Similarly, there is other class of compounds called alkalies; they have ability to produce soapy feel in water, ability to change the colour of indicators and they can neutralize acids to produce salt and water. In 1754 Rouelle introduced the term base for such class of compounds. So, let's go through Introduction to acids and bases by different theories like Arrehenius theory, Bronsted-Lowry theory, Lewis concept etc. And also the types of Acids and bases.
Types of Acids and Bases:
Arrhenius Theory:
According to Arrhenius theory:
Acid:
An acid is a hydrogen compound which gives hydrogen ions, H+, in aqueous solution.Examples: HCl, HNO3 etc.
Similarly,
Base:
An alkali is a hydroxide compound that gives hydroxyl ions, OH-, in aqueous solution.Examples: NaOH, KOH etc.
Advantages of Arrhenius concept:
The concept was extremely useful:1) To determine the acid-base properties of aqueous solution.
2) To explain various aspects such as neutralization, strength of acids and alkalies and hydrolysis of salts.
Limitations of Arrhenius theory:
1) The theory defines acids and alkalies in terms of the aqueous solutions only and not in solution of any other solvent, e.g. benzene, CCl4 etc.
2) The theory does not provide any explanation regarding acidic or basic properties of certain salts, e.g. NH4NO3 in liquid ammonia behaves as an acid though it does not contain H+ ion, similarly, NaNH2 in liquid NH3 behaves as a base though it does not contain OH- ion.
3) There are certain compounds which do not produce OH- ions in water solution still they exhibit basic properties, e.g. acetate ions(CH3COO-).
4) The theory does not explain the basic nature of substances like pyridine, ammonia which do not contain OH- ions.
5) The theory cannot explain acidic character of salts like AlCl3 which does not possess H+ ion.
6) Definition of neutralization is limited only to the reactions taking place in aqueous medium.
7) This theory assumes the presence of a bare portion. A bare portion, H+ cannot exist as such in water because of its high hydration energy. It can exist only in the hydrated form.
Bronsted -Lowry Theory (the protonic definition) :
Arrhenius theory applies only to reactions taking place in water. There are certain reactions that occur in solvents other than water or even without solvents. In order to overcome this difficulty, Bronsted and Lowry almost simultaneously in 1923 proposed a more general definition of acids and bases.According to Bronsted-Lowry theory:
Acid:
An acid is a proton donor i.e. an acid is a substance that is capable of donating a proton.Base:
Base is a proton acceptor i.e. A base is a substance that is capable of accepting a proton.Conjugate acid-base pair:
This theory suggests that every acid-base reaction involves formation of a conjugate acid-base pair. A pair of substance which is formed by loss or gain of a proton from each other is called a conjugate acid-base pair. |
| Conjugate acid-base pairs |
Advantages of Bronsted -Lowry Theory:
1) Acids and bases are defined in terms of substances themselves and not in terms of their aqueous solutions. Thus HCl is an acid because it can donate proton and NH3 is a base because it can accept a proton.
2) The concept recognises acid-base behavior of the substance irrespective of nature of solvent.
3) It helps in explaining the hydrolysis of salts in aqueous solutions.
Limitations of Bronsted-Lowry Theory:
1) This concept lays excessive emphasis on proton transfer. There are many substances like BF3, AlCl3 which do not possess proton but still behave as acid.
2) Number of acid-base reactions, not involving transfer of a proton, are known.
3) The definition can not explain acid-base reactions taking place in non-protonic solvents like COCl2, SO2, N2O4 etc.
Solvent system:
A new system was proposed by Cady and Elsey. They tried to define acids and bases in general way in order to get rid of weaknesses of Arrhenius theory of acids and bases. It applies equally well to both protonic and non-protonic solvents. It was based on auto or self ionization of solvents.Acid:
An acid is a solute that, either by direct dissociation or by reaction with the solvent increases the concentration of the cation (positive ion) characteristic of the solvent.Base:
A base is a solute or species that, either by direct dissociation or by reaction with the solvent increases the concentration of the anion (negative ion) characteristics of the solvent. |
| Protonic and non-protonic solvent |
Advantages of the solvent system:
1) It explains acid base reactions both in aqueous and non-aqueous solvents.2) It explains the acid-base character of a solute in relation to the solvent system
Drawback of the solvent system:
Acid -base reactions can be explained in the presence of solvent only. In the absence of any solvent such reaction cannot be explained.
Lux-Flood Concept:
The definition proposed by Lux and extended by Flood describes acid-base behavior in terms of the oxide ion.According to Lux-Flood:
Acid:
An Acid is an oxide ion acceptor.
Base:
A base is an oxide ion donor. |
| Lux-flood theory for acid and bases |
The Lewis Concept:
In 1923, G. N. Lewis proposed a new definition of acids and bases. It was based on experimental facts.Acid:
An acid is any substance or species that is capable of accepting an electron pair.Lewis acids are:
1) All cations, e.g.Ag+, Cu²+, Fe³+, etc.
2) Electron deficient molecules, in which the central atom has an incomplete octet. e.g. BF3, AlCl3, etc or can expand their valence shell, e.g. SiF4, SnCl4.. etc.
Base:
A base is any substance or species that is capable of donating an electron pair.Lewis bases are:
1) All anions, e.g. OH-, F-, CN- etc.
2) Electron pair donors, e.g. H2O, NH3, R2O... etc.
3) Olefinic compounds which can form complexes with transition metals through pi-bond, e.g. ethylene, acetylene, CO, NO etc.
Merits of Lewis concept:
1) This definition does not attribute acidity or Basicity to any particular element or compound but attributes it to a feature- electron configuration and electron distribution in a substance.2) A variety of compounds and ions are included under this theory. Thus, the properties of bases might be attributed to the availability of a pair of electrons and in case of acids the properties are attributed to the availability of an empty orbital for the acceptance of a pair of electrons.
3) Acids and bases are not defined in terms of their solutions or in terms of any one element.
4) This theory does not lower the status of any previous definition of acids and bases.
5) This theory does not lower the status of any previous definition of acids and bases.
Demerits or limitations of Lewis concept:
1) Lewis concept assumes that combination of acid and base is a rapid reaction. However, this is not true in all cases. For example, formation of a complex ion [Cr(NH3)6]³+ is not a rapid reaction.2) It fails to give any information regarding acid-base strength.
3) It is very wide concept. It includes all classifications (like redox reactions, complex formation reactions) under acid base reactions.
So, this is the small introduction to Acids and Bases.