Acid and base reaction play a major role in both industrial chemistry and biochemistry. Many substances we come across in our homes, supermarkets, and the pharmacy are either acids or bases. Eg: aspirin and vinegar are acids and antacids are bases. Lemon is acidic in nature and Kiwi is alkaline. The taste in the food is also due to the presence of acids and bases in them. Before we discuss the characteristics of how do acids and bases react with each other and what is the reaction between acid and base, let’s first understand What are Acids and Bases?
What are Acids and Bases?
We can define acids as substances that dissolve in water to produce H+ ions, it is capable of donating a proton (hydrogen ion) to another substance, whereas bases are defined as substances that dissolve in water to produce OH- ions and is a molecule or ion able to accept a hydrogen ion from an acid.
The general properties of acids and bases were known to people for more than a thousand years, The definitions of acid and base have changed dramatically as scientists have learned more about them.
Acids are usually identified by their sour taste. An acid is basically a molecule that can donate H+ ions and can remain favorable even after losing H+ion, Acids turn blue litmus red. On the other hand, A base is any substance that had a bitter taste, felt slippery to the touch, and caused color changes of red litmus paper to blue.
We encounter reactions between acid and base in our everyday lives. The orange, lemon, or grapefruit juice we drink contains citric acid. When milk is left for sometimes it turns sour, it contains lactic acid. The vinegar used contains acetic acid. According to this, a chemical bond is made by an acid-base combination. The properties of a molecule can be understood more easily by classifying them into acid and base fragments.
Theories of Acids and Bases
Three different theories define acids and bases. These different theories include the Arrhenius theory, the Bronsted-Lowry theory, and the Lewis concepts of acids and bases. A brief description of each of these theories with acid and base reaction examples are described below:
The Arrhenius Definition of Acids and Bases:
HCl(g) → H+(aq) + Cl- (aq) (HCl is Arrhenius acid)
NaOH(s) → Na+ (aq) + OH- (aq) (NaOH is Arrhenius base)
Although Arrhenius’s ideas were widely accepted and it successfully explains the reaction between acids and bases that yield salts and water.
The limitation in Arrhenius’s definitions of acids and bases is that it fails to explain how substances lacking hydroxide ions form basic solutions when dissolved in water, Eg: NO2-- and F--.
The reaction between ammonia (a base) with gaseous HCl (an acid) to give ammonium chloride is not an acid base reaction examples because it does not involve H+ and OH-- :
NH3(g) + HCl(g) → NH4Cl(s)
The Bronsted–Lowry Definition of Acids and Bases
Because of some limitations in the Arrhenius definition, a more general definition of acids and bases was required. According to Bronsted and Lowry, A Bronsted acids undergo dissociation to yield protons and therefore increase the concentration of H+ ions in the solution, and A base is defined as a proton acceptor (or H+ ion acceptor) by this theory. The Bronsted–Lowry definition of an acid is the same as the Arrhenius definition, except that it is not restricted to aqueous solutions only. The Bronsted–Lowry definition of a base, is far more general because the hydroxide ion is just one of many substances that can accept a proton. An advantage of the Bronsted-Lowry definition of acids and bases is its ability to explain the acidic or basic nature of an ionic species.
The main limitation of Bronsted–Lowry theory is that it fails to explain how compounds lacking hydrogen exhibit acidic properties, Eg: BF3 and AlCl3.
Lewis Concept of Acids and Bases
According to Lewis’s concept of an acid, it states that it is a species that has a vacant orbital and therefore, has the ability to accept an electron pair, and A Lewis base can hold a lone pair of electrons and can act as an electron-pair donor.
Lewis acids are electrophilic in nature and Lewis Bases are nucleophilic in nature.
A Lewis acid can accept an electron pair from a Lewis base and forms a coordinate covalent bond in the process. The theory does not involve the hydrogen and hydroxide atoms in its definition of acids and bases.
Lewis acids: Cu2+, BF3, Fe2+ ,Fe3+, Lewis bases: F-, NH3,AlCl3 ,C2H4.
The major advantage of this concept is that many compounds can be defined as acids and bases. However, it does not focus on the strength of acids and bases.
The major drawback of this theory is that it fails to explain the acid and base reaction that does not involve the formation of a coordinate covalent bond.