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Acids, Bases, and Salts

ACIDS
Physical Properties of Acids
  • They have a sour taste. Unripe fruits, vinegar, sour milk, have a sour taste because they contain acids
  • Acids turn blue litmus red
  • Concentrated forms of strong acids are corrosive
Chemical Properties of Acids
  • Acids react with metals above hydrogen in the electrochemical series to liberate hydrogen gas. However, dilute HNO3 reacts with metals to form water instead of hydrogen
  • Acids react with bases and alkalis to form salt and water only. This reaction is known as neutralization
  • Acids react with trioxocarbonates (iv) or hydrogen trioxocarbonates (iv) to liberate cardon (iv) oxide
  • Acids react with trioxosulfate (iv) salts to liberate SO2 gas
Methods of Preparation of Acids
  • By dissolving an acid anhydride in water to form its corresponding acid. An acid anhydride is the oxide of a non-metal which can dissolve in water to produce its corresponding acid
  • By direct combination of the heated constituent elements to produce a gas which is then dissolved in water to form the desired acid
  • By using hydrogen sulfide gas to precipitate an insoluble sulfide from a metallic salt solution
  • By displacing a weaker or more volatile acid from its salt by a stronger or less volatile acid
Uses of Acids
  • H2SO4 is used in car batteries and it is also used in the manufacture of fertilizers, detergents, and drugs
  • Fatty acids are used in the manufacture of soap
  • Some acids are used as preservatives in food industries
BASES AND ALKALIS
  • Bases are proton acceptors
  • Bases are oxides or hydroxides of metals which can react with acids to give salt and water only
  • Bases which dissolve in water are known as alkalis. These are NaOH, KOH, sllightly soluble Ca(OH)2 and and aqueous ammonia (NH4OH)
  • The insoluble bases are Mg(OH)2, Al(OH)3, Zn(OH)2, Pb(OH)2 and Fe(OH)3
  • The soluble bases are strong allkalis which ionize completely in aqueous solutions to produce negatively charged hydroxide and positively charged metallic ions
Preparation of Bases
  • By the reaction between a metal and the oxygen of the air to form a basic oxide
  • By heating the trioxonitrates (v) or trioxocarbonates (v) of metals
  • By the reaction between reactive metals and water or steam
  • By double decomposition reactions involving a salt solution of a metal and sodium hydroxide or potassium hydroxide solution
Physical Properties of Alkalis
  • They have a bitter taste
  • They turn red litmus paper or solution blue
  • They give a soapy feel when touched
  • Concentrated forms of NaOH and KOH are corrosive
Chemical Properties of Bases/Alkalis
  • Bases react with acids to give salt and water only
  • Alkalis precipitate metallic hydroxides, by a double decomposition reaction, when added to the solution of some metallic salts
  • Alkalis react with any ammonium salt to liberate ammonia gas
  • NaOH and KOH are stable to heat but the hydroxides of Mg, Al, Zn, Fe, Pb, and Cu decompose into their oxides and water on heating
Uses of Bases
  • NaOH is used in the manufacture of soap, paper, and sodium salts
  • Ammonia is used in manufacture of fertilizers and household cleaners such as bleach, etc.
  • Calcium hydroxide is used in the manufacture of mortar, cement, and plaster. It is also used in neutralizing acidic soils
  • Magnesium hydroxide is used in the manufacture of toothpaste
Ionic Product of Water, Kw
  • Even in its pure state, water ionizes feebly and reversibly as H2O(l) ↔ H+(aq) + OH(aq)
  • For this equilibrium process, the product of the concentrations of hydrogen ions and hydroxide ions in moldm-3 is equal to 1 × 10-14 mol2dm-6 at 25 °C. This product is known as the ionic product of water, Kw
  • Kw = [H+][OH] = 1 × 10-14 mol2dm-6
  • Kw is a constant at constant temperature for any aqueous solution – neutral, acidic, or basic solution
  • As the ionization of water is endothermic, Kw increases with an increase in temperature and decreases with a decrease in temperature
  • From the Kw concept, the concentration of H+(aq) or OH(aq) may be determined in any aqueous solution if either [OH] or [H+] is known as 25 °C
  • If [H+] > [OH], the aqueous solution is acidic
  • If [H+] < [OH], the aqueous solution is basic
  • If [H+] = [OH], the aqueous solution or water is neutral

Examples

  1. At 25 °C, the hydrogen ion concentration of an aqueous solution is 4.6 × 10-8 moldm-3. Calculate the hydroxyl ion concentration of this solution and decide if the solution is acidic, basic, neutral
    • Kw = [H+][OH] = 1 × 10-14
    • ∴ [OH] = 1 × 10-14 ÷ 4.6 × 10-8 = 2.2 × 10-7moldm-3
    • Thus, the solution is basic
  2. What is the hydrogen ion concentration of a 250 cm3 solution containing 1.48 g of dissolved calcium hydroxide? (Ca = 40, O = 16, H = 1)
    • 1.48 g Ca(OH)2 / 250cm3 solution is the same as 1.48/74 mol Ca(OH)2 / 250cm3 solution
    • ∴ concentration of Ca(OH)2 solution = 1.48/74 × 4 = 0.08 moldm-3
    • From Ca(OH)2 → Ca2+ + 2OH; the concentration of OH = 2 × 0.08 = 0.16 moldm-3
    • From [H+][OH] = 1 × 10-14
    • [H+] = 1 × 10-14 ÷ 0.16 = 6.25 × 10-14 moldm-3
pH and pOH
  • pH is defined as the negative logarithm of hydrogen ion concentration in moldm-3
  • pH = -log [H+]
  • For acidic solutions, pH > 7. For basic solutions, pH < 7. For neutral solutions, pH = 7
  • pH of a solution can be measured accurately by a pH meter
  • For all solutions, the higher the hydrogen ion concentration, the lower the pH
  • Like pH, pOH is defined as the negative logarithm of hydroxide ion concentration in moldm-3
  • pOH = -log [OH]
  • From Kw = [H+][OH] = 1 × 10-14
  • log Kw = log [H+][OH] = log(1 × 10-14)
  • log [H+] + log [OH] = -14
  • Multiplying through by -1: -log [H+] – log [OH] = 14
  • ∴ pH + pOH = 14

Examples

  1. Calculate the pH of 0.2 moldm-3 sodium hydroxide solution
    • NaOH → Na+ + OH
    • From the equation, [NaOH] = [OH]
    • pOH = – log (0.2) = 0.699
    • But pH + pOH = 14
    • ∴ pH = 13.301
  2. What is the mass of potassium hydroxide present in 200 cm³ of its solution whose pH is 11.5? (K = 39, O = 16, H =1)
    • pH = 11.5
    • ∴pOH = 2.5 = -log [OH]
    • ∴ [OH] = 3.16 × 10-3 moldm-3
    • From KOH → K+ + OH
    • [OH] = [KOH] = 3.16 × 10-3 moldm-3
    • Using mass/MM = CV/1000
    • mass/563.16 × 10-3 × 200 / 1000
    • ∴ mass = 0.035 g
Salts
  • A salt is a compound formed when all or part of the ionizable hydrogen atoms in an acid is replaced by metallic or ammonium ions
  • Examples of salts include, NaCl, K2SO4, (NH4)2SO4
  • A salt that contains water of crystallization is known as a hydrated salt. Examples include Iron (ii) tetraoxosulfate (vi) heptahydrate, Sodium trioxocarbonate (iv) – decahydrate
  • A salt without water of crystallization is known as an anhydrous salt or an anhydrate
  • Water of crystallization is the number of moles of water associated with one mole of a hydrated salt
Types of Salts
  • Normal Salt contains only metal ions (or ammonium ions) and the acid radical.
    • Examples include NaCl, (NH4)2SO4
    • A normal salt does not contain any replaceable hydrogen atom in its formula unit
  • An Acid Salt contains at least one replaceable hydrogen atom in its formula unit
    • Examples include NaHSO4, Ca(HCO3)2
    • An acid salt is formed when the replaceable hydrogen ions in the acid are only partially replaced by a metal
    • Only acids with a basicity of two or more can form acid salts
  • Basic Salt contains hydroxide or metal oxides in addition to metal ions and an acid radical
    • Examples include Zn(OH)Cl, Mg(OH)NO3
    • Only bases containing more than one OH group per mole can react with acids to form basic salts
  • Double Salt contains two different cations (either two different metal ions or a metal ion and an ammonium ion) and acid radicals
    • Examples include KAl(SO4)2⋅12H2O, (NH4)2Fe(SO4)2.6H2O
    • A double salt is formed when equimolar solutions of two normal salts react together at high temperature. On cooling, crystals of double salts are formed
    • Double salts dissolve in water to produce two different cations and acid radicals
    • A double salt will show the properties of its constituent salts
  • Complex Salt contains one complex positive ion and one simple negative ion or one simple positive ion and one complex negative ion
    • Examples include Potassium hexacyanoferrate (iii), Copper (ii) tetramine chloride
Uses of Salts
  • Na2CO3 is used in the removal of both temporary and permanent hardness in water
  • CaCl2 is used as a drying agent
  • CaSO4 is used for making Plaster of Paris
  • MgSO4 is used as a laxative
  • PbCO3 is used in making paint
Salt Solubility Rules
  • All salts of potassium, sodium, and ammonium are soluble
  • All trioxonitrates (v) salts are soluble
  • All chlorides are soluble except silver chloride, mercury (i) chloride, and lead (ii) chloride
  • All tetraoxosulfate (vi) salts are soluble except BaSO4, PbSO4, and slightly soluble CaSO4
  • All common trioxocarbonates (iv), trioxosulfates (iv) and sulfides are INSOLUBLE except those of sodium, potassium, ammonium
Preparation of Soluble Salts
  • The action of an acid on a metal
  • The action of an acid on insoluble
  • The action of an acid on a suitable trioxocarbonate (iv)
    This method is unsuitable if the trioxocarbonate (iv) and the salt formed are both insoluble because the insoluble salt forms precipitates on the unchanged trioxocarbonate (iv) and stops the reaction
  • The action of an acid on an alkali
Preparation of Insoluble Salts
  • Any insoluble salt may be prepared by mixing two aqueous solutions which can provide the oppositely charged ions which make up the insoluble salt
  • Both soluble and insoluble salts can be prepared by direct combination of their constituents
  • Chlorides of heavy metals are prepared in the anhydrous state by heating the metal in a current of dry chlorine or hydrogen chloride gas. This is because they usually crystallize with water of crystallization. An attempt to remove the water of crystallization converts the salt to a basic salt.
  • If an insoluble salt is to be prepared from an insoluble substance, the insoluble substance is first converted to a soluble substance which is then used to obtain the desired insoluble substance by double decomposition
Acid – Base Indicators
  • An acid base indicator is a weak organic acid or base which can exhibit one color in an acidic solution and another distinct color in a basic solution
  • The color observed when an indicator is added to a solution depends on the pH of the solution
  • Each indicator has a pH range over which it undergoes a change from one color to another

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