40DaysofCHEM 9.6.7 = Part 27

40DaysofCHEM 9.6.3 & 9.6.4 = Part 26

40DaysofCHEM 9.6.2 = Part 22

40daysofCHEM 9.6.1 = Part 21

9.6.7 HSC Feedback

2001 e)

EVALUATE means + / - and judgement (with criteria) – ie want to restore the artefact to as close to original condition as possible without further degradation. So for each step say the pros and cons. And when there is a decision to be made (ie a criteria/evaluation) say why eg mechanical vs acid concretion removal & stripping vs electrolysis of corrosion.
Remember to store in water first (why), leach (why – and NB leaching only removes SOLUBLE salts) remove concretions (here is a change to discuss pros and cons of mechanical vs chemical removal), electrolysis (pros and cons – cons mainly in terms of current and rate of electrolysis vs surface damage)

WHENEVER discussing electrolysis ALWAYS say a) It removes INSOLUBLE SALTS unlike leaching, b) It draws anions out of the artefact (eg Cl-) c) It restores the metal surface (ie metal ions reduced back to the surface)  AND d) WITHOUT FURTHER DAMAGE. ALWAYS use equations for the anode and cathode AND for the cathode reactions make sure you show the correct corrosion product being reduced (Fe(OH)Cl, Cu(OH)Cl, Ag₂S or PbS). NB in basic solutions the anode reaction will be 2OH^- => 1/2O₂ + H₂O + 2e^-

2002 cii)

Chemical procedures = leaching, concretion removal with acid, corrosion product stripping with citric acid/thiourea (Cu or EDTA (Pb), electrolysis and H-furnace (Fe) – make sure you show equations for the ones you chose.

If you have t speak about a specific artefact with chemical treatment choose the cannon over the anchor.

2003 e)

ASSESS = INTRO, +ve, -ve, judgement

INTRO = define electrolysis (perhaps brief history Davy/Faraday)

+ve = a) It removes INSOLUBLE SALTS unlike leaching, b) It draws anions out of the artefact (eg Cl-) c) It restores the metal surface (ie metal ions reduced back to the surface)  AND d) WITHOUT FURTHER DAMAGE.

-ve = trickier but   it requires energy, time, cannot remove concretions, cannot remove some oxides (eg why H-furnace is used for Fe artefacts sometimes)

Judgement = Has it developed artefact restoration? – ie before this could surface be restored? Could insoluble salts be removed?..

2004 aii)

Wood can be degraded and oxidised but not corroded (metals corrode)

2005 di)

The wax is a BARRIER to oxygen and electrolyte (water/humidity) preventing corrosion

2006 c)

The coins were encrusted (see diagram) thus you can’t just say that Cu artefacts wont have concretions

STEPS

1) Keep wet to avoid crystallisation of salt(and show what happens with equations)

2) Leach – Na2CO3 to prevent dissolution of Pb or Zn & Sn from Cu alloys

3) Concretion removal – with ACID for coins and don’t want to dameg surface engravings – perhaps only dilute/weak acid too else bubbling might damage surface

4) Leaching again?

5) For the Pb you might want to strip with EDTA as the surface is not important BUT for Cu the surface IS important (coins) thus you probably wont strip off the corrosion products as you want to RESTORE the Cu in the corrosion back to the surface

6) Electrolysis of coins – (and Pb?) make sure you use the correct equations – see notes for Q 2001e) above

VERY important – electrolysis removes corrosion NOT concretions

2010 a)

Don’t forget that the metal bands are part of the artefact and would be corroded!

2011 bi)

You need to know that the main corrosion product on silver is Ag2S and thus must show it being reduced back to silver (not just silver ion reduction)

2012 a)

Because the artefact material was not specified you should give SPECIFIC problems that can occur if salt crystallises in wood/leather, ceramics and metals. DON’T FORGET that acid can form on metals if salt crystallises and make sure you can write the equation for this.

9.6.3 HSC Question Feedback

2002 aii) – To reverse a galvanic cell and make it an electrolysis cell we have to reverse both the anode and cathode reactions. If we apply the exact opposite voltage produced by the galvanic cell all we do is stop the galvanic cell reaction. To reverse the cell we have to apply GREATER than the galvanic cell potential.

2003 bii) Any time you describe an experiment to measure the rate of electrolysis you MUST specify HOW you measured the rate – eg In our experiment we measured the mass change of the anode as an indication of the rate of reaction.

2006 bi) For the risk assessment and HOW it INFLUENCED our PROCEDURE– we used slightly toxic CuSO₄ – this may have limited our concentration to lower (safer) concentrations. We also used a power pack that limited the voltage to 12V to minimise the risk of electrocution but this limited our experiment to less than 12 V

2006 bii) When providing a conclusion from YOUR OBSERVATIONS you have to state the observations – eg the electrolysis cell that had the higher voltage had a greater mass loss at the anode in a fixed time. Thus it was concluded that higher voltages increase rates of electrolysis.

2007 ai) Graphite rods are used because they are INERT (meaning that they wont react in the redox reactions) CONDUCTORS (this allows them to transport re electrons required for electrolysis) – a lot of people forgot to mention that they are conductors. GRAPHITE IS NOT A METAL!

2008 ciii) It did say “Identify” the factors that affect rates of electrolysis but which of these two answers (that take basically the same time) is going to score more highly?:

·        Voltage, electrode spacing, electrode size and electrolyte concentration all affect electrolysis rates

OR

·        ­ voltage, ¯ electrode spacing, ­ electrode size and ­ electrolyte concentration all increase electrolysis rates.

2009 d) Usually w =e would use an Ag cathode so the Ag+ conc would stay constant in solution. This reaction gives a cell potential of 0V. However, i) if you think about you will see that a spontaneous metal displacement reaction would happen between the Ni and AgNO3. thus we do need to provide a -ve potential to the Ni to prevent it dissolving. ii) A potential of 0V actually means it is in equilibrium, the Ag will ppt and dissolve at both electrodes not plate the Ni. To actually get a silver plate we need to ensure that the reduction (deposition) of Ag only happens at the spoon (cathode) and oxidation (dissolution) of silver only happens at the silver anode - ie a voltage with the -ve terminal at the spoon is needed.

2010 bii) The cathode could be identified by the hydrogen gas or hydroxide ions  BUT HOW could you test for them – ie test with pop test or use a phenolphthalein indicator…

2012 di) Validity means measuring what you intended ie the effect of ONE variable (IV) on the dependent variable. As soon as you change multiple variables you can no longer validly say which changed variable affected the DV. Many people neglected to mention that in a valid test only one variable is changed.

2020 dii) Not the greatest Q for us. All through the Q student were drawing on the reactions at the electrodes until they get to the Q where it specifically states to draw the expected results on your diagrams and everyone stops drawing them on. You should have include 3 diagrams (control and two different variables changed – one in each of the other two pictures). You should have specified the constants and drawn on th results – ie bubbles and Cu deposition showing MORE (or less) depending on what variable you were testing.

Lastly – in their diagram they had a graphite and copper electrode. You should have used the sae chemicals in your example ie the Cu as the cathode th graphite as the anode.

9.6.5/6 HSC Q - Feedback

2002
di) Remember – we cleaned, dried & weighed all the identical samples. Then submerged them in pH buffers acidic (pH <7), neutral (pH =7) and basic (pH > 7). We observe the rate of corrosion via bubbling and colour of solution and by cleaning, drying and weighing at the end (basic = no corrosion (passivating), neutral = normal rust, acidic = no rust but iron dissolved = large mass loss.
dii) Remember that there are two reasons why metals corrode more in acidic environments i) MASH – hydrogen ions oxidize reactive metals (any metal with a –ve reduction potential). ii) Oxygen is a stronger oxidant in acidic conditions (give two oxygen reduction potential equations from the data sheet to prove) thus the rate of corrosion is higher.

2004

dii) Remember the experiment was as described in 2002di) above but we changed the oxygen concentration via having i) partially submerged, fully submerged, fully submerged with oil layer on top, fully submerged in pre-boiled water with oil layer on top. We observed qualitatively for the time until first sign of corrosion and fr the extent corrosion at the end of the experiment.
diii) To explain differences in rusting you must include the rusting equations. Even though the sunk ship is submerged the ship on the shore would be constantly wet and salty (perhaps even higher salt conc due to evaporation) so they both have an electrolyte. The temperature and oxygen concentration of the shore ship would be higher. Thus the shore ship rusted faster (as shown in the pic) NOTE – 30m is NOT deep enough for SRB and acidic microenvironments to have an impact. But you should explain why its temp and oxygen is lower (ie water high heat capacity keeps the ocean cooler and oxygen has low solubility in water as it is non-polar).

2006

bi) Yes CO₂ is more soluble that oxygen in the water but it is OXYGEN that is the oxidant in rusting.
bii) Be careful how you express yourself. Low temperatures and high pressures make gases MORE SOLUBLE. But (just like the deep ocean) low tem and high pressure does not mean that there will be ‘more dissolved’ gas.

2007

ci) See 2002 di above

2010

di) As described in 2004 dii) above except we also changed oxygen concentration, salt concentration (and pH) in three different experiments. Temp was changed via use of incubator, room temp, fridge and freezer (but problems…). Oxygen conc and pH as described in 2004 dii) & 2001di). If weighing – make sure you clean dry and weight then after the experiment, wipe off the corrosion product, clean, dry and weigh AND say the larger the mass loss the greater the rate of corrosion.
dii) keep it simple – to keep the electrolyte or oxygen away from the steel – paint! (or use some other barrier protection) – 1 mark  = keep it simple

2012

ci) The conditions would be anaerobic conditions where there are sulphate ions where SRB live = eg deep ocean

9.6.2 HSC Q - Feedbak

2001

c) You needed to answer this Q. You should have described the effect of increasing C% but also said why Mn, Si & Cu are added to steel in small amounts. You should have detailed what the Cr, Ni and Mo are for in stainless steel and what the Ti (and Nb…etc) are used for in HSLA steel. Tis would be a very good Q to practise.

2003

aii) You must note the properties of the passivating layer on passivating metals. For active metals you must say that they react with air/water to corrode but their corrosion product is often porous and non-tightly bound (eg rust)  = NO BARRIER to further corrosion.

2005

bii) You would get metal displacement between Mg and Fe ions – detail reactions & explain why in terms of defining metal displacement and relating to Fe and Mg reactivity. Also – you would get rusting of the nail – there is no electrical contact between the Mg and Fe so no galvanic (sacrificial anode) cell can form – thus detail the rusting equations. This is a question to redo!

2008

b) If you start describing an experiment we didn’t do you risk accidentally missing something that a student who DID the experiment you described would not = putting you at a disadvantage. We put iron and stainless steel in test tubes of equal conc salt water, same temp, pressure …. We weighed before and after 20 days. Visually observed (first appearance of rust and final extent of rust – qualitative comparison) and weighed mass of samples once corrosion had been wiped off. Stainless = no corrosion or mass loss. Iron – lots of orange/brown corrosion and small mass loss. Was it reliable – NO – but you can still demonstrate your understanding of reliability by noting that and defining reliability OR you can say these values were compared to the results of the previous year of HSC Chemistry classes and were found to be consist.

2010

c) There was  bit of a fuss that year because despite the fact that the Q says they are steels, #3 is clearly cast iron. You should pick up on this (ie anything > 2.1wt%C is no longer steel) and specify its uses in gates/cookware/hotplates etc – hard strong good heat retention. You need to explain what happens to the properties of steel when C is increases (ie compare properties and applications of #1 and #2 – NB you should explain why corrosion rates increase in terms of cathode sites). For Steel #4 you need to explain what both the Cr and the Ni do.

9.6.1 HSC Q - Feedback

2004

b) You need to analyse the IMPACT – thus say that he studies electrolysis, made the laws and invented the terminology but also say how this impacted the knowledge/use of electrolysis – ie make a statement that it allowed electrolysis to be applied to industry, or that it allowed for greater ‘collaboration’ and communication between scientists which increased out understanding further…

2009

c) The flow chart arrows in the question are a clue that the scientists research is linked so you need to make the connections between them clear.

Galvani – animal electricity – Volta - disproved animal electricity with Voltaic pile – Davy – used voltaic pile for electrolysis – Faraday – devised the laws of electrolysis.

Key points to include (italics = optional)

Galvani – iron and brass in circuit touching nerve in frogs leg = jump = concluded jump caused by animal electricity from the muscle. Circuit produced was first recorded continuous current. Realised that metals were electrical conductors

Volta – made voltaic pile (describe) produced electricity sans-frog = disproved animal electricity. Proposed contact between metals as source of electricity. Realised current produced depended on the combination of metals (start of activity series). Voltaic pile = precursor to modern batteries and first reliable source of continuous current.

Davy – Used the voltaic pile to perform electrolysis of compounds – decomposed water and acids (ie water a compound, disproved Lavoisier), decompose molten salts to extract reactive metals (eg K) for the first time thus showed potential for electrolysis to be applied to scientific research and metal extraction for industry. Realised that electricity was decomposing compounds = breaking bonds = bonding must be electrical in nature. Realised it was reactions between electrode and electrolyte that were producing current in voltaic pile.

Faraday – Davy’s student/apprentice. Developed a device to measure charge passing through an electrolytic cell. Used this to devise the two laws of electrolysis (give). Quantitative laws meant electrolysis could be applied to scientific and industrial projects. Developed the terminology we use for electrochemistry (eg cation cathode, catholyte, electrolyte) allowed scientists to communicate/collaborate and further develop understanding of electrochemistry.

 

Maritime Museum

The Maritime Museum runs a Shipwrecks, Corrosion and Conservation program that is not bad. Not good enough to take you all the way to Sydney for it though. However, that gives Dr Blurg the oppurtunity bring the best of the Maritime Museum to you. Three sets of images below. If you click on the images you should get a larger view where you can read the informative (and sometimes correct) captions.

SET 1 - History of Shipbuilding Materials

SET 2 - Corrosion

SET 3 - Artefacts: Conservation & Restoration