AT2 Depth Study

Question 1

Effect of Temperature on Solubility of Salts

Answer 1

Solubility is proportional to temperature, but differently for different salts.

Exception is sodium sulphate, which decreases in solubility past the 30°C.

Question 2

Effect of Temperature on Solubility of Gases

Answer 2

Solubility is indirectly proportional to temperature because increased kinetic energy breaks dispersion forces.

Carbon dioxide is more soluble than other gases because it reacts with the water.

Question 3

Effect of Gas Pressure on Solubility

Answer 3

Gas pressure is proportional to solubility of gas.

Question 4

What does gas concentration depend on?

Answer 4

Temperature, depth and salinity

Question 5

Concentration in sea water and fresh water?

Answer 5

Concentration is sea water is less than in fresh water because the presence of ions interferes with dispersion forces.

Question 6

Factors that Influence Gas Concentration (8)

Answer 6

Partial pressure of gases in troposphere.

Diffusion rate into water surface.
Temperature of ocean.

Diffusion into deeper layers.

Ocean currents.

Water salinity.

Photosynthesis/Respiration.

Carbonate-hydrogen carbonate equilibria.

Concentrations with increasing depth.

Question 7

Partial Pressure of Gases in Troposphere

Answer 7

Abundancy in troposphere: N > O > CO2

Proportional to shift in dissolution equilibria.

Question 8

Diffusion Rate into Water Surface

Answer 8

Affected by water turbulence and temperature.

Droplet created by turbulence increases rate of diffusion.

Question 9

Temperature of Ocean Water

Answer 9

Temperature is inversely proportional to concentration.

Temperature varies according to proximity to land and cold salt water is denser than warm, meaning that surface water near the poles has higher concentration.

Question 10

Diffusion into Deeper Layers

Answer 10

Surface and deep water do not mix much.

Question 11

Ocean Currents

Answer 11

Deep: Currents from Antarctica and Southern Ocean transport cold water.

Surface: Currents move into the Indian Ocean.

Ocean temperature decreases with depth and cold water has higher concentration of gases.

Question 12

Water Salinity

Answer 12

Solubility is inversely proportional to salinity.

Seawater salinity: 3.0-3.7% (w/w)
Warm currents have lower salinity.

Salinity is proportional to density and water is more saline near the poles.

Question 13

Photosynthesis and Respiration

Answer 13

The photic zone is the surface 150m where photosynthetic organisms can live.

Oxygen is both consumed and produced in photic zone.

Question 14

Carbonate-Hydrogen Carbonate Equilibria

Answer 14

Carbon dioxide is constantly dissolving to create carbonic acid, which can be stored in deeper layers of the ocean for thousands of years at a constant concentration, but currents can bring it to the surface.

Some organisms use dissolved CO2 in constructing shells and exoskeletons.

CO2(aq) + H2O(l) → H2CO3 (aq)

Question 15

Concentrations with Increasing Depth

Answer 15

Oxygen: Higher in surface waters because photosynthesis and diffusion.

Carbon dioxide: Higher in deeper water because respiration and decomposition.

Nitrogen: Not affected by depth but opposite of CO2.

Question 16

What affects rate of reaction in oceans? (4)

Answer 16

Temperature, depth, oxygen concentration and salinity.

Question 17

Evolution of Ship-Building Materials

Answer 17

European ships in 16th century were predominantly timber with metals used for fastenings, anchors or braces.

British ships in 18-19th centuries became iron and then steel when iron began to rust too much.

Question 18

Ancient Greek Ship

Answer 18

Oldest, intact shipwreck ever found (350-410 BCE).

Water in the Black Sea is either of high salinity (from the Mediterranean) or low salinity (land runoff), and lack of water currents makes these layers distinct.

Deep basin allows ships to be entirely submerged in an anaerobic environment.

Question 19

The Endurance

Answer 19

Sunk in the Antarctic in 1915.

Ice slowed the ships descent to the ocean floor.

Lack of light and oxygen prevent degrading organisms, such as parasitic worms and bacteria.

Question 20

Hull of the Titanic

Answer 20

Hull - watertight main body of a ship.

Overlapping steel plates held by iron rivets.

Steel was ten times more brittle than modern steel, especially at freezing temperatures due to higher content of S, O and P and lower Mn.

Iron rivets had high concentrations of slag, prone to fracturing and couldn’t handle the stress.

Question 21

Superstructure of the Titanic

Answer 21

Superstructure - parts that project above the main deck.

Two timber decks with steel framing and a navigating bridge in between.

Sealant to survive harsh weather.

Question 22

Decay of the Titanic’s Hull

Answer 22

North Atlantic Ocean has ideal conditions for protobacteria.

Presence of bacteria (Halomonas titanicae) consumed iron in hull and formed rust mounds called rusticles because of the solubility of iron.

Fe2O3 formed a porous layer on the surface, which diffused water into iron metal and increased corrosion rate.
2Fe(s) + O2(g) + H2O(l) → 2Fe2O3(s) + H2O(l)

Sulfate-reducing bacteria is one of the key microbes in metal corrosion, facilitating the reduction of sulfate under anaerobic conditions in the water. It occurs in the following redox reaction:
4Fe(s) + SO4(aq) + 10H(aq) → 4Fe2+(aq) + H2S(g) + 4H2O(l)
Hydrogen sulfide reacts with iron (II) ions to form iron (II) sulfide (rusticles), among other components.

Question 23

Decay of the Titanic’s Superstructure

Answer 23

Steel framing corroded and broke, collapsing decks on top of each other.

Water pressure and deep currents caused disintegration.

Thick layers of sediment allowed some areas of wood deck to be protected from damp environment and remain intact.

Teredo worm holes found.

Question 24

Corrosion in Shallow Waters

Answer 24

High oxygen concentration allows aerobic bacteria the thrive and cause concretion, and corrosion reactions on reactive metals.

Higher temperature and acidity, which can cause acid/metal reaction with metal salts and hydrogen gas.

Question 25

What is concretion?

Answer 25

The accumulation of local matter to form a mass of sediment or precipitate from a precipitation reaction between water minerals and iron oxide, separating the reactive metal surface from exposure to seawater and oxygen, slowing corrosion with a more stable, mineral layer.

Fe2+(aq) + CO3(aq) → FeCO3(s)
4Fe2+(aq) + O2(aq) + 6H2O(l) → 4Fe(OH)3(s)

Ca2+(aq) + CO3(aq) → CaCO3 (s)
5Ca2+(aq) + 3PO4(aq) + OH-(aq) → Ca5(PO4)3(OH)(s)

Question 26

Why does concretion not occur deeper than 300m?

Answer 26

Requires the presence of dissolved minerals and oxygen.

Conditions are not stable enough at great depths for concretion to form.

Question 27

Why do bones dissolve at depths greater than 300m?

Three contributing factors

Answer 27

A combination of temperature, presence of carbonic acid and organisms.

Cold temperature and acidity increase solubility of bone mineral, and the presence of bacteria causes rapid decay.

Question 28

Green rust

Answer 28

Lack of oxygen causes the formation of both iron (II) and iron (III) hydroxide.

4Fe(OH)2(s) + 2Fe(OH)3(s) + 2Cl−(aq) + 4H2O → [Fe2 + 4Fe3 + 2(OH)12]Cl⋅4H2O

Question 29

What happened to the iron nail in ferroxyl indicator?

Answer 29

Oxidation → blue at head
Fe(s) → Fe2+(aq) + 2e-

Reduction → pink at base
1/2O2(g) + H2O(l) +2e- → 2OH-(aq)

Question 30

What happened to the bent iron nail in ferroxyl indicator?

Answer 30

Oxidation → blue at bend
Fe(s) → Fe2+(aq) + 2e-

Reduction → pink at head and base
1/2O2(g) + H2O(l) +2e- → 2OH-(aq)

Question 31

What happened to the iron nail with copper wire in ferroxyl indicator?

Answer 31

Oxidation → everywhere with exposed iron
Fe(s) → Fe2+(aq) + 2e-

Reduction → around the wire
1/2O2(g) + H2O(l) +2e- → 2OH-(aq)

Question 32

What happened to the iron nail with magnesium strip in ferroxyl indicator?

Answer 32

Oxidation → no blue
Mg(s) → Mg2+(aq) + 2e-

Reduction → everywhere (magnesium in the best reductant)
1/2O2(g) + H2O(l) +2e- → 2OH-(aq)

Question 33

Sacrificial Anodes

Answer 33

Highly reactive metals attached to the hull of an iron or steel ship to corrode instead.

Cathodic protection system is the most effective way to protect against corrosion, and anode is usually made from zinc, aluminium or magnesium, which are all better reductants than iron.

Question 34

Electrolysis Experiment Redox Reaction

Answer 34

Reduction: Cu2+(aq) + 2e- → Cu(s)

Oxidation: H2O(l) → ½O2(g) + 2H+(aq) + 2e-

Net Ionic: Cu2+(aq) + H2O(l) → Cu(s) + ½O2(g) + 2H+(aq)

Question 35

How are wooden articles of artefacts restored?

Answer 35

Salt water penetrates the organic fibres of wood, hydrolysing it ad leaving salt in the internal spaces if left to dry. This can cause it to disintegrate, so wooden artefacts are stored in warm alcohol or polyethylene glycol solutions so that the artificial polymer can displace the penetrated salt, therefore stabilising its decomposition.

Question 36

Hydrolysis of Iron

Answer 36

The final product (rust) of the iron corrosion and oxidation process is solid iron (III) hydroxide.

This can be hydrolysed in water to release iron (III) ions and hydroxide ions. The ions then further hydrolyse into hydrogen ions, which increase the acidity of the solution. This acidity can further corrode the iron, producing iron (II) ions and hydrogen gas.

Fe(s) + 2H2O(l) + 1/2O2(g) → Fe(OH)2(s)
4Fe(OH)2 + O2(g) + 2H2O(l) → 4Fe(OH)3(s)
Fe(OH)3(s) → Fe3+(aq) + 3OH-(aq)
Fe3+(aq) + 3H2O(l) → Fe(OH)3(s) + 3H+(aq)
Fe(s) + 2H+(aq) → Fe2+(aq) + H2(g)

Question 37

Hydrolysis of Copper

5 chemical equations

Answer 37

The final product of the copper corrosion and oxidation process is solid copper (II) chloride.

This can be hydrolysed in water to release copper (I) hydroxide and hydrochloric acid. The HCl decomposes into hydrogen ions and chloride ions, increasing the salinity of the solution. This acidity can further corrode the copper, producing copper (II) ions and hydrogen gas.

Cu(s) + Cl-(aq) → CuCl(s)
2CuCl(s) + O2(g) + 2Cl-(aq) → 2CuCl2(aq)
CuCl(s) + H2O(l) → CuOH(s) + HCl(aq)
HCl(aq) → H+(aq) + Cl-(aq)
Cu(s) + 2H+(aq) → Cu2+(aq) + H2(g)

Question 38

Electrolysis of Iron Artefacts

Answer 38

A basic electrolyte, like NaOH, is used as the electrolyte.

The iron artefact acts as the cathode, where iron (II) ions will reduce into solid iron.
Fe2+ (aq) + 2e- → Fe(s)

An inert electrode will be used, but the water molecules will oxidise.
4OH-(aq) → 2H2O(l) + O2(g) + 4e-

2Fe2+(aq) + 4OH-(aq) → 2Fe(s) + 2H2O(l) + O2(g)

Question 39

Electrolysis of Copper Artefacts

Answer 39

A basic electrolyte, like sodium bicarbonate (NaHCO3) is used as the electrolyte.

The copper artefact acts as the cathode, where copper (II) ions will reduce into solid copper.
Cu2+(aq) + 2e- → Cu(s)

An inert electrode will be used, but the water molecules will oxidise.
4OH-(aq) → 2H2O(l) + O2(g) + 4e-

2Cu2+(aq) + 4OH-(aq) → 2Cu(s) + 2H2O(l) + O2(g)

Question 40

Desalination

Answer 40

The submersion of metal artefacts into basic solutions to remove ocean salts, such as chlorides and sulfates, from the metal.

The alkaline solution diffuses the salt out, the solution needing to be changed regularly to maximise diffusion rate.

Fe(OH)Cl(s) + OH-(aq) → Fe(OH)2(s) + Cl-(aq)

Question 41

How do you know when the desalination process is complete?

Answer 41

The salinity of the alkaline electrolyte solution can be tested, and once the chlorine concentration is less than 50ppm, the treatment is complete.

This is usually followed by titration or calibrated chloride meter analysis to ensure that it has been properly desalinated before the metal artefact is exposed to air again.