METALS: STRONTIUM
Strontium, a relatively rare element in the Earth's crust (0.04%),
occurs mainly as the sulfate or carbonate (e.g., in the ores celestite
and strontianite, respectively). Strontium compounds have neither any
large-scale industrial uses nor any known biological rôles. However,
partly because of the observed and predicted similarities between 
calcium and strontium ions, and partly because calcium ions have been 
shown to be essential to all living organisms, strontium ions could be 
expected to affect diverse biochemical processes.
[.. K > Ba > Sr > Ca > Na > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg ..]
1. Based on strontium's position in the reactivity series, suggest how the metal could be extracted from its chloride. By electrolytic reduction of the molten salt / By chemical reduction with potassium [2] 2. This first Table shows the temperatures necessary to induce, and the heat energy changes accompanying, the thermal decomposition of four Group 2 carbonates.
 
 BaCO3 
 SrCO3 
 CaCO3 
 MgCO3 
 Decomposition temperature / °C 
 1360 
 1290 
  900 
  400 
 Heat energy change (DH) / kJ mol-¹
  269 
  235 
  178 
  101 
A radiochemist used the carbon-14 isotope to prepare a small amount of
radioactive labelled strontium carbonate, Sr14CO3. Using the apparatus
shown, a sample of this labelled compound was heated; the evolved gas
passed into limewater, and the precipitate collected by filtration.
(a) Ensuring you indicate the radioactive-labelled carbon, complete and
label the energy level diagram for the endothermic decomposition of 
strontium carbonate.
                                                                    [5]
(b) Construct the symbol equation for the reaction of the evolved gas
with limewater.
                                                                    [2]

3.  Strontium carbonate could be reasonably viewed as an 'analogue' of
calcium carbonate, which is a major component of the exoskeletons of
various protoctistans, sponges, corals, crustaceans, and molluscs.
Temporary hard water results from the slow reaction of rainwater with
naturally occurring calcium carbonate; its 'strontium equivalent' is:
Complete this second Table to show further strontium analogues.
 
 Calcium compound 
 Strontium analogue 
 Mammalian endoskeleton   
  Ca5(PO4)3OH(s)
   Sr5(PO4)3OH(s)
 Crustacean exoskeleton 
  CaCO3(s)
   SrCO3(s)
 Permanent hard water
  CaSO4(aq)
   SrSO4(aq)
 Temporary hard water
  Ca(HCO3)2(aq)
   Sr(HCO3)2(aq)
 'Scale' or 'fur'
  CaCO3(s)
   SrCO3(s)
 Limewater
  Ca(OH)2(aq)
   Sr(OH)2(aq)
                                                                    [2]
Construct the symbol equation for each of these strontium equivalents.
Softening permanent hard water

Softening temporary hard water

Removing scale with aqueous methanoic acid, HCOOH(aq)
                                                                    [6]
4.  Aqueous strontium hydroxide contains the ions Sr2+(aq), OH1-(aq),
and H1+(aq). When this solution is electrolyzed, using carbon-graphite
electrodes, these reactions occur:
Explain, using a symbol equation, each of the following observations 
made during such an electrolysis experiment.
The mass of the anode decreased.  The evolved oxygen gas oxidized the 
carbon-graphite anode to carbon dioxide:
                                                                    [2]
The solution turned milky during the initial stages of electrolysis.
Aqueous strontium hydroxide reacted with the evolved carbon dioxide to
form a white precipitate of strontium carbonate:
                                                                    [3]
The milky suspension turned clear if electrolysis was continued.
Strontium carbonate reacted with more evolved carbon dioxide to form
soluble strontium hydrogencarbonate:
                                                                    [3]
5.  One radioactive isotope of strontium, Sr-90, has a half-life of 28 
years and is a b-emitter (i.e., it decays spontaneously, emitting 
radiation in the form of high speed electrons). This isotope, one of
the most dangerous waste products of the nuclear industry, is a fission
product in nuclear explosions and in reactors of nuclear power plants.
Suggest what might happen to any compounds of strontium-90 ingested by
a mammal.  Sr(II)-90 ions decay, and b-particles will promote blood and 
bone cancers. / Sr(II)-90 ions could be absorbed into bones as ... 
                                                                    [2]
If 0.16 g of strontium-90 was absorbed into the exoskeleton of a coral
(e.g., Corallum rubrum), how much would remain after 84 years?  0.16 g
= 0 yr; 0.08 g = 28 yr; 0.04 g = 56 yr; and so 0.02 g after 84 years
                                                                    [2]
Name one process in autotrophs or saprotrophs that might be affected by
strontium-90.  Cell division / DNA replication / Natural mutation rates
                                                                    [1]

Dr. R. Peters Next Contents' List & Teacher's Notes