Answers to comprehension questions

For the questions, see Shipwreck: science to the rescue! The hull timbers of both the Vasa and the Mary Rose have been found to contain about 2 tonnes of sulphur, as the element S. If 1000 kg sulphur in the wood were in the form of the pyrite, FeS2, how much sulphuric acid (H2SO4(aq)) would be…

For the questions, see Shipwreck: science to the rescue!

  1. The hull timbers of both the Vasa and the Mary Rose have been found to contain about 2 tonnes of sulphur, as the element S. If 1000 kg sulphur in the wood were in the form of the pyrite, FeS2, how much sulphuric acid (H2SO4(aq)) would be produced when all the pyrite is fully oxidised? Two pathways are common: to hydrated iron(II) sulphate:
         FeS2(s) + 7/2O2 + (n+1)H2O → FeSO4·n(H2O)(s) + H2SO4(aq)
    or to goethite, alpha-FeOOH (as in rust):
         FeS2(s) + 15/4O2 + 5/2H2O → FeOOH(s) + 2H2SO4(aq)

    Answer: 1000 kg S corresponds to 15.6 kmol FeS2. If hydrated iron(II) sulphate is the end product, 1.5 tonnes sulphuric acid would form. If goethite, alpha-FeOOH, forms, the amount would be double: 3.1 tonnes sulphuric acid.

 

  1. Sodium hydrogen carbonate (sodium bicarbonate, NaHCO3) has been added to the re-circulated sprayed conservation solution of the Mary Rose to keep its pH about 7. How much sodium hydrogen carbonate would be needed to neutralise the acid formed in Question 1 (from oxidised pyrite containing 1000 kg sulphur)? Answer: To neutralise 1.5 tonnes sulphuric acid, 2.6 tonnes sodium bicarbonate would be required; 3.1 tonnes of sulphuric acid would require 5.2 tonnes of sodium bicarbonate.
     
  2. Crystalline pyrite has a volume of 40 Å3 per FeS2 unit and expands dramatically when oxidised. For example, the volume per formula unit of the crystalline hydrated iron(II) sulphate melanterite, FeSO4·7(H2O)(s), is 243.5 Å3 and of rozenite, FeSO4·4(H2O)(s), 162.7 Å3. Also natrojarosite, NaFe3(SO4)2(OH)6, with a volume of 266.0 Å3 per formula unit, is commonly found on the Vasa’s wood. Estimate how many times the volume will increase when a pyrite crystal oxidises and a) FeSO4·7(H2O)(s), b) FeSO4·4(H2O)(s) or c) NaFe3(SO4)2(OH)6 salts crystallise as products. What effects could these processes have if they took place inside the wood structure?

    Answer: When solid FeSO4·7(H2O)(s) precipitates, the volume increases with a factor of 12.2 for FeSO4·7(H2O)(s); 8.1 times for FeSO4·4(H2O)(s); and 6.7 times for NaFe3(SO4)2(OH)6. This can cause outbursts of salts through the wood surface, or crack the wood structure from within.
     

  3. In a chemistry textbook, look up a schematic molecular orbital energy level diagram for the oxygen molecule O2 in its ground state. Explain how uptake of energy from light can produce singlet oxygen, 1O2, with all electrons paired. Answer: The O2 molecule is paramagnetic with the two outermost electrons unpaired, one in each of two degenerate (with same energy) anti-bonding molecular π-orbitals formed by combining the p-orbitals of the O atoms. With an energy uptake of ~92 kJ mol-1 from photons of light, the two outermost electrons can be paired (a 400-nm photon has energy of 300 kJ mol-1). The energy uptake process usually needs a sensitiser (e.g. light-absorbing dissolved organic matter). This excited singlet 1O2 (one single arrangement in space for a pair of electrons) oxygen molecule is a diamagnetic but reactive short-lived species, even though its energy is only sufficient for mild oxidation (Stumm W, Morgan J (1996) Aquatic Chemistry (3rd ed). New York, NY, USA: Wiley-Interscience).


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