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Concepts of Equilibrium

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Previously, acids, bases, and salts were identified through the testing of their aqueous solutions. An acid is defined as a substance that causes a sour taste in water, turns blue litmus red, and neutralises bases. If an aqueous solution of a substance tastes bitter, turns red litmus blue, or neutralises acids, it is referred to as a base. Salt is a non-reactive material that has no impact on litmus in an aqueous solution. Electrolytes, according to Faraday, are acids, bases, and salts. Furthermore, Liebig proposed that acids are hydrogen-containing compounds that can be replaced by metals.

  • Acids: Acidity is a distinguishing feature of acids. Acidic substances are typically extremely sour. Aside from hydrochloric acid, there are numerous other types of acids in our environment. Citrus fruits such as lemons and oranges contain citric and ascorbic acids, whereas tartaric acid is found in tamarind paste. 
  • Bases: Bases cause red litmus paper to turn blue, while blue litmus paper remains blue. They have a bitter taste and a soapy feel to them. Other common bases include sodium bicarbonate, which is used in cooking, and household bleach. 
  • Salts: Other common salts besides sodium chloride include sodium nitrate, barium sulphate, and others. Sodium chloride, also known as common salt, is a by-product of the reaction of hydrochloric acid (acid) and sodium hydroxide (base). A cluster of positively charged sodium ions and negatively charged chloride ions is held together by electrostatic forces in solid sodium chloride. Electrostatic forces between opposite charges are inversely proportional to the medium’s dielectric constant. 

In other words, combining an acidic and a basic compound may result in salts. Water, the universal solvent, has a dielectric constant of 80. As a result, when sodium chloride is dissolved in water, the water’s dielectric constant reduces the electrostatic force, allowing the ions to move freely in the solution. Because of hydration with water molecules, they are also well-separated.

Ionization and Dissociation

When a solid ionic compound dissolves in water, dissociation occurs, which is the separation of ions from an ionic crystal. Ionization, on the other hand, is the process by which a neutral molecule breaks down into charged ions when dissolved in a solution. The degree of ionisation is determined by the strength of the ionic bonds and the degree of ion solvation. The following are the three most important modern concepts of acids and bases.

Arrhenius Concept

According to the Arrhenius concept, acids are substances that produce H+ ions when dissolved in water, whereas bases are substances that produce OH ions when dissolved in water. Acid-base reactions, according to Arrhenius, are distinguished by acids dissociating in aqueous solution to form hydrogen ions and bases dissociating in an aqueous solution to form hydroxide ions.

HA →   H+ + A (Acid)

Limitations of Arrhenius Concept:

  1. Acids and bases cannot exist without the presence of water. Dry HCl cannot function as an acid. HCl only acts as an acid in water and not in any other solvent.
  2. The concept does not explain why substances in non-aqueous solvents are acidic or basic.
  3. Although reactions involving salt formation can occur in the absence of a solvent, the neutralisation process is only possible for reactions that can occur in aqueous solutions.
  4. Some salts, such as AlCl3, have an acidic character in an aqueous solution that cannot be explained.
  5. To define the fundamental nature of NH3, an extended as well as artificial explanation is required.

Bronsted-Lowry Concept

Bronsted and Lowry offered a broader concept of acids and bases in 1923. According to them, an acid is any hydrogen-containing material (molecule, anion, or cation) that may give a proton to another, whereas a base is any hydrogen-containing material (molecule, anion, or cation) that can absorb a proton from another. As a result, acids are proton donors and bases are proton acceptors.

Conjugate Acid-Base Pairs

Consider a reaction

H2O + HCl ⇔ H3O+  + Cl

HCl donates a proton to water in this reaction and is thus an acid. Water, on the other hand, accepts a proton from HCl and becomes a base as a result. The H3O+ ions donate a proton to the Cl ion in the reverse reaction, which at equilibrium proceeds at the same rate as the forward reaction, and thus H3O+, an ion is an acid. Cl ion is a base because it accepts a proton from the H3O+ ion. Conjugate acid-base pairs are acid-base pairs in which the members of the reaction can be formed from each other by the gain or loss of protons.

Limitations of Bronsted Lowry Concept:

  1. Bronsted Lowry was unable to explain the reaction that occurred in non-protonic solvents such as COCl3 and others.
  2. It cannot explain the reactions between acidic oxides such as etc. and basic oxides such as etc., which can easily occur in the absence of a solvent, for example (No proton transfer)
  3. Substances such as BF3, AlCl3, and others do not contain hydrogen, so they cannot donate a proton, but they still behave as acids.

Lewis Concept

The Lewis theory of acid-base reactions states that bases donate pairs of electrons, while acids accept pairs of electrons. As a result, a Lewis acid is an electron-pair acceptor. The Lewis theory has the advantage of being able to supplement the oxidation-reduction model. Oxidation-reduction reactions occur when electrons are transferred from one atom to another, resulting in a net change in the oxidation number of one or more atoms. The Lewis theory also proposed that when acids react with bases, they share a pair of electrons but no atoms’ oxidation numbers change. Either an electron is transferred from one atom to another, or the atoms share a pair of electrons.

Al(OH)3 + 3H+ → Al3+ + 3H2O (Aluminium hydroxide is acting as a base)

Al(OH)3 + OH → Al(OH)4 (Aluminium hydroxide is acting as an acid)

These reactions are clearly visible: Aluminium hydroxide acts as a base when it accepts protons. It acts as an acid when it accepts electrons. This Lewis acid-base theory also explains why non-metal oxides like carbon dioxide dissolve in water to form acids like carbonic acid H2CO3.

Limitations of Lewis Concept

  1. Lewis’ concept provided a generalised idea that encompassed all coordination reactions and compounds. This is not always the case.
  2. The Lewis concept does not provide an idea about the relative strength of acids and bases.
  3. The Lewis concept contradicts the acid-base reaction concept.
  4. The Lewis concept has not addressed the behaviour of protonic acids such as HCl.

Sample Questions 

Question 1: What is salt in acid-base and salt?


A salt is a substance formed by the reaction of an acid and a base in chemistry. Positive ions (cations) of bases and negative ions (anions) of acids make up salts. The neutralisation reaction occurs when acid and base react.

Question 2: Is NH4Cl a basic salt?


Because it is a salt of a strong acid (i.e. hydrochloric acid) and a weak base, ammonium chloride (chemical formula NH4Cl) is an acid salt (i.e. ammonium hydroxide).

Question 3: What are the types of acids?


There are two main types of acids: organic acids and inorganic acids.

Question 4: What happens when salt reacts with HCl?


The acid is weak hydrochloric acid, and the metal is iron. To generate iron (II) chloride and hydrogen, diluted hydrochloric acid is added to the iron filings. Iron replaces hydrogen from hydrochloric acid in this reaction, resulting in iron chloride and hydrogen. This is a simple displacement reaction in the case of gas.

Question 5: Is salt basic or acidic?


Only when a weak acid conjugate base is present in the salt does it become basic. Sodium chloride, for example, contains chloride (Cl), the conjugate base of HCl.

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Last Updated : 08 Jun, 2022
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