Bronsted-Lowry Acid and Base

Arrhenius’ definition of acid and base is limited to substances that can produce H+ and OH− ions in aqueous solutions. Although this is useful because water is a normal solvent, it does not define the acidity and basicity of all substances. For example, ammonia (NH3) forms a basic solution when dissolved in water. 

In 1923, the Danish chemist Johannes Bronsted and the English chemist Thomas Lowry independently proposed new definitions of acids and bases. Proton transfer was the key in these definitions.  

A Bronsted-Lowry acid can donate a proton (H+) to another molecule. On the other hand, the Bronsted-Lowry can receive a proton from another molecule. In short, Bronsted-Lowry acid is a proton donor (PD), while Bronsted-Lowry acid is a proton acceptor (PA).

Acid

An acid is a substance that shows the following behaviour in solution with water:

• Changes the colour of certain solvents
• Combines with other metals (e.g., iron) to release hydrogen
• Forms salts in reaction with bases 
• Induces certain chemical reactions (acid catalysis)

Examples of acids include inorganic acids such as hydrochloric acid and organic compounds in carboxylic acid, sulphonic acid, and phenol groups. Acids contain one or more hydrogen atoms, which, when dissolved, are released as positive hydrogen ions. 

The Bronsted-Lowry and Lewis theories give extensive definitions of acids that cover pure compounds exhibiting common acidic behaviours when dissolved in non-aqueous solvents. E.g., sulphur trioxide, boron trifluoride, and aluminium chloride.

Base

A base is any chemical substance that behaves in the following manner in an aqueous solution:

• Feels slimy when touched
• Has a bitter taste 
• Changes the colour of the indicators 
• Combines with acids to form salts
• Stimulates some chemical reactions (base catalysis)

Hydroxides of alkaline and alkaline iron (e.g., sodium hydroxide) and aqueous solutions of ammonia or its organic extracts (amines) are all bases. Such substances produce hydroxide ions (OH-) in aqueous solutions. 

As in the case of acids, the Lewis and Bronsted-Lowry theories include pure compounds that act as bases when dissolved in non-aqueous solutions.

Conjugate Acid-Base

Within the Bronsted-Lowry acid-base theory, conjugate acid is a chemical compound formed when an acid delivers a proton (H+) to a base. In other words, it is a base with added hydrogen ions in it. 

On the other hand, a conjugate base is what remains after the acid contributes the proton during the chemical reaction. Thus, it is an acid with lost protons.  

Because some acids can release more protons, the conjugate base of the acid may be acidic. 

In general, we can explain this concept as:

Acid + Base -> Conjugate base + Conjugate acid 

The members can be formed from one another by sharing or losing protons called conjugate acid-base pairs. A strong acid and weak base form one pair of the conjugate, while a strong base with a weak acid form another pair.

Conjugate acid combines one H atom and + charge with the base it has created. The conjugate base consists of one H atom and one charge, which is less efficient than a conjugate acid. Conjugates will always be listed on the product side of the reaction. 

Strong acid makes the conjugate base weak and vice versa.

Lewis Acid and Base

Gilbert Lewis (1875-1946) proposed a third theory of acid and base that is more common than those of Arrhenius or Bronsted-Lowry. Lewis acid is a substance that absorbs a pair of electrons to form a harmonious bond. Lewis’s base donates two electrons to create a harmonious bond.  

Therefore, the transfer of an electron pair from the base to the acid represents the Lewis acid-base reaction. Hydrogen ions, devoid of electrons, receive a pair of electrons. It is acidic under both Bronsted-Lowry and Lewis definitions. E.g., Ammonia contains a nitrogen atom as a central atom with a single pair of electrons.

Bronsted-Lowry Acid and Base: The Concept

To determine if a compound is acidic or basic, you need to calculate its hydrogens pre and post the reaction. Low hydrogen numbers indicate the substance is acid (it provides hydrogen ions). In contrast, an increase in hydrogen means that it is basic (receives hydrogen ions). 

You can apply these definitions to the reactants side. You can detect a new acid and a new base when you view the reverse reaction. Thus, the right-hand side has a conjugate acid and conjugate base. Remember, the original acid becomes a conjugate base on the completion of the reaction. 

Transfer of electrons is essential to balance a reaction. The acid will give the electron away, becoming a proton donor. On the other hand, the base becomes the proton acceptor by receiving the electron. Such a combination of acids and bases is called the conjugate acid-base pair.

Conclusion

From the basic understanding of acids and bases, we can conclude that the Bronsted-Lowry acid and base expands on Arrhenius’ definition of acid and base. The latter is limited to substances producing H + and OH− ions in aqueous solutions. Although it is useful because water is a normal solvent, it does not define the acidity and basics of all substances.  

The Bronsted-Lowry theory focuses on the proton transfer to broaden the horizons of what are acids (proton donor) and bases (proton acceptor).