• About Science in School
  • About EIROforum
  • Submit an article
Science in School
Science in School
  • Understand
    • Recent research and science topics
      • Astronomy / space
      • Biology
      • Chemistry
      • Earth science
      • Engineering
      • General science
      • Health
      • History
      • Mathematics
      • Physics
      • News from the EIROs
      • Science and society
  • Inspire
    • People, events and resources
      • Advertorials
      • Back in the staffroom
      • Event reports
      • Resource reviews
      • Science in films
      • Scientist profiles
      • Spotlight on education
      • Teacher profiles
  • Teach
    • Activities and projects
      • Astronomy / space
      • Biology
      • Chemistry
      • Earth science
      • Engineering
      • General science
      • Health
      • History
      • Mathematics
      • Physics
      • Science and society
  • Archive
  • Login
  • Contact
Issue 1
 -  04/08/2006

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.

 

  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).
 

Log in to post a comment

Print issues

  • Current issue
  • Archive

Tools

  • Print
  • Share

Related articles

  • Two hydrolytic enzymes and an epistemological–historical approach
  • Better milk for cats: immobilised lactase used to make lactose-reduced milk
  • Using news in the science classroom
  • Bad science: how to learn from science in the media
  • How water travels up trees

Login / My account

Create new account
Forgot password


Subscribe (free)

Please login or create an account to be able to subscribe.

Contact us

Please contact us via our email address editor@scienceinschool.org.

  • More contact details

Get involved

  • Submit an article
  • Review articles
  • Translate articles
  • Advertise

Support Science in School


Tweets by Science in School

Tweets by @SciInSchool
EIROforum members:
CERN European Molecular Biology Laboratory European Space Agency European Southern Observatory
European Synchrotron Radiation Facility EUROfusion European XFEL Institut Laue-Langevin


EIROforum
Published and funded by EIROforum


  • About Science in School
  • About EIROforum
  • Imprint
  • Copyright
  • Safety note
  • Disclaimer
  • Archive
  • Advertise
  • Donate
  • Contact
  • Facebook
  • Twitter
ISSN 1818-0361

CERN
European Molecular Biology Laboratory
European Space Agency
European Southern Observatory
European Synchrotron Radiation Facility
EUROfusion
European XFEL
Institut Laue-Langevin
EIROforum

Published and funded by EIROforum
  • About Science in School
  • About EIROforum
  • Imprint
  • Copyright
  • Safety note
  • Disclaimer
  • Archive
  • Advertise
  • Donate
  • Contact
  • Facebook
  • Twitter
ISSN 1818-0361