help with assignment question (1 Viewer)

[dead]

Hi , i need help with this particular question, and if anyone can help me it would be great.

The question is: "Describe what happens to artefacts that are saturated with salts if they are allowed to dry out. Emphasise the chemical properties involved in this process"

I know that the salts start to crystallise and therefore damage the artefact revealing cracks and holes. But i dont understand what they mean by the chemical properties involved in this process.

 

[beckles]

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hey, what core it that? its chem right? an assessment question or something?
when you think chemical properties, obviously dont think physical properties...they obviously want to know whats going on chemically, so what reactions are occurring, what evidence is there of this. what reactions are taking place between the artefacts when they're saturated with salts....
perhaps...i guess you probably knew that hey, oh well, all the best with it
catchya...

 

[dead]

is there an equation for crystallisation of salts?

 

[mitochondria]

ummm.. the answer is yes, let's assume the saturated salt is mainly NaCl:

Na⁺_(aq) + Cl^-_(aq) -----> NaCl_(s)

(The final product is probably a Sodium Chloride nHydrate.. but I left it out for simplicity.. the formula is: NaCl.nH₂O_(s))

Clearly, when the such artefacts are left to dry, NaCl crystal will start forming between the tiny space in the artefacts.. the salt crystal will cause the space they occupy to expand and possibly damaging/destroying the artefact..

Desalination must be done right after the artefact is extracted from sea water.. and I think this is done using NaOH.. or at least involve NaOH sorry.. I can't remember on top of my head.. but I'm certain about NaOH Hope that helps!

 

[Templar]

It involves NaOH as corrosion in this case releases OH- ions and increasing the pH pushes the reaction in the opposite direction, preventing further corrosion and help to remove chlorides from the metal (iron).

 

[dead]

ummm.. the answer is yes, let's assume the saturated salt is mainly NaCl:

Na⁺_(aq) + Cl^-_(aq) -----> NaCl_(s)

(The final product is probably a Sodium Chloride nHydrate.. but I left it out for simplicity.. the formula is: NaCl.nH₂O_(s))

so would the equation be: Na⁺_(aq) + Cl^-_(aq) + H₂0_(l) ------> NaCl.nH₂0_(s) ???????

Thnxs for your help and time

 

[dead]

It involves NaOH as corrosion in this case releases OH- ions and increasing the pH pushes the reaction in the opposite direction, preventing further corrosion and help to remove chlorides from the metal (iron).

So is the NaOH just poured onto the object or soak the object in the NaOH solution?? If u know the equation can u show me because it would help me alot.

Thanx for ur time and help, now i understand the chemical process behind crystallisation.

 

[Slidey]

You soak the artefact in the NaOH and wait until saturation point is reached, then you replace the solution and wait until saturation point is reached, then you replace the solution and wait until saturation point is reached...

 

[dead]

lol i get ur point, so there is no equation to represent this process?

It's just repeating the saturation process until all or most of the salt is gone.

 

[Slidey]

You don't have equations for physical processes. Well, not the chemical kind, anyway.

I believe the salt is simply being washed out - that's why the solution must be replaced, because it can no longer hold any salt (correct me if I'm wrong).

 

[dead]

its just that templar and mitochondria's posts above say that NaOH is used to reverse the corrosion process by breaking down the salt from the metal into it's ions and should't that be a chemical process?

anyways, i have enough for the answer to the question, thanx slide for ur help

 

[Budz]

I got this same question...
the process of hydrated crystallization causes whats know as acidic oxidaition

4FeCl2(s) + 4H2O(l) + O2(g) -------> 2Fe2O3(s) + 8HCl(aq)

of course we all know that HCl is acidic and that the reduction of oxygen in a high H+ concentration has a higher electrode potential and is more spontaneous

btw every thing else said in this forum is also to be considered

 

[Slidey]

its just that templar and mitochondria's posts above say that NaOH is used to reverse the corrosion process by breaking down the salt from the metal into it's ions and should't that be a chemical process?

anyways, i have enough for the answer to the question, thanx slide for ur help

Uh, isn't the NaOH just acting as a "catalyst" for the hydration and hence removal of the salt?

 

[xiao1985]

oxidation is catalysed by presence of H+ ions... so in a basic soln, it first will greatly slow down the furthur corrosion of artefacts...

2ndly, dip the artefacts in less concentrated water, the salt saturated in the artefact will slowly but spontaneously diffuse out... i am sure some one mentioned that above... and as mito said, once the conc of salt in the soln which the artefact is in, is the same as the salt conc in the artefact, no further diffusion will occur,.. hence u need to replace ur soln with clean tank of water... and so on until majority of the salt is removed...