Is there such a thing as ammonium carbonate? _Industrial additives

A problem we encountered in teaching is that ammonium bicarbonate generally refers to ammonium bicarbonate, not ammonium carbonate. Explain that ammonium carbonate is unstable. Is there any ammonium carbonate substance?

Ammonium carbonate, colorless translucent hard crystalline block or powder. There is a strong ammonia odor. It tastes spicy. It is a mixture of ammonium bicarbonate (NH4HCO3) and ammonium carbamate (NH2COONH4) . It decomposes in the air to release ammonia and carbon dioxide, turning into opaque ammonium bicarbonate powder, which completely volatilizes at 60°C.

The industrial product is actually a double salt of ammonium bicarbonate and ammonium carbamate. Contains 31% ammonia and 56% carbon dioxide. Soluble in water, insoluble in ethanol, carbon disulfide and concentrated ammonia. It is unstable in the air and will gradually turn into ammonium bicarbonate and ammonium carbamate.

The dried material is easily decomposed at 58°C, releasing ammonia and carbon dioxide. The aqueous solution begins to decompose at 70°C. It is unstable to light and heat and slightly hygroscopic.

So generally speaking, ammonium carbonate (NH4)2CO3 does not exist in an absolute sense. Generally, ammonium carbonate is divided into There are two types, both of which are mixtures. One is a mixture of ammonium bicarbonate and ammonium carbamate, and the other is ammonium bicarbonate and hydroxylammonium carbamate (which is the monohydrate of the former). What is generally available on the market is the latter. The latter, because the latter is more stable. Although it is a mixture, the general reagent company will still list the molecular weight for reference only, ammonium carbonate is 96, and hydrated ammonium carbonate is 157.

It can be seen that ammonium carbonate is an unusual binary weak acid and weak base salt. In addition, it has extremely special hydrolysis conditions and is generally not suitable for use in middle school chemistry teaching. However, question 26 of the 2019 National College Entrance Examination Volume 1, “leaching” boron magnesium ore powder at 95°C, the gas produced is “absorbed” with NH4HCO3, the chemical equation of the reaction is,


Question 19 of the 2019 Jiangsu College Entrance Examination Chemistry Examination: Mix ammonia water and NH4HCO3 solution to prepare (NH4 )2CO3 solution, its ionic equation is,

HCO3+ NH3·H2O= NH4+ + CO32- + H2O (or HCO3 + NH3·H2ONH4+ + CO 32- + H2O).

Testing this equation brings some trouble to teachers. So does this equation happen? Professor Wu Weifu specifically discussed this issue.

For 1.0 mol·L-1 of NH4HCO3 and (NH4)2CO3 solution, the actual (equilibrium) concentration of various ions (unit is mol·L-1) is as follows:

It can be seen that the original NH4HCO3 solution contains a large amount of HCO3 and NH A solution of 4+ ions (shown by the red number in the first row of numbers). The so-called (NH4)2CO3 solution also contains a large amount of HCO3– sup> and NH4+ ions (as shown by the red numbers in the second number row), while there are a large number of NH3 molecules (such as solution (indicated by the blue numbers in the second row of numbers). Although [CO32-] is more than 20 times larger than the number in the previous line, in the end it is only “0.07835 mol·L-1” (shown in bold purple font). It is basically a mixed solution composed of NH4HCO3 and NH3. It can also be considered that the hydrolysis reaction is “NH4++CO32-=HCO3 +NH3” was carried out quite thoroughly.

In other words, the mixing of ammonia water and NH4HCO3 solution is basically a physical process of mixing the two.

Are there any chemical reactions?

Of course there are chemical changes, and the reality of those changes can be seen from the bottom row of numbers. After adding more ammonia, the concentrations of three substances decreased (red font). The amounts of two substances increased significantly (indicated in blue fonts, which are the products of the reaction).

It is not difficult to distinguish the precipitation of calcium bicarbonate from it. The 1.0 mol·L-1 ammonia solution added has only 0.1107 mol·L-1 from the microscopic composition of the solution. Ammonia (11.1%) participates in the following two reactions.

Among them, 0.0403 mol·L-1 of NH3 participated in the reaction,

HCO3+NH3=CO32-+ NH4+&hpyridinium dichromate ellip;…(1)

Another 0.0704 mol·L-1 of NH3 participated in the reaction,

H2CO3+2NH3=CO32- +2NH4+……(2)

From a microscopic perspective, more NH3 participates in reaction (2) than NH3 participates in reaction (1).

However, as a chemical equation describing a macroscopic process, it can still only be written as formula (1).

Due to the mixing process of ammonia water and NH4HCO3 solution, the changes in the relative amounts of each substance are very limited. The so-called (NH4)2CO3 solution is basically just an NH3—NH 4+ buffer solution.

TAG: ammonium carbonate, ammonium bicarbonate,

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