SALTS - Chemistry Notes Form 2

• Introduction to salts
  • Table Showing Normal and Acid Salts Derived from Common Acids
  • Table Showing Some Examples of Salts.
  • Table Showing Some Hydrated Salts.
  • Table of Some Complex Salts
  • Table of Some Double Salts
  • Table of Solubility of Salts
• Preparation of Salts
  • Soluble Salts
  • Insoluble Salts
• Effect of Heat on Salts
  • Effect of Heat on Chlorides
  • Effect of Heat on Nitrate(V)
  • Effect of Heat on Sulphate(VI)
  • Effect of Heat on Carbonates(IV) and Hydrogen Carbonate(IV).

Introduction to Salts

• A salt is an ionic compound formed when the cation from a base combine with the anion derived from an acid.
• A salt is therefore formed when the hydrogen ions in an acid are replaced wholly/fully or partially/partly ,directly or indirectly by a metal or ammonium radical.
• The number of ionizable/replaceable hydrogen in an acid is called basicity of an acid.
• Some acids are therefore:
  1. monobasic acids generally denoted HX e.g.
     HCl, HNO₃,HCOOH,CH₃COOH.
  2. dibasic acids ; generally denoted H₂X e.g.
     H₂SO₄, H₂SO₃, H₂CO₃,HOOCOOH.
  3. tribasic acids ; generally denoted H₃X e.g.
     H₃PO₄.
• Some salts are normal salts while other are acid salts.
• A normal salt is formed when all the ionizable /replaceable hydrogen in an acid is replaced by a metal or metallic /ammonium radical.
• An acid salt is formed when part/portion the ionizable /replaceable hydrogen in an acid is replaced by a metal or metallic /ammonium radical.

Table Showing Normal and Acid Salts Derived from Common Acids

Table Showing Some Examples of Salts.

• Some salts undergo hygroscopy, deliquescence and efflorescence.
  • Hygroscopic salts /compounds are those that absorb water from the atmosphere but do not form a solution. Some salts which are hygroscopic include anhydrous copper(II)sulphate(VI), anhydrous cobalt(II)chloride, potassium nitrate(V), common table salt.
  • Deliquescent salts /compounds are those that absorb water from the atmosphere and form a solution. Some salts which are deliquescent include: Sodium nitrate(V),Calcium chloride, Sodium hydroxide, Iron(II)chloride, Magnesium chloride.
  • Efflorescent salts/compounds are those that lose their water of crystallization to the atmosphere. Some salts which effloresces include: sodium carbonate decahydrate, Iron(II)sulphate(VI)heptahydrate, sodium sulphate (VI)decahydrate.
• Some salts contain water of crystallization.They are hydrated.Others do not contain water of crystallization. They are anhydrous.

Table Showing Some Hydrated Salts.

• Some salts exist as a simple salt while some as complex salts. Below are some complex salts.

Table of Some Complex Salts

• Some salts exist as two salts in one. They are called double salts.

Table of Some Double Salts

• Some salts dissolve in water to form a solution. They are said to be soluble. Others do not dissolve in water. They form a suspension/precipitate in water.

Table of Solubility of Salts

Preparation of Salts

Salts can be prepared in a school laboratory by a method that uses its solubility in water.

Soluble Salts

• Soluble salts may be prepared by using any of the following methods:

(i) Direct displacement/reaction of a metal with an acid.

• By reacting a metal higher in the reactivity series than hydrogen with a dilute acid, a salt is formed and hydrogen gas is evolved.
• Excess of the metal must be used to ensure all the acid has reacted.
• When effervescence/bubbling /fizzing has stopped ,excess metal is filtered.
• The filtrate is heated to concentrate then allowed to crystallize.
• Washing with distilled water then drying between filter papers produces a sample crystal of the salt. i.e.
  M_(s) + H₂X → MX_(aq) + H_2(g)
  
  Examples
  Mg_(s) + H₂SO_4(aq) → MgSO_4(aq) + H_2(g)
  Zn_(s) + H₂SO_4(aq) → ZnSO_4(aq) + H_2(g)
  Pb_(s) + 2HNO_3(aq) → Pb(NO₃)_2(aq) + H_2(g)
  Ca_(s) + 2HNO_3(aq) → Ca(NO₃)_2(aq) + H_2(g)
  Mg_(s) + 2HNO_3(aq) → Mg(NO₃)_2(aq) + H_2(g)
  Mg_(s) + 2HCl_{(aq) →} MgCl_2(aq) + H_2(g)
  Zn_(s) + 2HCl_(aq) → ZnCl_2(aq) + H_2(g)

(ii) Reaction of an insoluble base with an acid

• By adding an insoluble base (oxide/hydroxide )to a dilute acid until no more dissolves, in the acid,a salt and water are formed.
• Excess of the base is filtered off. The filtrate is heated to concentrate, allowed to crystallize then washed with distilled water before drying between filter papers.
  
  Examples
  PbO_(s) + 2HNO_3(aq) → Pb(NO₃)_2(aq) + H₂O_(l)
  Pb(OH)_2(s) + 2HNO_3(aq) → Pb(NO₃)_2(aq) + 2H₂O_(l)
  CaO_(s) + 2HNO_3(aq) → Ca(NO₃)_2(aq) + H₂O_(l)
  MgO_(s) + 2HNO_3(aq) → Mg(NO₃)_2(aq) + H₂O_(l)
  MgO_(s) + 2HCl_(aq) → MgCl_2(aq) + H₂O_(l)
  ZnO_(s) + 2HCl_(aq) → ZnCl_2(aq) + H₂O_(l)
  Zn(OH)_2(s) + 2HNO_3(aq) → Zn(NO₃)_2(aq) + 2H₂O_(l)
  CuO_(s) + 2HCl_(aq) → CuCl_2(aq) + H₂O_(l)
  CuO_(s) + H₂SO_4(aq) → CuSO_4(aq) + H₂O_(l)
  Ag₂O_(s) + 2HNO_3(aq) → 2AgNO_3(aq) + H₂O_(l)
  Na₂O_(s) + 2HNO_3(aq) → 2NaNO_3(aq) + H₂O_(l)

(iii) Reaction of insoluble /soluble carbonate /hydrogen carbonate with an acid.

• By adding an excess of a soluble /insoluble carbonate or hydrogen carbonate to adilute acid, effervescence /fizzing/bubbling out of carbon(IV)oxide gas shows the reaction is taking place.
• When effervescence /fizzing/bubbling out of the gas is over, excess of the insoluble carbonate is filtered off.
• The filtrate is heated to concentrate ,allowed to crystallize then washed with distilled water before drying between filter paper papers 
  
  Examples
  
  PbCO_3(s) + 2HNO_3(aq) → Pb(NO₃)_2(aq) + H₂O_(l)+ CO_2(g)
  ZnCO_3(s) + 2HNO_3(aq) → Zn(NO₃)_2(aq) + H₂O_(l)+ CO_2(g)
  CaCO_3(s) + 2HNO_3(aq) → Ca(NO₃)_2(aq) + H₂O_(l)+ CO_2(g)
  MgCO_3(s) + H₂SO_4(aq) → MgSO_4(aq) + H₂O_(l)+ CO_2(g)
  CuCO_3(s) + H₂SO_4(aq) → CuSO_4(aq) + H₂O_(l) + CO_2(g)
  Ag₂CO_3(s) + 2HNO_{3(aq) →} 2AgNO_3(aq) + H₂O_(l) + CO_2(g)
  Na₂CO_3(s) + 2HNO_3(aq) → 2NaNO_3(aq) + H₂O_(l) + CO_2(g)
  K₂CO_3(s) + 2HCl_(aq) → 2KCl_(aq) + H₂O_(l) + CO_2(g)
  NaHCO_3(s) + HNO_3(aq) → NaNO_3(aq) + H₂O_(l) + CO_2(g)
  KHCO_3(s) + HCl(aq) → KCl_(aq) + H₂O_(l) + CO_2(g)

(iv) Neutralization/reaction of soluble base/alkali with dilute acid

• By adding an acid to a burette into a known volume of an alkali with 2-3 drops of an indicator, the colour of the indicator changes when the acid has completely reacted with an alkali at the end point.
• The procedure is then repeated without the indicator.
• The solution mixture is then heated to concentrate, allowed to crystallize, washed with distilled water before drying with filter papers.
  
  Examples
  NaOH_(aq) + HNO_3(aq) → NaNO_3(aq) + H₂O_(l)
  KOH_(aq) + HNO_3(aq) → KNO_3(aq) + H₂O_(l)
  KOH_(aq) + HCl_(aq) → KCl_(aq) + H₂O_(l)
  2KOH_(aq) + H₂SO_4(aq) → K₂SO_4(aq) + 2H₂O_(l)
  2NH₄OH_(aq) + H₂SO_4(aq) → (NH₄)₂SO_4(aq) + 2H₂O_(l)
  NH₄OH_(aq) + HNO_3(aq) → NH₄NO_3(aq) + H₂O_(l)

(iv) Direct synthesis/combination.

• When a metal burn in a gas jar containing a non metal, the two directly combine to form a salt. e.g.
  2Na_(s) + Cl_2(g) → 2NaCl_(s)
  2K_(s) + Cl_2(g) → 2KCl_(s)
  Mg_(s) + Cl_2(g) → MgCl_2(s)
  Ca_(s) + Cl_2(g) → CaCl_2(s)
• Some salts once formed undergo sublimation and hydrolysis.
• Care should be taken to avoid water/moisture into the reaction flask during their preparation.
• Such salts include aluminium(III)chloride(AlCl₃) and iron (III)chloride(FeCl₃)
• Heated aluminium foil reacts with chlorine to form aluminium(III)chloride that sublimes away from the source of heating then deposited as solid again
  2Al_(s) + 3Cl_2(g) → 2AlCl_3(s/g)
• Once formed aluminium(III)chloride hydrolyses/reacts with water vapour / moisture present to form aluminium hydroxide solution and highly acidic fumes of hydrogen chloride gas.
  AlCl_3(s)+ 3H₂O_(g) → Al(OH)_3(aq) + 3HCl_(g)
• Heated iron filings reacts with chlorine to form iron(III)chloride that sublimes away from the source of heating then deposited as solid again
  2Fe_(s) + 3Cl_2(g) → 2FeCl_3(s/g)
• Once formed , iron(III)chloride hydrolyses/reacts with water vapour / moisture present to form iron hydroxide solution and highly acidic fumes of hydrogen chloride gas.
  FeCl_3(s)+ 3H₂O_(g) → Fe(OH)_3(aq) + 3HCl_(g)

Insoluble Salts

• Insoluble salts can be prepared by reacting two suitable soluble salts to form one soluble and one insoluble.
• This is called double decomposition or precipitation.
• The mixture is filtered and the residue is washed with distilled water then dried.
  
  Examples
  CuSO_4(aq) + Na₂CO_3(aq) → CuCO_3(s) + Na₂SO_4(aq)
  BaCl_2(aq) + K₂SO_4(aq) → BaSO_4(s) + 2KCl_(aq)
  Pb(NO₃)_2(aq) + K₂SO_4(aq) → PbSO_4(s) + 2KNO_3(aq)
  2AgNO_3(aq) + MgCl_2(aq) → 2AgCl_(s) + Mg(NO₃)_2(aq)
  Pb(NO₃)_2(aq) + (NH₄)₂SO_4(aq) → PbSO_4(s) + 2NH₄NO_3(aq)
  BaCl_2(aq) + K₂SO_3(aq) → BaSO_3(s) + 2KCl_(aq)

Effect of Heat on Salts

• Salts may lose their water of crystallization, decompose, melt or sublime on heating on a Bunsen burner flame.
• The following shows the behavior of some salts on heating gently /or strongly in a laboratory school burner:

Effect of Heat on Chlorides

• All chlorides have very high melting and boiling points and therefore are not affected by laboratory heating except ammonium chloride.
• Ammonium chloride sublimes on gentle heating.
• It dissociate into the constituent ammonia and hydrogen chloride gases on strong heating.
  NH₄Cl_(s) ⇌ NH₄Cl_(g) ⇌ NH_3(g) + HCl_{(g)
           Sublimation        Dissciation}

Effect of Heat on Nitrate(V)

• Potassium nitrate(V)/KNO₃ and sodium nitrate(V)/NaNO₃ decompose on heating to form Potassium nitrate(III)/KNO₂ and sodium nitrate(III)/NaNO₂ respectively and producing Oxygen gas in each case.
  2KNO_3(s) → 2KNO_2(s) + O_2(g)
  2NaNO_3(s) → 2NaNO_2(s) + O_2(g)
• Heavy metal nitrates(V) salts decompose on heating to form the oxide and a mixture of brown acidic nitrogen(IV)oxide and oxygen gases. e.g.
  2Ca(NO₃)_2(s) → 2CaO_(s) + 4NO_2(g) + O_2(g)
  2Mg(NO₃)_2(s) → 2MgO_(s) + 4NO_2(g) + O_2(g)
  2Zn(NO₃)_2(s) → 2ZnO_(s) + 4NO_2(g) + O_2(g)
  2Pb(NO₃)_2(s) → 2PbO_(s) + 4NO_2(g) + O_2(g)
  2Cu(NO₃)_2(s) → 2CuO_(s) + 4NO_2(g) + O_2(g)
  2Fe(NO₃)_2(s) → 2FeO_(s) + 4NO_2(g) + O_2(g)
• )Silver(I)nitrate(V) and mercury(II) nitrate(V) are lowest in the reactivity series. They decompose on heating to form the metal(silver and mercury)and the Nitrogen(IV)oxide and oxygen gas. i.e.
  2AgNO_3(s) → 2Ag_(s) + 2NO_2(g) + O_2(g)
  2Hg(NO₃)_2(s) → 2Hg_(s) + 4NO_2(g) + O_2(g)
• Ammonium nitrate(V) and Ammonium nitrate(III) decompose on heating to Nitrogen(I)oxide(relights/rekindles glowing splint) and nitrogen gas respectively. Water is also formed.i.e.
  NH₄NO_3(s) → N₂O_(g) + H₂O_(l)
  NH₄NO_2(s) → N_2(g) + H₂O(l)

Effect of Heat on Sulphate(VI)

• Only Iron(II)sulphate(VI), Iron(III)sulphate(VI) and copper(II)sulphate(VI) decompose on heating.
• They form the oxide, and produce highly acidic fumes of acidic sulphur(IV)oxide gas.
  2FeSO_4(s) → Fe₂O_3(s) + SO_3(g) + SO_2(g)
  Fe₂(SO₄)_3(s) → Fe₂O_3(s) + SO_3(g)
  CuSO_4(s) → CuO_(s) + SO_3(g)

Effect of Heat on Carbonates(IV) and Hydrogen Carbonate(IV).

• Sodium carbonate(IV)and potassium carbonate(IV)do not decompose on heating.
• Heavy metal nitrate(IV)salts decompose on heating to form the oxide and produce carbon(IV)oxide gas.
• Carbon (IV)oxide gas forms a white precipitate when bubbled in lime water.
• The white precipitate dissolves if the gas is in excess. e.g.
  CuCO_3(s) → CuO_(s) + CO_2(g)
  CaCO_3(s) → CaO_(s) + CO_2(g)
  PbCO_3(s) → PbO_(s) + CO_2(g)
  FeCO_3(s) → FeO_(s) + CO_2(g)
  ZnCO_3(s) → ZnO_(s) + CO_2(g)
• Sodium hydrogen carbonate(IV) and Potassium hydrogen carbonate(IV)decompose on heating to give the corresponding carbonate (IV) and form water and carbon(IV)oxide gas. i.e.
  2NaHCO_3(s) → Na₂CO_3(s) + CO_2(g) + H₂O_(l)
  2KHCO_3(s) → K₂CO_3(s) + CO_2(g) + H₂O_(l)
• Calcium hydrogen carbonate (IV) and Magnesium hydrogen carbonate(IV) decompose on heating to give the corresponding carbonate (IV) and form water and carbon(IV)oxide gas. i. e.
  Ca(HCO₃)_2(aq) → CaCO_3(s) + CO_2(g) + H₂O_(l)
  Mg(HCO₃)_2(aq) → MgCO_3(s) + CO_2(g) + H₂O_(l)