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Sulphuric Acid

A molecule of Sulphuric Acid, H2SO4, consists of two atoms of hydrogen, one atom of sulphur and four atoms of oxygen

Sulphuric acid is a colourless viscous corrosive oily liquid, which has

  • Melting Point : 10.3 degC
  • Boiling Point : 338 degC
  • Formula weight 98.08
  • Specific gravity or density 1.94
  • Flash point none

Sulphuric acid is the strong acid produced by dissolving sulphur trioxide in water.

SO3 + H2O ==> H2SO4

The Strength of Acids is determined by the degree to which they are ionised in aqueous solution.

For example, Sulphuric Acid, H2SO4, which is a strong acid is fully dissociated, and all the displaceable hydrogen in the acid is present in solution as Hydrogen Ion, H(+). H2SO4 ==> H(+) + SO4

100% as H(+)

In contrast, the weak acids ethanoic acid, CH3COOH, is partially ionised in solution, and only approximately 5% of the displaceable Hydrogen in the acid is present in solution as hydrogen ion, H(+).

Preparation of Sulphuric Acid

Combustion of Sulphur

When a small amount of Sulphur, S, is kindled on a deflagrating spoon, it burns with a bright blue flame when introduced into a gas jar containing Oxygen, O2. A gas, Sulphur Dioxide, SO2, is the main product of the combustion. However, a little Sulphur Trioxide, SO3, is also formed, which makes the gas slightly cloudy.

S + O2 ==> SO2

Sulphur Dioxide

2S + 3O2 ==> 2SO3

Sulphur Trioxide

When shaken with water, the products of combustion dissolve, forming an acidic solution which turns litmus red.

Reactions of Sulphuric acid

Electrolysis of a Solution of dilute Sulphuric Acid

The Electrolysis of an Aqueous Solution of dilute Sulphuric Acid is often carried out in a Hofmann Voltammeter, an apparatus in which the gases evolved at the anode and cathode can be collected in separate graduated tubes. When the solution is electrolyzed hydrogen is produced at the cathode and oxygen at the anode. These gases can be shown to be present in a 2 to 1 ratio and result from the electrolysis of water under acidic conditions.

Sulphuric acid is a strong electrolyte is fully dissociated in aqueous solution.

H2SO4 ==> 2 H(+) + SO4(2 -) Water is a weak electrolyte and is only slightly dissociated H2O ==> H(+) + OH(-) During electrolysis, the Hydrogen Ions, H(+), migrates towards the cathode, and are discharged there (i.e. they gain an electron and are converted to hydrogen gas). 2 H(+) + 2 e(-) ==> H2-

At the anode the concentration of Hydroxyl Ions, HO(-),is too low to maintain a reaction and the Sulphate Ions, SO4(2 -) are not oxidized but remain on in solution at the end. Water molecules must be the species reacting at the anode.

2 H2O ==> O2 + 4 H(+) + 4 e(-)

The overall reaction is

Cathode Reaction :

2 H(+) + 2e(-) ==> H2 4 H(+) + 4e(-) ==> 2H2

Anode Reaction :

2 H2O ==> O2 + 4 H(+) + 4 e(-)

Overall Cell Reaction:

4 H(+) + 2 H2O ==> 2 H2 + O2 + 4 H(+)

For every Hydrogen Ions, H(+), discharged at the anode, another hydrogen ion is formed at the cathode. The net result is that the concentration of the Sulphuric Acid, H2SO4, remains constant and this electrolysis consists of the decomposition of water with the overall reaction

2H2O ==> 2H2- + O2-

Ferrous Sulphate, Fe(II)SO4, is the salt formed when Iron, Fe, is dissolved in Sulphuric Acid, H2SO4.

Hydrogen Chloride, HCl, may be prepared in the laboratory by heating Concentrated Sulphuric Acid, H2SO4, with Sodium Chloride, NaCl.

NaCl + H2SO4 ==> NaHSO4 + HCl

Many Metallic Chlorides liberate Chlorine, Cl2, when treated with Sulphuric Acid, H2SO4, and Manganese Dioxide, MnO2).

Many Metallic Chlorides liberate Hydrogen Chloride gas, HCl, when warmed with concentrated Sulphuric Acid, H2SO4.

Sulphur Trioxide, SO3, is prepared by heating concentrated Sulphuric Acid, H2SO4, with a large excess of Phosphorus Pentoxide, P2O5.

H2SO4 + P2O5 ==> SO3 + 2 HPO3

Sulphur Dioxide, SO2, is usually made in the laboratory by heating concentrated Sulphuric Acid, H2SO4, with Copper turnings, Cu.

Cu + 2 H2SO4 ==> CuSO4 + SO2 + 2 H2O

Hydrogen Fluoride, HF, can be prepared in the laboratory by heating Concentrated Sulphuric Acid, H2SO4, with Calcium Fluoride, CaF2.

H2SO4 + CaF2 ==> 2 HF + CaSO4

Hydrogen Iodide, HI, can be prepared by direct combination of the elements using a platinum catalyst. In the laboratory it is prepared by heating Concentrated Sulphuric Acid, H2SO4, with Sodium Iodide, NaI.

H2SO4 + 2 NaI ==> 2 HI + Na2SO4

Methanol, CH3OH, does not undergo dehydration reactions. Instead, in reaction with Sulphuric Acid, H2SO4, the ester, Dimethyl Sulphate, (CH3)2SO4, is formed.

concentrated H2SO4 2CH3OH ==> (CH3)2SO4 + H2O Methanol Dimethyl Water Sulphate

Sulphuric Acid, H2SO4, absorbs Ethylene, C2H4, at room temperature to form Ethyl Hydrogen Sulphate, C2H5.HSO4, with much evolution of heat.

C2H4 + H2SO4 ==> C2H5.HSO4

If this is treated with Water, H2O and warmed, Ethanol, C2H5OH, is formed.

heat C2H5.HSO4 + H2O ==> C2H5OH + H2SO4

Zinc

Zinc, Zn, is a transition metallic element found in Group IIb of the Periodic Table.

Discovery

Zinc was known from ancient times, when Brass (i.e. an alloy of Copper and Zinc) was used. Lohneyes was the first to apply the term "Zinc" correctly to the metal that we know today in 1697AD.

Occurrence

Zinc is found in different ore forms, including

  • Zinc Spar, ZnCO3,
  • Calamine, ZnCO3,
  • Zinc Blande, ZnS,
  • Zincite, ZnO,
  • Willemite, 2ZnO.SiO2,
  • Franklinite, (ZnFe)O.Fe2O3, and
  • Zinc Spinel, ZnO.Al2O3.
Extraction

Zinc is extracted using two process :

  • Roasting, which involves the preparation of zinc oxide and
  • Reduction, with charcoal which involves the release of the zinc from the oxide. PropertiesZinc is
    • a bright bluish-white metal, which is brittle at room temperature.
    • is slowly oxidised in moist air.
    • is an good conductor of heat and electricity.
    Reactions
    • Zinc burns in air at 1000 degC to form a bulky mass of zinc oxide which is known as Philosopher's Wool.
    • Zinc reacts with dilute acids, with the liberation of hydrogen.

      Zn + H2SO4 ==> ZnSO4 + H2

    • Zinc in the presence of dilute acid is a powerful reducing agent, probably due to Nascent Hydrogen, H*, liberated in the reaction being a better reducing agent than hydrogen gas.
    • Zincis amphoteric and forms salts with alkalis. For example, Zinc dissolves in hot concentrated caustic soda solution with the liberation of hydrogen gas and the formation of sodium zincate.

      Zn + 2 NaOH ==> Na2ZnO2 + H2

      UsesZinc is in widespread use
      • in the manufacture of electric cells (i.e. Dry Cells),
      • in the manufacture of galvanized iron sheets, to protect against corrosion of the iron,
      • in the manufacture of die-castings objects (e.g. the carburettor in car engines),
      • in alloys,
      • in medicine (as the Oxide), and
      • as a filler for Rubber and in Paints.
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