Chemistry- Shipwrecks and salvage
9.6.1.1 Identify the origins of the minerals in oceans as; Leaching by rainwater from terrestrial environments; hydrothermal vents in mid ocean ridges.
-As rainwater moves through soil and rock it picks up ions such as; Na+, K+,Mg+, Ca2+,Cl- and (SO4)2-. This then makes its way into rivers and creeks and eventually the ocean. This is the process of leaching. HCO3- is present if the water is acidic and NO2, PO4 is present from decaying plant and animal matter. Aquifers greatly concentrate these ions.
- Hydrothermal vent exist on mid ocean ridges. They are cracks or fissures which allow water to percolate and come close the upwelling magma. This hot water is then released back to the ocean rich in minerals. Many of these minerals precipitate when the hot water reaches the cold water.
9.6.1.2 Outline the role of electron transfer in oxidation-reduction reactions.
OIL RIG
Oxidation is loss
Reduction is gain (of electrons)
RED CAT; Reduction at the cathode
AN OX; Oxidation at the anode.
Electron transfer can occur directly between reactants or from the oxidation reaction site through a conductor to the reduction reaction site.
9.6.1.3 Identify that oxidation-reduction reactions can occur when ions are free to move in liquid electrolytes.
The reductant undergoes oxidation
The oxidant undergoes reduction.
Oxidation- reduction reactions will only occur if ions are free to move and a electrochemical cell is produced. It needs a circuit.
Sea water is an electrolyte so can be used in an electrochemical cell.
9.6.1.4 Describe the work of Galvani, Volta, Davy and Faraday in increasing understanding of electron transfer reactions.
Galvani first demonstrated a generated electric current in frogs legs using two different metals. (he explained with animal electricity)
Volta did not agree and eventually developed the first electric battery from two different metals separated by a conducting solution of salt. (voltaic cell)
Davy used electricity in his experiments. He decomposed water by electrolysis and isolated many metals in pure form.
Faraday developed the laws of electrolysis and developed terms such as anion and cation to explain what was happening in terms of particle theory. (refer to assessment task)
9.6.2
9.6.2.1 Account for the differences in corrosion of active and passivating metals.
Generally the more active a metal is the more likely it is to corrode.
Some metals called passivating metals readily form an uncreative surface coating with substances such as oxygen or water, which adheres to the surface thus protecting the metal from further corrosion. E.g. Aluminium, chromium, (zinc and lead to a lesser extent).
9.6.2.2 Identify iron steel as the main metals used in ships.
Iron andsteel are used in ships because they are; relatively hard, mechanically strong, can be worked into different shapes and structures, can be welded, and are abundant and cheap.
9.6.2.3 Identify the composition ofsteel and explain how the percentage composition of steel can determines its properties.
Steel is and allow of iron with no more than 2% carbon with varying amounts of other metals and silicon. The carbon may form the cathodic sites where reduction reactions take place. Deposits of carbon impurities may lead to mechanical stress of the iron.
Mild steel<0.2% carbon soft, malleable and corrodes readily.
Structural steel 0.2<x<0.5% carbon, hard but malleable with high tensile strengthcorrodes easily.
Stainless steel <0.2% carbon, but alloyed with10 to 20% chromium and 5 to 10% nickel. It is hard and resistant to corrosiondue to the formation of a passivating chromium oxide layer.
BDS systems is used to get rid of excess carbon from pig iron (oxygen is blown over at molten state making CO)
-Quickly cooled steel is quenched (cementite Fe3C is formed) hard but brittle steel.
-cooled slowly is annealing, very ductile but not as hard or tough.
Other elements such as Ni, Cr, tungsten W, and titanium Ti can also be alloyed to produce other properties.
-Tempering holds steel at high temps below melting point then cooled making quenched steel tougher and less brittle whilst retaining hardness.
-working; rollingor hammering at high temps improves properties such as tensile strength.
9.6.2.4 Describe the conditions under which resting of iron occurs and explain the process of rusting.
Conditions under which iron will rust;
-Iron will rust when In contact with oxygen andwater.
-rusting is faster if water is acidic or if It contains dissolved salts (electrolyte solution e.g. sea water.
-rusting is accelerated if the iron is impure, is in contact with a less active metal or under mechanical stress.
4Fe(s) + 3O2 + 2x. H2O(l) --- 2Fe2O3. xH2O
9.6.3
9.6.3.1 Describe, using half equations what happens at the anode and cathode during electrolysisof selected aqueous solutions.
- Reduction occurs at the negative electrode, the cathode.
- Oxidation occurs at the positive electrode, the anode.
- The charge on the electrodes is different for an electrolytic cell and a galvanic cell. The cathode is negative in a n electrolytic cell while the cathode is positive in a galvanic cell.
- Anions carry charge towards the anode.
- Cations carry charge towards the cathode.
- Electrolysis only occurs when the voltage applied to the cell is greater than the calculated cell potential.
- The reactions that occur at the anode and cathode can depend on the applied voltage.
(refer to book for reactions)
9.6.3.2 Describe factors that affect an electrolysis reaction.
-Effect of concentration;Increased concentration of ions increases the current and consequently the rate of electrolysis. E.g. NaCl; if [Cl-] > 0.5mol.L^-1, Cl2 (g) is produced at the anode. If the [Cl-] is <0.1mol.L^-1 water is oxidizedrather than Cl-ions.
-Nature of electrolyte; Electrolytes that have more ions dissociated will have a higher rate of electrolysis. GET MORE INFO
-Nature of the electrodes; Increased surface area of the electrodes used increases the currentand the rate of electrolysis. Decreased distancebetweenthe electrodes increases the current and the rate of electrolysis.
9.6.4
9.6.4.1
Identify the ways in which a metal hull may be protected including;
-corrosion resistant metals
-development of surface alloys
-new paints.
Stainless steels are favoured corrosion resistant metals becauseof a passive film of chromium (III) oxide on the surface that resists corrosion.
Surface alloys can be created by various methods to give ordinary steel a protective surface similar to stainless steel. The hull has the corrosion protection of stainless without the expense.
Polymer paints protect against rust by forming a film over the surface of the steel that is impervious to oxygen and water. These paints also form a layer of a very insoluble ionic substance call pyroaurite. This ionic layer bonds strongly to the surface of the steel and well into the polymer layer. It prevents movement of ions on the surface of the steel.
9.6.4.2 Predict the metal which corrodes when two metals form an electrochemical cell using a list of standard potentials.
To predict which metal will corrode;
-select the reduction half reactions for each metal.
Reverse the half reaction that has the most negative E0 value(lowest reduction potential). This will now have positive E0 value.
This metal will be the one that is oxidizedand the metal that corrodes.
9.6.4.3 Outline the process of cathodic protection; describing examples of its use in both marine and wet terrestrial environments.
Cathodic protection basically works by turning the structure to be protected into a Galvanic cell. It uses a metal of higher activity to become the cathode and connects these metals with a wire to turn the structure to be protected into a cathode which does not corrode. The metal at the anode undergoes oxidation where it is ionised or corroded instead of the metal at the cathode which has an excess of electrons and becomes reduced, regaining any corroded molecules. There are three main types of cathodic protection; Galvanising, Sacrificial anode and impressed current.
(refer to assessment)
9.6.4.4 Describe the process of cathodic protection in selected examples interms of the oxidation/reduction chemistry involved.
e.g. an iron tank buried in the ground may be protected using sacrificial anode of zinc. The zinc needs to be monitored and is easily replaced when corroded away as it is more active. Zn Zn2+ + 2e-.
The electrons flow into the iron preventing the formation of Fe2+ ions. The electrons produce reduce and fe2+ ions formed back to Fe atoms. This means that any scratchon the zinc surface does not affect its protective properties.
9.6.5
9.6.5.1 Outline the effect of: temperature and pressure on the solubility of gases an salts.
- Solubility of a gas increases as the pressure of that gas in contact with the solution increases.
- The solubilityof gases decreases as temperature increases.
- The solubility of salts generally increases as the temperature rise. (there are a number of exceptions)
- The solubility of salts is largely unaffected by increasing the applied pressure.
9.6.5.2 Identify that gases are normally dissolved in the oceans and compare their concentrations in the oceans to their concentration in the atmosphere.
Gas / Formula / Air% (v/v) / Sea water % (v/v)Nitrogen / N2 / 78.1 / 0.8-1.5
Oxygen / O2 / 20.9 / 0-0.9
Carbon dioxide / CO2 / 0.03 / 4.5-5.4
Carbon dioxide is greater because CO2 molecules can react with water to form soluble ions lowering the pH.
Oxygen concentration is greatest near the surface , minimum at 500-1000m and steadily increases as depth increases.
9.6.5.3 Compare and explain the solubility of selected gases at increasingdepths in the oceans.
Solubility tends to increase with depth because of the increasing pressure.
Carbon dioxidereacts and is produced by respiration increasing its solubility.
CO2(g) + H2O(l)H2CO3H+ + HCO32H+ + CO32–
9.6.5.4 Predict the effect of low temperatures at great depths on the rate of corrosion of a metal.
Water at great depth will be about 4 degrees C (maximum densityat this temperature). Water colder than this is of lower density and will rise upwards.
The rate of corrosion of metals decreases as temperature decreases.
9.6.6
9.6.6.1 Explain that ship wrecks at great depths are corroded by electrochemical reactions and by anaerobic bacteria.
In contrast to predictions already made shipwrecks at great depths are corroded heavily. This is a result of anaerobic bacteria (do not require oxygen) as well as electrochemical reactions.
9.6.6.2 Describe the action of sulfate reducing bacteria around deep wrecks
Sulfate reducing bacteria near deep shipwrecks produce the compound H2S from the sulfate ions in sea water
SO42– + 10H+ + 8e– --> H2S +4H2O
(oxidation state of sulfur reduced from 6+ to -2)
Sea water pH is normally around 8 but with increased solubility of CO2 it becomes slightly acidic favouring corrosion.
This corrosion produces ion which can undergo hydrolysis(reaction with water) to produce more H+ ions. SRB are able to change H2 to 2H+ which they use to reduce sulfide ions.
Acidic environments as low as pH 4 can be produced around a shipwreck.
The sulfide ions from H2S can precipitate Fe2+ ions to from black insoluble iron (III) sulfide. Releasing more H+
9.6.6.3 explain that acidic environments accelerate corrosion in non-passivating metals.
Hydrogen ions react with non-passivating metals in displacement reactions. They will react with any metal above H+ in the standard potentials. All other metals except gold are also corroded as H2S can react with any metal.
9.6.7.1 Explain that artifactsfrom long submerged wrecks will be saturated with dissolved chlorides and sulfates.
Artifacts removed from shipwrecks are usually in poor condition because;
-Metals are corroded
-usually encrusted with calcium carbonate (CaCO3, or limestone) or coral.
-Porous objects (e.g. wood and leather) are soaked in sea water rich in chloride and sulfate salts.
9.6.7.2 Describe the processes that occur when a saturated solution evaporates and relate this to the potential damage to drying artifacts.
The solution becomes more concentrated as water evaporates, until it becomes saturated and eventually salt crystals are producted. These from throughout the artifact and the slower the evaporation the large the crystals formed.
This formation of crystals can damage the artifact by pushing it out of shape causing it to crack . It also may react chemically with the artifact.
Leeching is a process used to reduce the salt in the wet artifact. It works by running clean water through the object for a long time period effectively reducing theconcentrationof salt.
9.6.7.3 Identify the use of electrolysis as a means of removingsalt.
Chlorideis difficult to remove by leeching. Insoluble hydroxylchlorides such as Fe(OH)Cl can be trapped in Fe(OH)2 or Fe2O3. xH2O deposits. Electrolysis is used to free the chloride ions into the solution. The iron object to be restored is made the cathode and a stainless steel anode is used usually with a 0.5mol.L-1 solution.
9.6.7.4 Identify the use of electrolysisas a means of cleaning and stabilizingiron, copper and lead artifacts.
Artifacts made from iron, copper and lead (and their alloys) can be cleaned and stabilizedby electrolytic reduction.
The artifact is used as the cathode. The metal ions in the insoluble corrosion products are reduced to metals atoms. Forming a deposit on the artifact.
An inert electrode such as stainless steel Is used as the anode.
Various oxidation may occur at the anode depending on the voltage of applied current and concentration of anions. Alkaline solutions e.g sodium hydroxide or sodium carbonate are used as electrolyte.(high pH discourages furthercorrosion)
9.6.7.5 Discuss the rand of chemical procedures which can be used to clean, preserve and stabilizeartifacts from wrecks and where possible, provide and example of the use of each procedure.
- Surface deposits are removed by physical chipping or dissolving in dilute acid (both methods need care as damage can occur)
- Electrolytic reduction is use to remove chloride ions and sulfate ions and reduce metal oxides and sulfides.
- Artifact is preserved by coating in a clear polyurethane polymer or microcrystalline wax.