Reactions of calcium carbonate

[Moogie]

Hi

What is the reaction of calcium carbonate and water. I thought it was insoluble but I was looking up some notes from a chemistry practical i did and it says

CaCO3(s) + H2O (l) → Ca(OH)2(s) + CO2(g)

Can someone please explain this reaction for me as I can't find it on the internet.

I'm familiar with the reverse reaction

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)

but I would appreciate an explanation of why this reaction occurs.

Many thanks

 

[Borek]

Equilibrium of this reaction is shifted far to the solid calcium carbonate side. However, assuming the mixture is in contact with air free of carbon dioxide (not that such thing exist in nature) dissolution reaction is not impossible (hint: LeChatelier's principle, you are removing one of the products). But this is more theoretical construct than something that may happen in the reality.

 

[Moogie]

Thanks for your reply but I don't know what you mean. Which reaction are you describing? I think you mean the first reaction as you mention dissolution but why does it matter how much CO2 is in the air when the calcium carbonate is in the water? But I am only really interested in what happens under normal conditions, not extreme conditions. So is it correct to say calcium carbonate is generally insoluble? The notes i have say it formed a milky white ppt in water with ph 11 but i don't know what is supposed to be happening. The best i could imagine was that the carbonate might abstract a proton from water to make bicarbonate

 

[Borek]

Which reaction are you describing?

I have referred to the equilibrium lying far on the side with solid calcium carbonate, just to make the reaction irrelevant - note, that they are identical, just reversed.

why does it matter how much CO2 is in the air when the calcium carbonate is in the water?

Because carbon dioxide is between the dissolution product. Do you know LeChatelier's principle?

But I am only really interested in what happens under normal conditions, not extreme conditions. So is it correct to say calcium carbonate is generally insoluble?

Yes, unless you lower pH.

 

[lightarrow]

Hi

What is the reaction of calcium carbonate and water. I thought it was insoluble but I was looking up some notes from a chemistry practical i did and it says

CaCO3(s) + H2O (l) → Ca(OH)2(s) + CO2(g)

Can someone please explain this reaction for me as I can't find it on the internet.

I'm familiar with the reverse reaction

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)

but I would appreciate an explanation of why this reaction occurs.

Many thanks

Where did you find that reaction? Non considering very pathological situations (described by Borek), you can confidently consider it incorrect.

If, instead, you were considering this other reaction:

CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(s)

It would be different.

 

[Moogie]

I thought it was wrong. That was why I asked the question. :)

So what does happen when you add CaCo3 to water? Please can you give reaction and observations. Does it form a milky white solid ppt and an alkaline pH?

I believe this reactions only occurs in exess CO2?
CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(s)

 

[Borek]

So what does happen when you add CaCo3 to water? Please can you give reaction and observations. Does it form a milky white solid ppt and an alkaline pH?

No milky white, unless you use powdered carbonate and mix it with water. Just a piece thrown into water will dissolve a little bit, to the extent described by several equilibrium constants - solubility constant, carbonic acid dissociation constants. pH will go up to almost 10 (that is assuming there is no carbon dioxide in the air, if it is present, pH won't go that high).

I believe this reactions only occurs in exess CO2?
CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(s)

Depends on what you mean by 'excess'. Amount present in the air is enough for the reaction to occur and carve karst landscape.

 

[Moogie]

Thanks, so when you add the lump of CaCo3 to water and it dissolves a little bit, what is the reaction that is occurring. Wikipedia says the water has to be saturated with CO2 for this to occur so does that mean it ISN'T this reaction as water isn't saturated with co2?

CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(s)

If it isn't this reaction, what reaction is it please?

 

[Borek]

CaCO₃ in water dissolves according to the most obvious reaction equation:

CaCO₃ -> Ca²⁺ + CO₃^2-

However, this goes only till the solution is saturated, which happens quite fast, as CaCO₃ is weakly soluble.

This reaction is followed by CO₃^2- hydrolysis (to HCO₃^-) which slightly increases solubility.

Now, I haven't read the wikipedia article you refer to, but either it is misleading or you are reading it wrong. Presence of carbon dioxide substantially increases carbon carbonate solubility, but is not necessary for the carbon carbonate dissolution.

 

[Moogie]

Thanks. I presume then that the alkaline ph is because the carbonate ion is the conjugate base of a weak acid so it abtracts a proton from water to give hydrogen carbonate as you say and OH-

 

[Borek]

Yes, that's hydrolysis that I referred to.

 

[Moogie]

this is the wikipedia link

http://en.wikipedia.org/wiki/Calcium_carbonate:

Calcium carbonate will react with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.

CaCO3 + CO2 + H2O → Ca(HCO3)2

What role does CO2 play in that reaction because, as you have just shown me, that reaction happens anyway in two steps without co2:

1) dissolution of ca/carbonate ions
2) carbonate hydrolysis to bicarbonate

In other words, why does CO2 increase the solubility of carbonate? I tried to think in terms of le chateliers principle but couldn't see why. carbon dioxide and water gives carbonic acid which dissociates to hydrogencarbonate. This is one of the productions of the hydrolysis of carbonate which makes me think it would reduce solubility. So obviously my logic is warped. Please can you correct it for me :)

 

[Borek]

CO₃^2- + CO₂ + H₂O -> 2HCO₃^-

Try to apply LeChatelier's principle now.

 

[Moogie]

Ah right, i see. That's brilliant I get it now.

However, i don't understand why carbon dioxide features in the hydrolysis reaction. Can i ask the following:

a) being as carbonate is the conjugate base of a weak acid it would do this reaction if carbond dixoide wasn't present?

CO3[2-] + H20 <=> HCO3[-]

b) why does the carbon dixoide participate in the reaction (and please don't say, I don't know, you'll have to ask it :-D). Perhaps there is no simple reaction to that one but i thought i would ask anyway

 

[Borek]

It is all in equilibrium.

Presence of CO₂ means presence of carbonic acid, so you have a mixture of CO₃^2- and H₂CO₃. Obviously one is an acid, the othe rone is a base, so they will react, producing HCO₃^-.

First reaction you wrote - hydrolysis of CO₃^2- - takes place whenever carbonates are dissolved in water, but only to some extent. Presence of carbon disoxide automatically shifts the equilibrium right.