OPTION 2
13
WORKSHEET
SHIPWRECK CORROSION
AND SALVAGE
Syllabus references 9.6.6 & 9.6.7
Read the following article then answer the questions at the end of the article.
(Adapted from ‘Sunken Treasure’ by David P. Robson, ChemMatters, April 87, vol 4.)
On 4 September 1622 the Spanish galleon, Nuestra Senora de Atocha, loaded with treasure left Havana
harbour (Cuba) for Spain. The weather, fine at the beginning of the voyage, soon changed and on the
second day a gale and huge waves caused the Atocha to be driven onto a reef. The ship rapidly sank and
all but five of the 260 people aboard drowned.
The bulk of the treasure aboard the ship came to rest in one spot where, over time, it was covered by
drifting sand. The wood and other organic matter decayed, consuming oxygen and creating an anaerobic
environment. Within this environment sulfur-reducing bacteria thrived and attacked the buried silver
treasure.
The bacteria take in plentiful sulfate ions (SO42) and expel hydrogen sulfide (H2S) by the reaction:
SO42(aq)  6H2O(l)  8e → H2S(aq)  10OH(aq)
Although silver is a relatively unreactive metal, it is susceptible to attack by hydrogen sulfide
2Ag(s)  H2S(aq) → Ag2S(s)  H2(g)
producing a black precipitate of silver sulfide. The silver sulfide layer tightly adheres to the surface of the
silver coin or bar and eventually forms a protective layer preventing further attack by H2S.
The rotting of the organic material also causes further reactions producing CO2 in the enclosed
environment. Carbon dioxide is normally dissolved in sea water and the increased concentration causes
the equilibrium reaction
H2O(l)  CO2(aq)
H(aq)  HCO3(aq)
to shift to the right, which in turn causes the equilibrium reaction
Ca2(aq)  HCO3(aq)
CaCO3(s)  H(aq)
to shift to the right producing more CaCO3. This calcium carbonate crystallised around everything
creating a single rock-like structure.
Over three hundred years later in 1968 treasure hunter Mel Fisher, following rumours, began to hunt for
the Atocha. In 1973 after searching for years Mel found the evidence of the wreck—necklaces, muskets,
swords and several brass cannons, but not the tons of treasure originally carried by the galleon. For 12
more years Mel continued to search whenever weather and finances permitted.
Copyright © 2007 McGraw-Hill Australia
CONQUERINGCHEMISTRY HSC
OPTION 2 WS 13
In 1985 the persistent treasure hunter finally located the bulk of the treasure, hundreds of silver bars and
dozens of chests of coins locked on a ‘coral reef’. The silver bars and coins, covered by a black corrosion
and marine growth, didn’t appear to be a spectacular treasure.
It was the job of head conservator of the project, Jim Sinclair, to clean the coins and bars by a process
of cathodic reduction. The encrusted CaCO3 first needed to be removed from the coins; this was done
by immersing them in an acid bath of several organic acids and buffers.
2H(aq)  CaCO3(s) → Ca2(aq)  CO2(g)  H2O(l)
After rinsing the coins were individually clipped to a wire
that carries direct current, then immersed in a 5% NaOH
electrolyte solution. Electrons flow through the coins,
acting as the cathode, where they combine with silver
ions in the Ag2S producing silver atoms:
Ag2S
S2–
Cathode
(silver coin)
e–
Ag  e → Ag(s)
The silver atoms adhere to the coin and the negative
sulfide ions (S2) migrate away from the negative
electrode, thus the layer of Ag2S disintegrates.
Simultaneously, hydrogen ions in the water are reduced
to bubbles of hydrogen gas that expand and break off
flakes of the sulfide resulting in a clean coin.
Ag
Ag+
H2
bubble
A stainless steel plate anode is used to complete the circuit. At the anode water is decomposed and
electrons are released.
2H2O(l) → O2(g)  4H(aq)  4e
Using the article and your knowledge of chemistry answer the following questions.
QUESTIONS
1
Describe the action of sulfur-reducing bacteria around the wreck.
The sulphur-reducing bacteria produce H2S in the anaerobic environment around the wreck.
2
Explain, using appropriate equations, how the silver coins were corroded.
Silver coins react with H2S producing AgS.
2Ag(s)  H2S(aq) → Ag2S(s)  H2(g)
3
Explain why an increase in CO2 concentration caused the production of more CaCO3.
Increase in CO2 causes an increase in HCO3. This in turn reacts with Ca2 ions producing
CaCO3.
Copyright © 2007 McGraw-Hill Australia
CONQUERINGCHEMISTRY HSC
OPTION 2 WS 13
4
a
Draw a labelled diagram of the electrolytic cell used to restore the silver coins.
–
+
stainless steel
plate anode
coins
b
5% NaOH
electrolyte
solution
Write the two cathode and one anode half-reactions occurring in this cell.
Cathode: Ag(aq)  e → Ag(s)
2H2O(l)  2e → H2(g)  2OH(aq)
Anode: 2H2O(l) → O2(g)  4H(cq)  4e.
5
In the restoration nearly all the silver is recovered. Describe what happens to the silver in the silver
sulfide layer (if it isn’t lost) and compare this method to removal of the sulfide layer using abrasion.
Using electrolysis the silver ions in the silver sulfide are converted back to silver atoms. Using
abrasion the silver sulfide layer is mechanically scraped off causing the silver ions to be lost.
Copyright © 2007 McGraw-Hill Australia
CONQUERINGCHEMISTRY HSC
OPTION 2 WS 13