13.3    Sulfur Compounds

Uses and formation of  sulfur dioxide, SO2  and its consequences

• Atmospheric sulfur dioxide is formed during the burning of fossil fuels.  Fossil fuels like coal and oil all contain sulfur compounds, and when the coal or the oil product are burned, sulfur dioxide is produced
• As discussed above, sulfur dioxide in the atmosphere can cause acid rain. The consequences of acid rain include:
  1. the corrosion of limestone buildings as the calcium carbonate reacts with the acid
  2. the corrosion of ironwork as the iron reacts with the acid
  3. the acidification of lakes and rivers leading to the death of  aquatic life.  This is complicated by the fact that a fall in pH dissolves aluminium ions from the soil. Aluminium ions are toxic to fish
  4. damage to trees. This again is partly the result of aluminium ions being toxic to plants
• On the other hand, sulfur dioxide can be used as food preservative in, for example, wine and dried fruit and It has two functions:
  1. It slows oxidation of the food by oxygen in the air
  2. It also kills bacteria.

Manufacture of  sulfuric acid – the Contact process

• The Contact process:
  1. makes sulfur dioxide
  2. converts the sulfur dioxide into sulfur trioxide
  3. converts the sulfur trioxide into concentrated sulfuric acid
• The sulfur dioxide, SO2 can be made by using two methods:
  1. Burning sulfur in an excess of
     S(s) + O2(g) → SO2(g)
  2. Heating sulfide ores like pyrite in an excess of air
     4FeS2(s) + 11O2(g) → 2Fe2O3(s) + 8SO2(g)
• The sulfur dioxide made is then converted into sulfur trioxide. This is the key process in the Contact process
  2SO2(g) + O2(g) ⇌ 2SO3(g)      ; ΔH = -196 kJ mol⁻¹
• The required conditions to achieve this are:
  i. (400 – 450) ºC.
  ii.   (1 – 2) atm (equivalent to about 101 kPa).
  iii. Presence of vanadium(V) oxide, V2O5 as catalyst.
• i. Sulfur dioxide and oxygen gas are fed into the reactor in a ratio of 1:1. An excess of oxygen is used to shift the position of equilibrium to the right.
  ii. Higher proportions of oxygen are not used because it wastes the space in the reactor and decrease the efficiency of the catalyst, since the excess oxygen will have nothing to react with.
• i. The production of sulfur trioxide is an exothermic reaction in equilibrium. According to Le Chatelier’s principle, in order to shift the position of equilibrium to the right as much as possible(to increase the yield), a low temperature should be used. However, (400 – 450) ºC is not a low temperature.
  ii. A low temperature will decrease the rate of reaction albeit having a high yield. The reaction will take a long time to complete and it is not economically plausible
  iii. Hence, (400 – 450) ºC is the compromise temperature that produces a good enough yield in a short time
• i. According to Le Chatelier’s principle, the position of equilibrium will shift to the right if the pressure is increased because there are less molecules on    the right of the equation. Besides, a high pressure can also increase the rate of   reaction.
  ii. However, the reaction is done almost at atmospheric pressure. This is because even at this relatively low pressure, the conversion rate is already about 99.5 %. Increasing the pressure will only result in minor improvements.
  iii. Higher pressures are not used because:
  • it is expensive to build and maintain the pipes and generators to withstand the pressure, this increases the production cost
  • there is a risk of the pipes exploding
• A catalyst of vanadium(V) oxide, V2O5 is used to increase the rate of  reaction. Although it has no effect on the position of equilibrium, it is essential because without it, the reaction will too long to complete
• i. Then, sulfur trioxide is first converted into oleum or fuming sulfuric acid. This is done by dissolving sulfur trioxide in concentrated sulfuric acid.
  H2SO4(l) + SO3(g) → H2S2O7(l)ii. It is then reacted with water to produce sulfuric acid.
  H2S2O7(l) + H2O(l) → 2H2SO4(l)
• In the last step, water cannot just react with sulfur trioxide. This is because a mist of poisonous and uncontrollable sulfuric acid will be formed. Dissolving it in concentrated sulfuric acid is a more gentle and safe way