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What are acids and bases?
In chemistry, Acids and bases have a very important place because they have been very important in determining the nature of the reaction as well as the medium in which the reactions take place.
A common definition of acids is that they have hydrogen present in it.
A base or an alkaline substance is a substance that has an OH- group present in it.
Definitions of Acids and Bases based on different theories
The most acceptable theories regarding acids and bases are:
Acid-Base Theory Put forward by Arrhenius
Arrhenius, in the 1880s with his experiments, had put forward his theory regarding acids and bases. The concept of Arrhenius’s theory was based on Water.
As per the theory put forward by Arrhenius,
- Acids are known to be substances that form hydrogen ions in aqueous solutions. They have hydrogen present in them
- Bases are known to be substances that form hydroxide ions (OH-) ions in them.
Examples of acids as per the Arrhenius theory are:
- Hydrochloric Acid (HCl)
- Sulphuric Acid (H2SO4)
Examples of bases as per the Arrhenius Theory are:
- Sodium Hydroxide (NaOH)
2.Potassium hydroxide (KOH)
It is to be noted that the definition put forward by Arrhenius was only applicable in Water.
Acid-Base Theory Put forward By Bronsted and Lowry
Another famous acid-base theory in chemistry is Bronsted and Lowry. Bronsted and Lowry were the two Scientists who studied acids and bases. They studied the work of Arrhenius and did some improvisation and proposed a new theory by their name in the year 1923.
As per the Bronsted and Lowry Theory,
- Acids can be defined as the compound that tends to lose the hydrogen ions (H+ Ions)
- Bases can be defined as the compound that tends to gain Hydroxide ions (OH-) ions
Since Arrhenius’s theory was only applicable in an Aqueous medium, it was found that Bronsted and Lowry’s theory was applicable in aqueous as well as in non-aqueous medium too.
Bronsted and Lowry had also introduced a new term known as Conjugate Acids and Conjugate Bases in their theory which made their theory different from that of Arrhenius.
According to Bronsted and Lowry, if an acid loses its H+ ions, then the substance formed by such a loss is a base by definition. So, for every Conjugate acid, there is a conjugate base present for it.
They were thought to exist in pairs.
Some examples of Conjugate Acids and Conjugate Bases are:
- H3O+ is a conjugate acid and its conjugate base is H20
- H2O and its conjugate base are OH-
It was observed that any substance that had a pair of electrons could behave as Bronsted Base
It was also observed that the same compound tended to behave as a Bronsted Base as well as a Bronsted Acid too.
Lewis Acid-Base Theory
Lewis’s Acid-Base theory is another distinguished theory put forward to study the nature of acids and bases.
In 1923, A scientist named G.N Lewis proposed a new theory and a vision to look at the acids and bases.
His theory said that bases are commonly the substances that tend to donate electrons and acids are the substances that tend to accept the electrons.
- So according to him, Lewis’s acids are the substances that can accept the pair of nonbonding electrons.
- Lewis Bases are substances that can donate a pair of bonding electrons.
So as per the Lewis Acid Theory, Bases are known as Electron pair donors and Acids are known as Electron Pair Acceptors.
The Lewis Acid Theory suggests that acids tend to react with the bases and that’s how the sharing of the electrons takes place.
The main advantage of the Lewis Theory is that it has expanded the number of acids and therefore there are many acid-base reactions possible.
Theories of Acid-Base Indicators
Acid-base indicators are substances that have been widely used in quantitative analysis of the reaction. It is mainly used to check the verge of the completion of a chemical reaction. A pH scale is used to determine whether the solution is acidic or basic.
Color criteria are also there to determine the same.
Phenolphthalein is used as an indicator. The pH range is 8.0-9.8.
The color of the solution is pink if the solution is strongly alkaline whereas the color of the phenolphthalein is colorless if it’s added to the weak alkaline solution.
This was concluded from Ostwald’s Theory that says that Phenolphthalein is a weak acid.
Quinonoid Theory
The Quinonoid Theory of acids and bases says that aromatic compounds are used as indicators in acid bases titrations.
The benzoid form of compounds appears light as compared to the quinoid forms of compounds that appear darker.
Conclusion
Both Acids and Bases have their importance in a chemical reaction. Acids and Bases find their importance in our daily life too. Acids like Citric Acid give a sour flavor to our drink. NaOH solution is used in medicines.
