Acid base concept

An acid is any substance that in water solution tastes sour, changes blue litmus paper to red, reacts with some metals to liberate hydrogen, reacts with bases to form salts, and promotes chemical reactions (acid catalysis).
A base is a substance that can neutralize the acid by reacting with hydrogen ions. Most bases are minerals that react with acids to form water and salts.
Salt is a chemical compound consisting of an ionic assembly of cations and anions.

1. Presentation on Acid, Base and Salts: Basic concept Presented by: Shisam Neupane BPH, 1 st semester

2. Acids: Latin word “ acidous ” means sour 3 concepts have been supported till date for acid and base: Arrhenius concept Bronsted -Lowry concept Lewis concept

3. 1. Arrhenius concept (given in 1887) An acid is a substance, which give hydrogen ion in water solution ( hydronium ion). i.e. H+ + H2O  H3O+ ii. A base is a substance which give OH- in water solution. iii. Acid and base combine to form neutral molecule i.e. salt and water (neutralization) . Neutralization is exothermic process here.

4. Limitations of Arr. concept Only for aqueous solution Acidic or basic nature of oxides isn’t explained Acidic or basic nature of salts isn’t explained No explanations for Amphoteric ions

5. Modified Arr. Concept Basicity of acid: no. of replaceable H+ ion in aq.solution Acidity of base: no. of replaceable OH- in aq. solution

6. Classification of acids (on the basis of basicity of acid) Monobasic acid Dibasic acid Tribasic acid Tetrabasic acid

7. Classification of base (on the basis of acidity of base) Monoacidic base Diacidic base Triacidic base Tetraacidic base

8. 2. Bronsted -Lowry Concept (given in 1923) It’s a protonic concept. Defination : A substance is known as an acid if it can donate a proton and base if it can accept protein H2O + NH3 ↔ OH- + NH4+ Here H2O is losing a proton(H+), so is Bronsted acid and NH3 is accepting H+, so it’s a Bronsted base. Strength of acid is determined by it’s tendency to lose a proton and of base is the tendency to accept a proton. Proton is always in solvated form; dry HCl dry HNO3 : They aren’t B ronstate acid though the donate protons Non-aqueous solvent can also be used for the study of acid-base reaction.

9. On the basis of proton interaction, solvents are classified as: Protonic or protogenic solvent: donate proton. eg . HCl , H2O etc. Protophilic or Protic : accept proton. eg . H2O, alcohol etc. Amphiprotic or Amphoteric : can accept or donate proton. Eg . H2O, alcohol, CH3COOH etc Aprotic : can neither acccept nor donate proton. Eg . CCl4, Benzene etc

10. Limitations of Bronsted -Lowry Concept The most serious limitation of this concept is that it can’t explain the acid-base reactions taking place in non- protonic solvents like liq. SO2, liq.BF3, AlCl3, POCl3 etc., in which no transfer of protons take place.

11. 3. Lewis Concept (given in 1923) A more general and fundamental concept of acid base behavior was given by Lewis (1923) same year in which Bronsted concept is given. It’s electronic concept. Electron acceptor: acid electron donar : base Donation and acceptance takes place in lone pairs. Lewis acid base neutralization takes place by formation of co-ordinate bond. NH3 + BF3  NH3  BF3 base acid

12. Conditions for Lewis Acids Positively charged ions- Fe2+, Cu2+, Fe3+, Ag+, H+ etc Molecules in which the central atom has incomplete octet Molecules in which the central atom has vacant d- orbitals Molecules having multiple bond (or non metallic oxides) Acid salts: ZnCl2, CuSO4, FeCl3 etc.

13. Conditions for Lewis base Negatively charged ion: CN-, NO3-, SO3– etc Neutral molecules having at least one lone pair of electrons, ammonia, amines, alcohol, ether etc. Multiple bond compounds which form complex with transition metals ( eg . CO, NO, ethylene etc). Basic salts are Lewis bases Metallic oxides- bases

14. Limitations It has several limitations; It’s too general and includes all reactant which can form coordinate bonds. Relative strengths of acid and base can’t be explained on the basis of L ewis concept.

15. Salt Salt is the chemical substance formed by the neutralisation of acid and base HCl + NaOH  Nacl + H2O acid base salt

16. Types of salt

17. A. Normal salt Normal salt is formed by complete rxn between acid and base and can be neutral, acidic, or basic. Normal neutral salt= strong acid + strong base eg . NaCl , KCl , NaNO3 etc Acidic normal salt= strong acid + weak base eg . CuSO4, FeCl3, NH4Cl, BeCl2 etc Basic normal salt= strong base + weak acid eg . Na2CO3, K2CO3, CH3COONa etc.

18. B. Acid Salt It’s formed by incomplete replacement of H+ of di , tri, or tetra basic acid. It may be acidic, basic, or neutral. It’s also called bi- salt. Neutral acid salt: NaHSO4 Acidic acid salt: NH4HSO4 Basic acid salt: NaHCO3, NH4HCO3

19. C. Base Salt It’s formed by the incomplete replacement of OH- from polyacidic base and is always basic in nature. Eg . 2CuCO3, Cu(OH)2, Pb (OH) Cl Some base salt: Malachite= CuCO3.Cu(OH)2 Azurite= 2CuCO3.Cu(OH)2 Basic copper carbonate

20. D. Mixed Salt In it there’s the presence of more than one cation or more than one anion, when dissolved in water it gives simple ion. eg . Bleaching powder: CaOCl2 Ca2+, OCl -, Cl - = 3 ions Microcosmic salt: NaNH4.HPO4. 4 H2O Na+, NH4+, HPO4 = 3 ions

21. E. Double salt Mixed salt is formed by chemical rxn but double salt is formed by only mixing of 2 simple salts in equimolar solution, not by chemical rxn . It dissolves in water to give simple ions. eg . FeSO4+ (NH4)2SO4  FeSO4.(NH4)2SO4.6H2O 1 : 1 Mohr’s salt(Ferrous ammonium sulphate ) Note: All double salts are mixed salts but all mixed salts are not double salt.

22. F. Complex salt It’s formed by Lewis acid base rxn where central atom is lewis acid while others are lewis base. It may be ionic (cationic or anionic) or neutral. eg . K4[Fe(CN)6], alkaline K2HgI4 ( Nessler’s Reagent)

23. Thank you!