Theories of acid-base indicators
Acid-base indicators are substances that change color depending on the acidity or basicity of a solution. There are several theories that explain how acid-base indicators work, including the Arrhenius theory, the Brønsted-Lowry theory, and the Lewis theory.
- Arrhenius Theory: This theory, proposed by Swedish chemist Svante Arrhenius in 1884, defines an acid as a substance that produces hydrogen ions (H+) in solution, and a base as a substance that produces hydroxide ions (OH-) in solution. According to this theory, acid-base indicators are substances that change color when there is a change in the concentration of H+ or OH- ions in solution.
- Brønsted-Lowry Theory: This theory, proposed by Danish chemist Johannes Brønsted and English chemist Thomas Lowry in 1923, defines an acid as a substance that donates a proton (H+) in a chemical reaction, and a base as a substance that accepts a proton. According to this theory, acid-base indicators are substances that can be either weak acids or weak bases, and they change color when they undergo a protonation or deprotonation reaction.
- Lewis Theory: This theory, proposed by American chemist Gilbert Lewis in 1923, defines an acid as a substance that accepts a pair of electrons, and a base as a substance that donates a pair of electrons. According to this theory, acid-base indicators are substances that can act as Lewis acids or Lewis bases, and they change color when they undergo a reaction that involves the transfer of electrons.
Overall, these theories help explain how acid-base indicators work by describing the chemical properties of acids and bases and their interactions with other substances.
Arrhenius Theory:
The Arrhenius Theory is a chemical theory proposed by Swedish chemist Svante Arrhenius in 1884 to explain the behavior of acids and bases. According to the theory, an acid is a substance that releases hydrogen ions (H+) in aqueous solution, while a base is a substance that releases hydroxide ions (OH-) in aqueous solution.
Arrhenius proposed that the reaction between an acid and a base results in the formation of water and a salt. For example, the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) produces sodium chloride (NaCl) and water (H2O):
HCl + NaOH → NaCl + H2O
In this reaction, HCl is an acid because it releases H+ ions in aqueous solution, while NaOH is a base because it releases OH- ions in aqueous solution.
Arrhenius theory was a significant advancement in the understanding of acids and bases, but it has some limitations. For example, it does not account for the behavior of substances that do not release H+ or OH- ions in aqueous solution, such as ammonia (NH3), which behaves as a base in water even though it does not release OH- ions. Later, other theories were developed to better explain acid-base behavior, including the Bronsted-Lowry theory and the Lewis theory.
Brønsted-Lowry Theory:
The Brønsted-Lowry theory is a chemical theory proposed independently by Danish chemist Johannes Nicolaus Brønsted and English chemist Thomas Martin Lowry in 1923 to provide a more comprehensive explanation of acid-base reactions than the Arrhenius theory.
According to the Brønsted-Lowry theory, an acid is a substance that donates a proton (H+) to another substance in a chemical reaction, while a base is a substance that accepts a proton. In other words, an acid is a proton donor, and a base is a proton acceptor.
For example, when hydrochloric acid (HCl) is dissolved in water, it donates a proton to a water molecule to form hydronium ion (H3O+), which acts as the acid in this reaction:
HCl + H2O → H3O+ + Cl-
In this reaction, HCl is the acid because it donates a proton (H+) to a water molecule, and the water molecule is the base because it accepts the proton.
Similarly, when ammonia (NH3) is dissolved in water, it accepts a proton from a water molecule to form ammonium ion (NH4+), which acts as the base in this reaction:
NH3 + H2O → NH4+ + OH-
In this reaction, NH3 is the base because it accepts a proton (H+) from a water molecule, and the water molecule is the acid because it donates the proton.
The Brønsted-Lowry theory is more general than the Arrhenius theory because it does not require that the acid and base be dissolved in water, and it can explain the behavior of substances that do not release H+ or OH- ions in aqueous solution. It also allows for the existence of conjugate acid-base pairs, which are related to each other by the transfer of a proton.
Lewis Theory:
The Lewis theory of acids and bases is a chemical theory proposed by American chemist Gilbert N. Lewis in 1923. The theory provides a broader definition of acids and bases than the Brønsted-Lowry theory and the Arrhenius theory.
According to the Lewis theory, an acid is a substance that can accept a pair of electrons (an electron pair acceptor), while a base is a substance that can donate a pair of electrons (an electron pair donor). This theory considers a wider range of chemical reactions beyond proton transfer reactions.
For example, when boron trifluoride (BF3) reacts with ammonia (NH3), BF3 acts as a Lewis acid by accepting a pair of electrons from NH3, which acts as a Lewis base:
BF3 + NH3 → F3B-NH3
In this reaction, BF3 accepts a pair of electrons from NH3 to form a coordinate covalent bond between the two molecules. The electron pair donor (NH3) is the Lewis base, and the electron pair acceptor (BF3) is the Lewis acid.
The Lewis theory also allows for the existence of multiple Lewis acids and bases in a reaction, which can form complexes. These complexes can be stable, as in the case of metal coordination compounds, where the metal ion acts as a Lewis acid, and the ligands act as Lewis bases by donating electron pairs.
The Lewis theory provides a more general explanation of acid-base reactions than the Arrhenius and Brønsted-Lowry theories, and it is particularly useful in the study of coordination chemistry and organometallic chemistry.
First Year B Pharm Notes, Syllabus, Books, PDF Subjectwise/Topicwise
Suggested readings:
