Acids, Bases and Salts Definition, Properties, and Concepts

Acid: The word acid is derived from the Latin word acids which means sour. Those substances which give Hydrogen ions (H+ )in an aqueous solution is called acid.

Base: Those substances which give hydroxyl ions (OH¯) in an aqueous solution is called a base.

Salt: The chemical compound which is formed due to the neutralization reaction of acid and base is called salt. A salt consists of positive ions from the base and negative ions from an acid, both positive and negative ions combine and form is salt.

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Acids, Bases and Salt class notes, Type, Properties and Concept

Properties of Acid

The following are some important properties of acid. (i) Acids have a sour taste. (ii) It turns blue litmus paper into the red. (iii) It changes the color of acid-base indicators. (iv) Aqueous solution of acid conduct electric current. (v) Strong acids are corrosive and have skin burning effect. (vi) They have a pH value of less than 7.0. (vii) They react with metals from salt and hydrogen gas.

Properties of Bases

The following are some important properties of bases. 

(i) Bases are bitter in taste. 

(ii) They have a soapy or slippery feel. 

(iii) It turns red litmus paper into the blue. 

(iv) It changes the color of acid-base indicators. 

(v) Aqueous solution of bases conducts electric current. 

(vi) Strong bases are very corrosive. 

(vii) They have a ph value greater than 7.0.

Alkalis: A water-soluble base is called alkalis, all alkalis are bases but all bases are not alkalis.

Various concepts about Acid and Bases

Arrhenius concept of acid and bases

Arrhenius concept of acids:- according to this concept those substances that ionized in an aqueous solution and produce (H+) ions is called acids. Example:- HCl, HNO3, H2SO4, HCN, CH3COOH, etc.

Arrhenius concept of acids

Arrhenius concept of bases: Those substances that are ionized in an aqueous solution and produce hydroxyl ions (OH¯) is called acid. Example:- KOH, NaOH, NH4OH, Ca(OH)2, etc. 

Arrhenius concept of bases

Limitation of Arrhenius concept

The following are the limitation of the Arrhenius concept. 

(i) This concept is only limited to aqueous solutions. 

(ii) It does not explain the acidity and basicity of the non-aqueous solution. 

(iii) It can not explain the acidic character of AlCl3, CO2, SO2, etc., and the basic nature of MgO, NH3, etc.

Bronsted and Lowery concept of Acid and Bases

Acid:- Those substances that donate or tend to donate proton [H+] ions are called Bronsted and Lowery acids.

Bases:- Those substances that accept or tend to accept proton [H+] ions are called Bronsted and Lowery bases. 

Example:- Considered the reaction between hydrochloric acid (HCl) and ammonia in the aqueous medium.   HCl + NH       ⟶   NH4 + Cl .

In this reaction, HCl donates the proton (H+) ion and act as an acid while NH3 accepts proton (H+) ion to form an ammonia ion (NH4) and act as a base.

Limitation of Bronsted and Lowery concept

The following are the limitation of Bronsted and Lowery's concept of acid and bases

(i) This concept can not explain the reactions which occur in a non-photonic solvent such as SO2, CCl4, C6H6. etc.

(ii) It can not explain the behavior of acidic oxides such as CO2, SO3, NO2, etc., and oxide such as MgO, BaO, etc.

Lewis Concept of Acid and Bases

Acid: According to Lewis's concept. Those species (molecules or ions) which can accept a pair of electrons are called acids.

Bases: Those species (molecules or ions) which can donate a pair of electrons is called bases.

Explanation of Lewis Concept

An acid-base reaction involves the donation of electrons pair from a base to an acid-forming coordinate covalent bond, the coordinate covalent bond is always formed between electron donor and electron acceptor species. So the compound which is electron deficient behaves as lewis acid while the compounds which have lone pair of electrons or negatively charged ions behave as Lewis base.

Example of Lewis acid and bases: Considered the reaction between BF3 and NH3

Example of Lewis acid and bases

In this reaction, BF3 accept a pair of electron act as a Lewis acid while in NH3 nitrogen donate a pair of electron forming a coordinate covalent bond and act as Lewis base.

Types of Acids

The following are the types of acids. 

(i) Monoprotic Acid:- The acids which donate only one proton per molecule is called monoprotic acid.

Example:-  HCL    ⟶   H+ + Cl¯

               HNO3  ⟶  H+     + NO3¯

(ii) Polyprotic acids:- Those acids which donate more than one proton per molecule are called polyprotic acid. It may be divided into diprotic acid and triprotic acids.

Diprotic Acids:- Those acids which donate two protons per molecule are called diprotic acids.

Example:   H2SO4 ⟶ 2H+ + SO4¯²

Triprotic Acids:- Those acids which donate three protons per molecule are called triprotic acids.

Example:  H3PO   ⟶ 3H+    + PO4 ̄3^

Type of Bases

The following are the types of bases. (i) Monoacidic bases (ii) Poly-acidic bases

(i) Monoacidic bases:- Those base that produces one hydroxyl ion per molecule is called monoacidic bases.

Example:- NaOH ⟶ Na+ + OH¯

(ii) Poly-acidic bases:- Those bases which produce more than one hydroxyl ion per molecule are called poly-acidic bases. It may be divided into di-acidic bases and tri-acidic bases.

Example of di-acidic.  Mg(OH)2  ⟶ Mg+ +   +  2OH¯

Example of Tri-acidic.  Al(OH)3  ⟶  Al+++    + 3OH¯

Conjugate of Acid and Base

Conjugate Acids:- A species formed from an acid by the loss of a proton is called the conjugate base of that acid.

Explanation:- When an acid donates a proton it forms a negative charge species which can accept proton and act as a base called the conjugate base of the corresponding acid. 

Exam.  CH3COOH + H2O  ⟶ CH3COO¯ + H3O+

Conjugate Bases:- A species formed from a base by gaining a proton is called conjugate acid of that base.

Explanation:- When a base accepts proton then it forms positive charge species which can donate a proton and act as acids called conjugate acid of the corresponding base.

Exam. NH3 + H2O ⟶ NH4 ๋ + OH¯

Strength of acid and base

Strong acid:- Those acids which completely dissociate or ionize completely in an aqueous solution and produce a higher concentration of (H+) ions is called strong acid. Hydrochloric acid, Sulphuric acid, Nitric acid. etc.

Weak Acid:- Those acids which partially dissociate or ionize in an aqueous solution and produce a low concentration of (H+) ions is called weak acid. Acetic acid, Carbonic acid, Propanoic acid. etc.

Strong Bases:- Those bases which completely dissociate or ionize in an aqueous solution and produce higher concentrations of hydroxyl ions (OH¯) are called strong bases. Sodium hydroxide, Potassium hydroxide, Lithium hydroxide. etc.

Weak bases:- Those bases which partially dissociate or ionize in an aqueous solution and produce a low concentration of hydroxyl ions (OH¯) is called a weak base. Ammonium hydroxide, Calcium hydroxide, Aluminum hydroxide, etc.

Preparation of Salt

There are two types of salt preparation. (i) Soluble salt and (ii) Insoluble salt. 

(i) Preparation of soluble salt:- 

Soluble salt can be prepared by the following four methods

(a) By the action of an acid and a metal:- In this method highly reactive metals such as Mg, Ca, Zn, etc. replaced hydrogen ions of acid and produce salts and hydrogen gas.

Exam.     2HCl  + Mg   ⟶  MgCl + H2

(b) By the reaction of acid and bases:- In this method acid react with base and neutralizes the effect of each other and produces salt and water.

Exam.  HCl + NaOH     ⟶   NaCl + H2O

(c) By the reaction of an acid and metallic oxide:- The metallic oxide such as (CuO, CaO, MgO) react with dilute acid to form salt and water.

Example:  H2SO + 2CaO   ⟶  CaSO4 + H2O

(d) By the reaction of an acid and carbonate:- Dilute acid reacts with carbonates such as sodium carbonate or sodium hydrogen carbonate to produce salt and water and carbon dioxide.

Example:  2HCl + Na2CO3    ⟶   2NaCl + H2O + CO2

(ii) Preparation of Insoluble Salts

Insoluble salt is usually obtained during the chemical reaction by mixing two soluble salts, during this reaction exchange of ions takes place and two new salts are produced one is soluble and the other is insoluble the insoluble salt precipitate at the bottoms, of the solution.

Example:   AgNO3 + NaCl    ⟶      AgCl(3) + NaNO3

Different types of salt

They are three main types of salts. (i) Neutral salts (ii) Acidic salts (iii) Basic salts.

(i) Neutral Salts 

The type of salts that are formed when the hydrogen ion of acid are completely replaced by metal ions or a group of atoms that behave like metal ions is called neutral salts.

Explanation of natural salts:- Neutral salts are formed when a strong acid and a strong base is neutralized in the reaction for example sodium chloride is are formed by the neutralized reaction of hydrochloric acid and sodium hydroxide.

HCl  +  NaOH  ⟶ NaCl + H2O

The following are examples of neutral salts. Potassium sulfate, Sodium Sulphate, Sodium carbonate, etc.

(ii) Acidic Salts

The type of salts that are formed when the hydrogen ions of acid are partially replaced by metal ions or a group of atoms that behave like metal ions are called acidic salts, acidic salts are formed by polybasic acid only. These salts further react with bases and form neutral salts.

H2SO4 + KOH  ⟶ KHSO4 + H2O

(iii) Basic Salts

This type of salt which is formed when partially neutralized of a strong base by weak acid is called basic salts. These salts contain ionized H+ ions along with another cation. Mostly the ionized H+ comes under the part of an anion. A few acid salts can be used in baking. For example, KH2PO4, NaHSO4, etc.

H2SO4 + NaOH → NaHSO4 + H2O

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