to play. You must have come across acid and bases when you tasted lemon or washed your hands with soaps. Acids are sour and can be dangerously corrosive; bases are slippery and can be corrosive as well. Scientists usually classify substances as acids, bases or neutral. The strength of the acid or base is measured by the pH scale.

What are Acids?

Acids are compounds that give out hydrogen ions or a proton when mixed in an aqueous solution. They turn blue litmus red and have a pH below 7.

An acid then generally has a dissociable proton or hydrogen group. It is called acidic hydrogen that readily dissociates in solution or in presence of bases (Arrhenius Acid). Acids can also be termed as compounds which are hydrogen or proton donors (Bronsted-Lowry acid).

HA + H2O → A- + H3O+

According to Lewis' description of acids, they are compounds which can accept electron pairs (Lewis Acid). Lewis acids broaden the definition of acids and incorporate all those compounds which do not contain any proton but have electron-accepting capacity.

In compounds where the acidic hydrogen is directly attached to an oxygen atom, they are called oxoacids or oxyacids. If the hydrogen is with some other element the name of such a compound has hydro- in the beginning, and ends with -ide. Organic acids usually contain a carboxyl group, -COOH.

The strength of an acid is related to its degree of dissociation in solution.

Table: Inorganic Acids

Table: Organic Acids

What are Bases?

Bases are compounds that give out hydroxyl ions when mixed in an aqueous solution. They turn red litmus blue and have a pH above 7.

A base, therefore, bears a dissociable hydroxyl group or a basic group that dissociates in an aqueous medium (Arrhenius base). Bases are also compounds that can steal or accept hydrogen ions, thus, bases can also be defined as compounds which are hydrogen acceptors (Bronsted-Lowry Base).

Metal hydroxides readily furnish hydroxyl ions in an aqueous solution and are predominantly basic. Alkali metals and some alkaline earth metals form the maximum basic hydroxides.

BOH(aq) → B+ (aq) + OH- (aq)

Lewis model of bases, attributes them as electron pair donors. Thus, compounds need not accept a proton or produce hydroxyl ion to be termed a base, any compound that can donate a pair of electrons can be considered a base (Lewis base).

Table: Base

Neutralisation Reaction

The reaction between acid and base produces salt and the reaction is named as neutralisation reaction. In the reaction, the acid releases the proton while the base is the source of the hydroxyl group. The hydrogen ion and the hydroxyl ion combine to form water molecules. The resultant anion after losing a proton combines with the cation formed from the dissociation of the hydroxyl group from the base to produce the salt (AB).

AH(aq) + BOH(aq) → AB(aq) + H2O(l)

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

One of the simplest neutralisations is the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH), to form sodium chloride salt (NaCl).

In the reaction between HCl and Mg(OH)2, the salt formed is MgCl2. This is an example of neutralization that takes place when one intake antacids that are bases while suffering from acidity.

2HCl(aq) + Mg(OH)2(aq) → MgCl2(aq) + 2H2O(l)

Metal surfaces with a coating of oxides can be removed by cleaning with acids. Rust or stains developed over iron can be removed by treating it with acid like HCl. The acid reacts with the basic ferrous hydroxide that forms the part of rust to neutralise it, producing ferrous chloride.

Certain similarities between acids and bases that can be pointed out are: both acids and bases release ions in an aqueous solution; both respond to the litmus test and both conduct electricity. When either acid or base is added to water, heat is released (exothermic reaction). It is due to this reason acids or bases should always be poured into the water not the other way around. Pouring acid or base into the water can cause accidents.

Interesting Facts

• The sting on ant bite is due to the secretion of an acid called Formic acid.

• Vinegar used for cooking has acetic acid.

• Guavas have oxalic acid.

Key Features

• Arrhenius Acids release hydrogen ion or proton in aqueous solution.

• Bronsted-Lowry Acids are proton donor.

• Lewis Acids are electron acceptor.

• Arrhenius Bases releases Hydroxyl ion in aqueous solution.

• Bronsted-Lowry Bases are Proton or hydrogen acceptor.

• Lewis Bases are electron donor.