Accident at factory, how to solve it?

Studiot

OK to reach a conclusion I'll start at the beginning.

When I first read this I thought that this had been a real accident at a factory the OP worked at. Now I wonder if it is really an academic exercise?

Anyway I should observe something obvious viz that to neutralise something is to render it neutral ie neither acid nor alkaline.

Now nitric acid is an acid liquid, whilst calcium and sodium carbonates are white neutral solids, usually powders. Only the sodium carbonate is soluble in water.

So if we consider our puddle of spilt nitric acid and shovel on some white powder until it turns from acid to neutral.

The reaction is

metal carbonate (solid) + nitric acid [tex]\rightarrow[/tex] metal nitrate (solution) + water + carbon dioxide (gas)

Since both nitrates are soluble, either powder dissolves in the liquid, and the reaction gives off carbon dioxide as a gas.
The liquid turns from acid to neutral.

At this point the action is the same whichever carbonate is used.

However once the acid is neutralised no further powder will dissolve if calcium carbonate is used. That white powder will simply settle to the bottom of the puddle.

Since there are no further chemical changes the liquid never becomes alkaline, meeting the condition of the question.

If, however, sodium carbonate is added to the neutral liquid this dissolves adding sodium and carbonate ions to the liquid.

Some carbonate ions now react with water to form bicarbonate and hydroxyl ions. The hydroxyl ions turning the liquid alkaline, since the matching proton is now part of the bicarbonate ion and so does not form a hydroxonium ion that would maintain the liquid neutrality.
The more excess sodium carbonate is added the more alkaline the liquid becomes.

Kyoma, are you able to write symbolic chemical reactions for this?

Kyoma

CO₃^2- + H2O -------> HCO₃^- + OH^-

And then,

OH^- + Na⁺ -----> NaOH

But now, I have another question:

If I remember correctly, there would be some hydroxide ions in nitric acid due to water dissociation. Why is my above quote incorrect?

Borek

That's OK.

No, these will be just dissolved as separate ions, no reaction between them.

They are always present because of water autodissociation. But the solution is dominated by the presence of H⁺ ions, so it can't be called "alkaline". It works also in other direction - in alkaline solutions there is always some small amount of H⁺, but it doesn't make solution acidic.

Nemus

I am not sure that I should put more fuel onto this fire but, since I have tried this in real life first in an accident ( I coose calium carbonate because it was in a sack next to me) and then playing with it later, I think I'll say it anyway. I think calcium carbonate is the better choice because it reacts less violently and therefore doesn't cause so much aerosol to form. Nitric acid aerosol is very bad for the person trying to save that day and also for everything in the room. The most common form of sodium carbonate contains some crystal water (10?) and I think that's why it reacts much more violently and boils like crazy when you pour it on concentrated nitric acid.

SpectraCat

The crystal water may indeed have an effect, but I am pretty sure that it is the solubility that causes the major difference in the reactivity. Sodium carbonate is soluble in water (21.6 g/100g) , so it dissolves extremely quickly when it comes into contact with the aqueous nitric acid solution, so that *all* the carbonate ions are almost immediately available to react with the acid. On the other hand, calcium carbonate is about 20,000 times less soluble (~0.001g/100g), so only the surface of the calcium carbonate particles are undergoing reaction at any given time, and so the local volume around the particles quickly becomes saturated, and you have to wait for more reactant to diffuse to the surface, and the overall reaction proceeds more slowly.

Studiot

Are you sure?

Nemus

Well, concentrated HNO3 is hardly aqueous. There is very little free water around actually so even if I don't dispute that sodium salts generally are more soluble than calcium salts I think we are dealing with surface reactions in both cases. Since surface reactivity is often dominated by buildup of insoluble passivating surface coating, it could actally be that the solubility of sodium nitrate versus calcium nitrate in concentrated nitric acid is more to the point. I would be very much impressed if anybody could dig those up!

SpectraCat

All nitrate salts are soluble. So I don't think that is an issue in this case. Also, no one specified that it was concentrated nitric acid, or what the concentration actually was.

SpectraCat

Fair enough .. I should have qualified that better. I was referring to case where the nitric acid is in excess .. of course we have the opposite case here.

What I really feel sure of is that, due to the difference in solubilities, there will be more a lot more carbonate available to react in the early parts of the reaction when sodium carbonate is used as opposed to calcium carbonate. That was the point and context of my comment above.

Nemus

That is a very strong statement for the behaviour in such an exotic medium. I would be surprised if all nitrates are highly soluble in concentrated (or strong) nitric acid. Compare with the effect of concentrated sulfuric acid on steel (none) because the iron sulfate is insoluble in the acid.

SpectraCat

Ok, I think the confusion here is that you are thinking about 68% nitric acid, while I have been thinking about an aqueous solution of nitric acid with a relatively high concentration, say 5 M or so. I agree that metal nitrates are unlikely to be very soluble in 68% nitric acid, but I would think they would be plenty soluble in 5 M nitric acid.

However, even in 68% nitric acid, I would not necessarily expect formation of protective nitrate layers on the surface of calcium (or sodium) carbonate crystals. This is because the reaction is exothermic and produces water and CO2 locally, which I would expect to displace the nitrate from the surface. I think I see why you mentioned the crystalline water now though, since I guess that would be helpful in locally solubilizing the nitrates in 68% nitric acid.