Sulfuric Acid : Production, Properties and Uses

Properties of Sulfuric Acid (H₂SO₄)

• Sulfuric acid is a diprotic acid
  (can donate 2 protons to a base)
• Sulfuric acid ionises (dissociates) in water in two stages:

  (i)   H₂SO₄(l) + H₂O(l) → HSO₄^-(aq) + H₃O⁺(aq)

  (ii)   HSO₄^-(aq) + H₂O(l) SO₄^2-(aq) + H₃O⁺(aq)

• Sulfuric acid is a strong acid
  (complete dissociation in water, K_a approaches infinity)
• Sulfuric acid reactions:

  sulfuric acid + metal → metal sulfate + hydrogen gas

  sulfuric acid + carbonate → metal sulfate + carbon dioxide gas + water

  sulfuric acid + base → salt + water

  sulfuric acid + ammonia → ammonium sulfate

• Sulfuric acid can take part in redox reactions.

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Uses of Sulfuric Acid (H₂SO₄)

• Sulfuric acid is the electrolyte used in lead-acid batteries (accumulators)
• Sulfuric acid is important in the production of fertilizers such as ammonium sulfate (sulfate of ammonia), (NH₄)₂SO₄, and superphosphate, Ca(H₂PO₄)₂, which is formed when rock phosphate is treated with sulfuric acid.
• Sulfuric acid is used to remove oxides from iron and steel before galvanising or electroplating
• Concentrated sulfuric acid (18 M) is used as a dehydrating agent, that is, to remove water, since it has a tendency to form hydrates such as H₂SO₄.H₂O, H₂SO₄.2H₂O, etc.

  Sulfuric acid is often used to dry neutral and acidic gases such as N₂, O₂, CO₂ and SO₂

  Sulfuric acid will "suck" water out of carbohydrates and some other organic compounds which contain oxygen and hydrogen. For example, sulfuric acid will "suck" water out of sucrose, C₁₂H₂₂O₁₁(s), (cane sugar) to produce a spongy mass of carbon:

  C₁₂H₂₂O₁₁(s) + 11H₂SO₄ → 12C(s) + 11H₂SO₄.H₂O

• Sulfuric acid is used in the production of nitroglycerine, an inorganic ester and organic nitrate, which is used as an explosive but can also be used as a vasodilator, a substance that dilates blood vessels and can be used in the treatment of certain types of heart disease:

  CH2ONO2

  |

  CHONO2

  |

  CH2ONO2

Manufacture of Sulfuric Acid (H₂SO₄)

The Contact Process is a process involving the catalytic oxidation of sulfur dioxide, SO₂, to sulfur trioxide, SO₃.

1. Solid sulfur, S(s), is burned in air to form sulfur dioxide gas, SO₂

   S(s) + O₂(g) → SO₂(g)

2. The gases are mixed with more air then cleaned by electrostatic precipitation to remove any particulate matter
3. The mixture of sulfur dioxide and air is heated to 450^oC and subjected to a pressure of 101.3 - 202.6 kPa (1 -2 atmospheres) in the presence of a vanadium catalyst (vanadium (V) oxide) to produce sulfur trioxide, SO₃(g), with a yield of 98%.

   2SO₂(g) + O₂(g) → 2SO₃(g)

Any unreacted gases from the above reaction are recylced back into the above reaction

4. Sulfur trioxide, SO₃(g) is dissolved in 98% (18 M) sulfuric acid, H₂SO₄, to produce disulfuric acid or pyrosulfuric acid, also known as fuming sulfuric acid or oleum, H₂S₂O₇.

   SO₃(g) + H₂SO₄ → H₂S₂O₇

   This is done because when water is added directly to sulfur trioxide to produce sulfuric acid

   SO₃(g) + H₂O(l) → H₂SO₄(l)

   the reaction is slow and tends to form a mist in which the particles refuse to coalesce.

5. Water is added to the disulfuric acid, H₂S₂O₇, to produce sulfuric acid, H₂SO₄
   

   H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)

The oxidation of sulfur dioxide to sulfur trioxide in step III above is an exothermic reaction (energy is released), so by Le Chatelier's Principle, higher temperatures will force the equilibrium position to shift to the left hand side of the equation favouring the production of sulfur dioxide.
Lower temperatures would favour the production of the product sulfur trioxide and result in a higher yield.
However, the rate of reaching equilibrium at the lower temperatures is extremely low.
A higher temperature means equilibrium is established more rapidly but the yield of sulfur trioxide is lower.
A temperature of 450^oC is a compromise whereby a faster reaction rate results in a slightly lower yield.

Similarly, at higher pressures, the equilibrium position shifts to the side of the equation in which there are the least numbers of gaseous molecules.

2SO₂(g) + O₂(g) → 2SO₃

On the left hand side of the reaction there are 3 moles of gaseous reactants, and the right hand side there are 2 moles of gaseous products, so higher pressure favours the right hand side, by Le Chatelier's Principle.
Higher pressure results in a higher yield of sulfur trioxide.

A vanadium catalyst (vanadium(V) oxide) is also used in this reaction in order to speed up the rate of the reaction.