Answers to comprehension questions

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.

2. 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.
    
3. Crystalline pyrite has a volume of 40 ų 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 ų and of rozenite, FeSO₄·4(H₂O)(s), 162.7 ų. Also natrojarosite, NaFe₃(SO₄)₂(OH)₆, with a volume of 266.0 ų 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.
    
4. 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).