SALTS - Form 2 Chemistry Notes

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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

 Acid name  Chemical formula  Basicity  Normal salt  Acid salt
 Hydrochloric acid  HCl  Monobasic  Chloride(Cl-)  None
 Nitric(V)acid  HNO3  Monobasic  Nitrate(V)(NO3-)  None
 Nitric(III)acid  HNO2  Monobasic  Nitrate(III) (NO2-)  None
 Sulphuric(VI)acid  H2SO4  Dibasic  Sulphate(VI) (SO42-)  Hydrogen sulphate(VI) (HSO4-)
 Sulphuric(IV)acid  H2SO3  Dibasic  Sulphate(IV) (SO32-)  Hydrogen sulphate(IV) (HSO3-)
 Carbonic(IV)acid  H2CO3  Dibasic  Carbonate(IV)(CO32-)  Hydrogen carbonate(IV) (HCO3-)
 Phosphoric(V) acid  H3PO4  Tribasic  Phosphate(V) (PO43-)

 Dihydrogen phosphate(V) (H2PO42-)

Hydrogen diphosphate(V) (HP2O42-)

Table Showing Some Examples of Salts.

 Base/alkali  Cation  Acid  Anion  Salt  Chemical name of salts
 NaOH  Na+  HCl  Cl-  NaCl  Sodium(I)chloride
 Mg(OH)2  Mg2+  H2SO4  SO42-  MgSO4
 Mg(HSO4)2
 Magnesium sulphate(VI)
Magnesium hydrogen sulphate(VI)

 Pb(OH)2  Pb2+  HNO3  NO3-  Pb(NO3)2  Lead(II)nitrate(V)
 Ba(OH)2  Ba2+  HNO3  NO3-  Ba(NO3)2  Barium(II)nitrate(V)
 Ca(OH)2  Ca2+  H2SO4  SO42-  CaSO4  Calcium sulphate(VI)
 NH4OH  NH4+  H3PO4  PO43-  (NH4)3PO4
(NH4)2HPO4
NH4H2PO4
 Ammonium phosphate(V)
 Diammonium phosphate(V)
 Ammonium diphosphate(V)
 KOH  K+  H3PO4  PO43-  K3PO4  Potassium phosphate(V)
 Al(OH)3  Al3+  H2SO4  SO42-  Al2(SO4)2  Aluminium(III)sulphate(VI)
 Fe(OH)2  Fe2+  H2SO4  SO42-  FeSO4  Iron(II)sulphate(VI)
 Fe(OH)3  Fe3+  H2SO4  SO42-  Fe2(SO4)2  Iron(III)sulphate(VI)
  • 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.

 Name of hydrated salt  Chemical formula
 Copper(II)sulphate(VI)pentahydrate  CuSO4.5H2O
 Aluminium(III)sulphate(VI)hexahydrate  Al2(SO4)3.6H2O
 Zinc(II)sulphate(VI)heptahydrate  ZnSO4.7H2O
 Iron(II)sulphate(VI)heptahydrate  FeSO4.7H2O
 Calcium(II)sulphate(VI)heptahydrate  CaSO4.7H2O
 Magnesium(II)sulphate(VI)heptahydrate  MgSO4.7H2O
 Sodium sulphate(VI)decahydrate  Na2SO4.10H2O
 Sodium carbonate(IV)decahydrate  Na2CO3.10H2O
 Potassium carbonate(IV)decahydrate  K2CO3.10H2O
 Potassium sulphate(VI)decahydrate  K2SO4.10H2O
  • Some salts exist as a simple salt while some as complex salts. Below are some complex salts.

Table of Some Complex Salts

 Name of complex salt  Chemical formula   Colour of the complex salt
 Tetraamminecopper(II)sulphate(VI)  Cu(NH3)4SO4H2O  Royal/deep blue solution
 Tetraamminezinc(II)nitrate(V)  Zn(NH3)4(NO3)2  Colourless solution
 Tetraamminecopper(II) nitrate(V)  Cu(NH3)4(NO3)2  Royal/deep blue solution
 Tetraamminezinc(II)sulphate(VI)  Zn(NH3)4SO4  Colourless solution
  • Some salts exist as two salts in one. They are called double salts.

Table of Some Double Salts

 Name of double salts  Chemical formula
 Trona(sodium sesquicarbonate)  Na2CO3NaHCO3.2H2O
 Ammonium iron(II)sulphate(VI)  FeSO4(NH4)2SO4.2H2O
 Ammonium aluminium(III)sulphate(VI)  Al2(SO4)3(NH4)2SO4.H2O
  • 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

 Soluble salts  Insoluble salts
 All nitrate(V)salts  
 All sulphate(VI)/SO42- salts except the ones on the right  Barium(II) sulphate(VI)/BaSO4, Calcium(II) sulphate(VI)/CaSO4, Lead(II) sulphate(VI)/PbSO4
 All sulphate(IV)/SO32- salts except the ones on the right  Barium(II) sulphate(IV)/BaSO3, Calcium(II) sulphate(IV)/CaSO3, Lead(II) sulphate(IV)/PbSO3
 All chlorides/Clsalts except the ones on the right  Silver chloride/AgCl, Lead(II)chloride/PbCl2(dissolves in hot water)
 All phosphate(V)/PO43-  
 All sodium,potassium and ammonium salts  
 All hydrogen carbonates/HCO3-  
 All hydrogen sulphate(VI)/ HSO4-  
 Sodium carbonate/Na2CO3, potassium carbonate/ K2CO3, ammonium carbonate (NH4)2CO3 All carbonates/CO32- except the ones on the left
 All alkalis(KOH,NaOH, NH4OH)  All bases

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) + H2(g)

    Examples
    Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)
    Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
    Pb(s) + 2HNO3(aq) → Pb(NO3)2(aq) + H2(g)
    Ca(s) + 2HNO3(aq) → Ca(NO3)2(aq) + H2(g)
    Mg(s) + 2HNO3(aq) → Mg(NO3)2(aq) + H2(g)
    Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
    Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(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) + 2HNO3(aq) → Pb(NO3)2(aq) + H2O(l)
    Pb(OH)2(s) + 2HNO3(aq) → Pb(NO3)2(aq) + 2H2O(l)
    CaO(s) + 2HNO3(aq) → Ca(NO3)2(aq) + H2O(l)
    MgO(s) + 2HNO3(aq) → Mg(NO3)2(aq) + H2O(l)
    MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
    ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l)
    Zn(OH)2(s) + 2HNO3(aq) → Zn(NO3)2(aq) + 2H2O(l)
    CuO(s) + 2HCl(aq) → CuCl2(aq) + H2O(l)
    CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
    Ag2O(s) + 2HNO3(aq) → 2AgNO3(aq) + H2O(l)
    Na2O(s) + 2HNO3(aq) → 2NaNO3(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

    PbCO3(s) + 2HNO3(aq) → Pb(NO3)2(aq) + H2O(l)+ CO2(g)
    ZnCO3(s) + 2HNO3(aq) → Zn(NO3)2(aq) + H2O(l)+ CO2(g)
    CaCO3(s) + 2HNO3(aq) → Ca(NO3)2(aq) + H2O(l)+ CO2(g)
    MgCO3(s) + H2SO4(aq) → MgSO4(aq) + H2O(l)+ CO2(g)
    CuCO3(s) + H2SO4(aq) → CuSO4(aq) + H2O(l) + CO2(g)
    Ag2CO3(s) + 2HNO3(aq) → 2AgNO3(aq) + H2O(l) + CO2(g)
    Na2CO3(s) + 2HNO3(aq) → 2NaNO3(aq) + H2O(l) + CO2(g)
    K2CO3(s) + 2HCl(aq) → 2KCl(aq) + H2O(l) + CO2(g)
    NaHCO3(s) + HNO3(aq) → NaNO3(aq) + H2O(l) + CO2(g)
    KHCO3(s) + HCl(aq) → KCl(aq) + H2O(l) + CO2(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) + HNO3(aq) → NaNO3(aq) + H2O(l)
    KOH(aq) + HNO3(aq) → KNO3(aq) + H2O(l)
    KOH(aq) + HCl(aq) → KCl(aq) + H2O(l)
    2KOH(aq) + H2SO4(aq) → K2SO4(aq) + 2H2O(l)
    2NH4OH(aq) + H2SO4(aq) → (NH4)2SO4(aq) + 2H2O(l)
    NH4OH(aq) + HNO3(aq) → NH4NO3(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) + Cl2(g) → 2NaCl(s)
    2K(s) + Cl2(g) → 2KCl(s)
    Mg(s) + Cl2(g) → MgCl2(s)
    Ca(s) + Cl2(g) → CaCl2(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) + 3Cl2(g) → 2AlCl3(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.
    AlCl3(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) + 3Cl2(g) → 2FeCl3(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.
    FeCl3(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
    CuSO4(aq) + Na2CO3(aq) → CuCO3(s) + Na2SO4(aq)
    BaCl2(aq) + K2SO4(aq) → BaSO4(s) + 2KCl(aq)
    Pb(NO3)2(aq) + K2SO4(aq) → PbSO4(s) + 2KNO3(aq)
    2AgNO3(aq) + MgCl2(aq) → 2AgCl(s) + Mg(NO3)2(aq)
    Pb(NO3)2(aq) + (NH4)2SO4(aq) → PbSO4(s) + 2NH4NO3(aq)
    BaCl2(aq) + K2SO3(aq) → BaSO3(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) NH3(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.
    2KNO3(s) → 2KNO2(s) + O2(g)
    2NaNO3(s) → 2NaNO2(s) + O2(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) + 4NO2(g) + O2(g)
    2Mg(NO3)2(s) → 2MgO(s) + 4NO2(g) + O2(g)
    2Zn(NO3)2(s) → 2ZnO(s) + 4NO2(g) + O2(g)
    2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)
    2Cu(NO3)2(s) → 2CuO(s) + 4NO2(g) + O2(g)
    2Fe(NO3)2(s) → 2FeO(s) + 4NO2(g) + O2(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.
    2AgNO3(s) → 2Ag(s) + 2NO2(g) + O2(g)
    2Hg(NO3)2(s) → 2Hg(s) + 4NO2(g) + O2(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.
    NH4NO3(s) → N2O(g) + H2O(l)
    NH4NO2(s) → N2(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.
    2FeSO4(s) → Fe2O3(s) + SO3(g) + SO2(g)
    Fe2(SO4)3(s) → Fe2O3(s) + SO3(g)
    CuSO4(s) → CuO(s) + SO3(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.
    CuCO3(s) → CuO(s) + CO2(g)
    CaCO3(s) → CaO(s) + CO2(g)
    PbCO3(s) → PbO(s) + CO2(g)
    FeCO3(s) → FeO(s) + CO2(g)
    ZnCO3(s) → ZnO(s) + CO2(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.
    2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(l)
    2KHCO3(s) → K2CO3(s) + CO2(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) → CaCO3(s) + CO2(g) + H2O(l)
    Mg(HCO3)2(aq) → MgCO3(s) + CO2(g) + H2O(l)

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