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Solubility of CaCO3 in oceans

So i understand all of Le Chatelier's principles etc. etc. but why is it that when you go deeper into the ocean, i.e. to a higher pressure, the solubility of CaCO3 (s) increases?

My understanding thus far is:

[CO2(aq)] will increase which means the [H+] and [HCO3-] will increase, but surely they increase by the same proportion so for the dissociation of HCO3- to H+ and CO3^2- the equilibrium will shift to the right, increasing the [CO3^2-] which would shift the equilibrium of CaCO3 to its dissolved form back to CaCO3?

I'll try and attach the equilibriums

Original post by BDunlop

As you go deeper the solubility of CO₂ increases.

This drives your equation (2) to the RHS

With more HCO₃^- this affects the equilibrium:

HCO₃^-<==> H⁺ + CO₃^2-

pushing this one the the RHS (and also pulling equilibrium (2) more to the RHS)

Reply 3

Dat1Guy

Original post by charco

As you go deeper the solubility of CO₂ increases.

This drives your equation (2) to the RHS

With more HCO₃^- this affects the equilibrium:

HCO₃^-<==> H CO₃^2-

pushing this one the the RHS (and also pulling equilibrium (2) more to the RHS)

thanks, I understand that, but if we relate this to the equilibrium of image-01d5bcc7-f26d-463c-91e1-32f87c122d60469668317-compressed.jpg.jpeg

why does the solubility increase, if we increase the concentration of CO3 then by this, wouldn't the solubility decrease for the CaCO3 solids present as equilibrium shifts to left?

(edited 6 years ago)

Reply 4

charco

Study Forum Helper

Original post by BDunlop

thanks, I understand that, but if we relate this to the equilibrium of image-01d5bcc7-f26d-463c-91e1-32f87c122d60469668317-compressed.jpg.jpeg

why does the solubility increase, if we increase the concentration of CO3 then by this, wouldn't the solubility decrease for the CaCO3 solids present as equilibrium shifts to left?

This is the equation of the solubility product as it shows the dissolution of solid calcium carbonate.

Ksp = [Ca²⁺][CO₃^2-]

The only thing that can affect an equilibrium constant is temperature. If you increase the concentration of the carbonate ion then you must decrease the concentration of calcium ions.

This will be effected by precipitation of calcium carbonate until Ksp is restored.

Reply 5

Dat1Guy

Original post by charco

Yes exactly, that is my point. But everywhere i look online states that the solubility of CaCO3 increases as the pressure increases?

Which is converse to what this explanation would suggest?

Reply 6

charco

Study Forum Helper

Original post by BDunlop

Yes exactly, that is my point. But everywhere i look online states that the solubility of CaCO3 increases as the pressure increases?

Which is converse to what this explanation would suggest?

The actual solubility cannot increase, that is governed by Ksp.

This must mean that one of the species (the carbonate ion) is being removed by one of the other equilibria.

The only thing that the carbonate can be removed by is reaction with hydrogen ions forming bicarbonate ions.

So, it must be an increase in H+ ions caused by increased carbon dioxide which drives it.