What Determines the Products of Heating (NH4)2CO3?

[Drakkith]

Homework Statement

We performed a lab experiment where we heated (NH₄)₂CO₃ in a test tube and observed the results. The solid compound turned into a gas and evaporated, leaving condensation of a clear liquid on the inside of the test tube.

The lab report asks:
What is the chemical equation for the reaction when heating (NH₄)₂CO₃?

3. The Attempt at a Solution

At first I thought the answer would be:
(NH₄)₂CO₃ = 2NH₄ + CO₃

But when I looked online to double check my answer, it turns out to be:
(NH₄)₂CO₃ = 2NH₃ + H₂O + CO₂

My question is how was I supposed to know the products of this reaction? I can't find any rules regarding this in the chapters we've covered in my book. What are the rules regarding CO₃ and NH₄ when it comes to heating them?

For reference, we've just learned about redox, precipitation, decomposition, synthesis, and single/double replacement reactions. My textbook is: Chemistry: The Central Science, 13th Edition, by Brown, LeMay, Bursten, Murphy, Woodward, and Stoltzfus.

 

[Suraj M]

2NH4 + CO3

NH₄ and CO₃ don't exist, its## NH_{4}^{+}~and~CO_{3}^{2-}##
When you heat the compound you get the above ions, right? they then decompose to more stable forms, think of what ##NH_{4}^{+}## can form, NH₃ is quite stable, and for ##CO_{3}^{2-}## CO₂ is quite stable, this leaves out 2H⁺ and one O^2-.

 

[Borek]

When you heat the compound you get the above radicals, right?

First, these are not radical but ions. Second, they are not produced by heating. They are what the salt dissociates to when dissolved. I strongly doubt you deal here with H⁺ or O^2- at any moment (even if technically you are right they would produce a water molecule).

@Drakkith carbonates - when roasted - in general decompose into metal oxides and carbon dioxide. Carbon dioxide goes away. In the case of ammonium carbonate you would be left with "ammonium oxide". No such compound, but if you know how ammonia reacts with water (and how volatile ammonia is in general, which expresses itself by its solutions being smelly), it is quite obvious ammonia would run away from the solution too, leaving just water.

Actually I am a bit surprised you were left with water, I guess a very gentle heating. It is pretty easy to just evaporate water and be left with the empty test tube Ammonium carbonate - if left in an open container - simply disappears.

 

[Suraj M]

strongly doubt you deal here with H+ or O2- at any moment (even if technically you are right they would produce a water molecule).

Actually i was just trying to help him in predicting the product, not that these ions are actually formed.

 

[Merlin3189]

Borek & Suraj have given the chemistry for you and there's not much I could say on the theory. But this is an interesting question:

My question is how was I supposed to know the products of this reaction? I can't find any rules regarding this in the chapters we've covered in my book. What are the rules regarding CO₃ and NH₄ when it comes to heating them?

For reference, we've just learned about redox, precipitation, decomposition, synthesis, and single/double replacement reactions. My textbook is: Chemistry: The Central Science, 13th Edition, by Brown, LeMay, Bursten, Murphy, Woodward, and Stoltzfus.

I suspect there aren't simple rules (though I'm sure, when one learns all the details of atomic structure and bonding, rules could be made), that what is needed here is experience. You have a vast knowledge of what compounds exist and their properties even before you learn any rules about them. I expect, if you are studying chemistry, you will have done lots of simple "experiments", heating metal salts to decompose them, detecting carbon dioxide by blowing through limewater, dissolving metals, oxides and carbonates in acid, mixing solutions to produce precipitates, etc. And you've probably been writing down chemical equations for them, so that you are familiar with the common entities, the metals, oxides, halides, common compound anions and ammonia. (You have my sympathy with ammonia. It's always struck me as an oddity, but its pungence does make it very memorable.)

So when you come to a question like this, you are looking at these experiences and trying to find something that fits. I'm not a chemist (beyond school 47 years ago) and afaik I've never seen this reaction (though I've come across "smelling salts", which I think are ammonium carbonate), but I guessed it would decompose, that I'd end up with carbon dioxide (what you always got in carbonate experiments) and ammonia (given a bit of encouragement ammonium salts always seem to produce ammonia and turn the damp litmus blue .) Then it's just a matter of making the equation balance - which turns out to be water, as in many other reactions. Maybe the condensation was a clue to water (I didn't notice that!) and you start with CO₂ and water, then find the ammonia (or nitrogen and hydrogen) left over.
And I may well be wrong! Maybe the ammonia decomposes (at least a bit) to give nitrogen and hydrogen. I'd bet if you got it hot enough it would (based on the Haber process.)

Your attempted solution was near, but you did not recognise the products, nor that the formulae you wrote down were compounds that you'd never seen before. That I think is where you should have taken a second look and said, what compounds DO I know that are near what I've written.
Perhaps you are still a bit shaky on chemical formulae and need to practice this. I would say though, don't carry on happily writing down formulae as if it were just a maths exercise. Look at what you have written and ask yourself, "what is it?"

I can't tell if you are just struggling through a compulsory chemistry course, or are really keen to understand it. If the latter and your course is like so many now, almost devoid of practical work, you might consider doing some simple experimentation at home. I think there is nothing as good as doing experiments to make them stay in your mind. I can still remember some of the simple science experiments I did at school more than 50 years ago. (You won't be able to do some that require complicated apparatus or dangerous substances, but there are plenty of things you can do safely with very little equipment.)

 

[Drakkith]

NH₄ and CO₃ don't exist, its## NH_{4}^{+}~and~CO_{3}^{2-}##
When you heat the compound you get the above ions, right? they then decompose to more stable forms, think of what ##NH_{4}^{+}## can form, NH₃ is quite stable, and for ##CO_{3}^{2-}## CO₂ is quite stable, this leaves out 2H⁺ and one O^2-.

Huh. I was not aware that both compounds decomposed into 'more stable' forms. Thanks!

Actually I am a bit surprised you were left with water, I guess a very gentle heating. It is pretty easy to just evaporate water and be left with the empty test tube Ammonium carbonate - if left in an open container - simply disappears.

I wasn't directly left with liquid water. The solid decomposed and simply disappeared, leaving the test tube empty except for some condensation towards the top.

carbonates - when roasted - in general decompose into metal oxides and carbon dioxide.

Hmmm... after further investigation I found something in chapter 3 that mentions this. Thanks, Borek!

No such compound, but if you know how ammonia reacts with water (and how volatile ammonia is in general, which expresses itself by its solutions being smelly), it is quite obvious ammonia would run away from the solution too, leaving just water.

Actually I don't know how ammonia reacts with water. And oddly enough, while heating the compound I wafted some vapors into my face but couldn't really smell anything. Just a mild 'burnt' smell. I certainly didn't smell a pungent, rotten eggs odor. I hope I don't have points taken from my lab report because I couldn't smell anything.

I suspect there aren't simple rules (though I'm sure, when one learns all the details of atomic structure and bonding, rules could be made), that what is needed here is experience. You have a vast knowledge of what compounds exist and their properties even before you learn any rules about them.

(Bolding and underlining mine)

I certainly don't feel that way. I have some simple rules regarding solubility, acids and bases, oxidation states, and the ionic charge of different compounds, but I certainly don't have anything like what you seem to be suggesting. Can you elaborate on what you think I should have?

Your attempted solution was near, but you did not recognise the products, nor that the formulae you wrote down were compounds that you'd never seen before. That I think is where you should have taken a second look and said, what compounds DO I know that are near what I've written.
Perhaps you are still a bit shaky on chemical formulae and need to practice this.

Taken a second look? I spent about half an hour flipping through the two most recent chapters (3 and 4) trying to figure out what to do here. And it's not that I didn't recognize the compounds. I've done plenty of questions that involved either carbonate or ammonium. I think the key thing I wasn't understanding here is that the reaction products need to be neutral (more of that below). Chapter 4, the most recent chapter, was all about reactions in aqueous solutions, while chapter 3 never dealt with ions at all, so I didn't realize that the products should be neutral.After thinking about this more, it appears to me that in the absence of water you can't separate these compound into ions, they have to be neutral compounds. Is that correct?
If so, that would mean that NH₄ would either need to get rid of a hydrogen or take on an oxygen. But, taking on an oxygen would put it at -1 charge and leave the other NH₄ with a +1 charge still. So the only option to keep them all neutral is to take an oxygen from carbonate and combine it with 1 hydrogen from each of the ammoniums, forming 1 water and 2 ammonia molecules. Is my thinking correct?

 

[Suraj M]

I certainly didn't smell a pungent,

Thats a bit surprising.

So the only option to keep them all neutral is to take an oxygen from carbonate and combine it with 1 hydrogen from each of the ammoniums, forming 1 water and 2 ammonia molecules. Is my thinking correct?

Yeah! you're thinking is right, just move towards the stable compounds you can find, close to the ions you have recognized.

 

[Borek]

Huh. I was not aware that both compounds decomposed into 'more stable' forms.

In a way that's the most basic description of all decomposition reactions.

I wasn't directly left with liquid water. The solid decomposed and simply disappeared, leaving the test tube empty except for some condensation towards the top.

That's more like what I would expect.

I certainly didn't smell a pungent, rotten eggs odor.

That would be hydrogen sulfide, not ammonia. Ammonia has an unpleasant odor as well, but it is not that easy to describe. Horse urine smells of ammonia, but who knows teh smell of the horse urine?

After thinking about this more, it appears to me that in the absence of water you can't separate these compound into ions, they have to be neutral compounds. Is that correct?

More or less. It is definitely possible to have ions outside of the solution (think ionized gas, think molten ionic salts) but these are rather exotic conditions. In typical situations you deal with neutral molecules or ions in the water solution (you can rather safely assume there will be no ions in other solutions - that's again not exactly true, but it again would require exotic solvents/conditions).

If so, that would mean that NH₄ would either need to get rid of a hydrogen or take on an oxygen. But, taking on an oxygen would put it at -1 charge and leave the other NH₄ with a +1 charge still. So the only option to keep them all neutral is to take an oxygen from carbonate and combine it with 1 hydrogen from each of the ammoniums, forming 1 water and 2 ammonia molecules. Is my thinking correct?

The final conclusion is OK, but there is another path that will get you there, one that doesn't even require ions. When ammonia dissolves in water it reacts to some extent with water:

NH₃ + H₂O ↔ NH₄OH ↔ NH₄⁺ + OH^-

The part that is important here is the ammonium hydroxide in the middle. I have added the dissociation part just because that's what we really observe, it is not possible to isolate the ammonium hydroxide, although it must exist as an intermediate.

You should probably know that salts can be produced by a direct reaction between hydroxides and acid anhydride, like

2NaOH + CO₂ → Na₂CO₃ + H₂O

combining these will tell you that ammonium carbonate can be produced from ammonia and carbon dioxide:

2NH₃ + H₂O + CO₂ → (NH₄)₂CO₃

and the decomposition is just a trivial reversal of this reaction, made easy by the fact both ammonia and carbon dioxide are gases, so they easily leave the reaction vessel.

 

[NascentOxygen]

Has it been pointed out that ammonium carbonate is the active ingredient in smelling salts? One good whiff of the ammonia it gives off makes you gasp, taking in a deep breath and thereby getting air into your lungs and rousing you to alertness. So it is constantly decomposing and liberating some ammonia gas.

 

[Suraj M]

ammonia it gives off makes you gasp, taking in a deep breath and thereby getting air into your lungs and rousing you to alertness. So it is constantly decomposing and liberating some ammonia gas.

Surprising that the OP did not get any kind of pungent odour.