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CHM 101 Dr. (Mrs). F.Y. Falope, ACID, BASE AND SALT, INTRODUCTION, Elements combine to form numerous compounds. On the basis of their chemical properties, compounds can be classified into three categories:, Acids, Bases, Salts, ACID AND BASE, Acid: An acid is a substance which produces hydrogen ions (H+) when dissolved in water. For example, in its aqueous solution hydrochloric acid HCl (aq) dissociates as:, HCl (aq) ⎯⎯→ H+(aq) + Cl–(aq), Some examples of acids are:, (i) Hydrochloric acid (HCl) in gastric juice (ii) Carbonic acid (H2CO3) in soft drinks (iii) Ascorbic acid (vitamin C) in lemon and many fruits (iv) Citric acid in oranges and lemons (v) Acetic acid in vinegar (vi) Tannic acid in tea (vii) Nitric acid (HNO3) used in laboratories (viii) Sulphuric acid (H2SO4) used in laboratories, Bases: A base is a substance which produces hydroxide ions (OH–) when dissolved in water. For example, sodium hydroxide NaOH (aq), in its aqueous solutions, dissociates as:, NaOH (aq) ⎯⎯→ Na+(aq) + OH–(aq), The term ‘alkali’ is often used for water soluble bases. Some examples of bases are:, (i) Sodium hydroxide (NaOH) or caustic soda used in washing soaps. (ii) Potassium hydroxide (KOH) or potash used in bathing soaps. (iii) Calcium hydroxide (Ca(OH)2) or lime water used in white wash. (iv) Magnesium hydroxide (Mg(OH)2) or milk of magnesia used to control acidity. (v) Ammonium hydroxide (NH4OH) used in hair dyes., PROPERTIES OF ACIDS AND BASES, Physical Properties of Acids and Bases, Some physical properties of acids Some physical properties of bases:, 1. They are sour. 1. They are bitter, 2. They turn blue litmus red. 2. They turn red litmus blue., 3. They are corrosive in nature. 3. They are soapy in nature, Physical Properties of Acids and Bases, Reaction of acids and bases with metals, Acids and bases react with metals to produce salt by displacing hydrogen, Acid + active metal →  salt + hydrogen + heat, 2HCl+Mg→MgCl2+H2 (↑), Base + metal → salt + hydrogen + heat, 2NaOH+Zn→Na2ZnO2+H2 (↑), Reaction of acids with metal carbonates and bicarbonates, Acids react with metal carbonates or bicarbonates to form salt and water with the evolution of carbon dioxide gas while bases have no reaction, Acid + metal carbonate or bicarbonate →  salt + water + carbon dioxide., 2HCl+CaCO3→CaCl2+H2O+CO2, H2SO4+Mg (HCO3)2→MgSO4+2H2O+2CO2, Effervescence indicates liberation of CO2 gas., Base + Metal carbonate or bicarbonates → No Reaction, c) Neutralisation reaction, 1. Reaction of metal oxides and hydroxides with acids, Metal oxides or metal hydroxides are basic in nature., Acid + base → salt + water + heat, H2SO4+MgO→MgSO4+H2O, 2HCl+Mg (OH) 2→MgCl2+2H2O, 2. Reaction of non-metal oxides with bases, Non-metal oxides are acidic in nature, Base + Non-metal oxide →  salt + water + heat, 2NaOH+CO2→Na2CO3+H2O, d) Acid and base in water, Acids produce H+ ions when dissolved in water. H+ ions cannot exist alone. They combine with water molecule (H2O) to form H3O + (hydronium ions). It conducts electricity., HCl (aq) ⎯⎯→ H+(aq) + Cl–(aq), Bases in water → Bases produce OH– ions when dissolved in water. Bases soluble in water are called alkalis. It conducts electricity., NaOH (aq) ⎯⎯→ Na+(aq) + OH–(aq), CLASSIFICATION OF ACIDS, 1. Depending upon the source from which they are obtained:, 2. Depending upon the strength of acids:, 3. Depending on the basis of water content:, MANUFACTURE OF ACIDS AND BASES, a) Non-metal oxide + water → acid, SO2(g)+H2O(l)→H2SO3(aq), SO3(g)+H2O(l)→H2SO4(aq), 4NO2(g)+2H2O(l)+O2(g)→4HNO3(aq), Non-metal oxides are thus referred to as acid anhydrides., b) Hydrogen + halogen → acid, H2(g)+Cl2(g)→2HCl(g), HCl(g)+H2O(l)→HCl(aq), c) Metallic salt + conc. sulphuric acid → salt + more volatile acid, 2NaCl(aq)+H2SO4(aq)→Na2SO4(aq)+2HCl(aq), 2KNO3(aq)+H2SO4(aq)→K2SO4(aq)+2HNO3(aq), d) Metal + oxygen → metallic oxide (base), 4Na(s)+O2(g)→2Na2O(s), 2Mg(s)+O2(g)→2MgO(s), e) Metal + water → base or alkali + hydrogen, Zn(s) + H2O(steam)→ ZnO(s)+ H2(g), f) Few metallic oxides + water → alkali, Na2O(s)+ H2O(l)→2NaOH(aq), g) Ammonia + water → ammonium hydroxide, NH3(g)+H2O(l)→NH4OH(aq), SALTS, Salts are ionic compounds made of a cation other than H+ ion and an anion other than OH– ion. A salt is a combination of an anion of an acid and a cation of a base., Examples – KCl, NaNO3, CaSO4,etc., Salts are usually prepared by the neutralisation reaction of an acid and a base., Formation of salts Salts are formed in many reactions involving acids and bases., 1. By Neutralization of acids and bases Salts are the product (besides water) of a neutralization reaction. For example,, Base Acid Salt Water, NaOH + HCl ⎯→ NaCl + H2O, KOH + HNO3 ⎯→ KNO3 + H2O, In general, MOH + HX ⎯→ MX + H2O, 2. By action of acids on metals: In a reaction between an acid and a metal, salt is produced along with hydrogen,, Metal Acid Salt Hydrogen, Zn + H2SO4 ⎯→ ZnSO4 + H2, 3. By action of acids on metal carbonates and hydrogen carbonates Salts are produced in reactions between acids and metal carbonates and hydrogen carbonates (bicarbonates) along with water and carbon dioxide., Metal/hydrogen, carbonate Acid Salt Water Carbon dioxide, CaCO3 + 2HCl ⎯→ CaCl2 + H2O + CO2, NaHCO3 + HCl ⎯→ NaCl + H2O + CO2, Type of salt and the nature of its aqueous solution, ACTION OF ACID, BASE AND SALT ON INDICATORS, An indicator tells us whether a substance is acidic or basic in nature, by the change in colour., DETERMINATION OF ACIDITY OR BASICITY OF A SOLUTION BASED ON PH CONCEPT, The term pH means “power of hydrogen” is the logarithm of the reciprocal of the hydrogen ion concentration. It is written as:, pH = –log [H+]., pH + pOH = 14 (where pOH = –log [OH-]., The method of calculation of pH used are valid for (i) solutions of strong acids and bases only and (ii) the solutions of acids or bases should not be extremely dilute and the concentrations of acids and bases should not be less than 10–6 mol L–1., Example 1: Calculate the pH of 0.001 molar solution of HCl., Solution: HCl is a strong acid and is completely dissociated in its solutions according to the process: HCl(aq) ⎯→ H+(aq) + Cl–(aq), From this process it is clear that one mole of HCl would give one mole of H+ ions. Therefore, the concentration of H+ ions would be equal to that of HCl i.e. 0.001 molar or 1.0 × 10–3 mol L–1, Thus, [H+] = 1 × 10–3 mol L–1, pH = –log[H+] = –(log 10–3) = –(–3 × log10) = –(3 × 1) = 3, Thus, pH = 3, Example 2: Calculate the pH of 1 × 10–4 molar solution of NaOH., Solution: NaOH is a strong base and dissociate in its solution as:, NaOH(aq) ⎯→ Na+(aq) + OH–(aq), One mole of NaOH would give one mole of OH– ions., Therefore, [OH–] = 1 × 10–4 mol L–1, pOH = –log[OH–] = –log × 10–4 = –(–4) = 4, Since pH + pOH = 14, pH = 14 – pOH = 14 – 4 = 10, Solve:, a) Calculate the pH of a solution in which the concentration of hydrogen ions is 1.0 × 10–8 mol L–1., b) What would be the pH of an aqueous solution of sulphuric acid which is 5 × 10–5 mol L–1 in concentration?