Is there such a thing as ammonium carbonate? _Industrial additives

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.

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,

NH₄HCO₃+NH₃=(NH₄)₂CO₃.

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⁺ + CO₃^2－ + H₂O (or HCO₃ ^－ + NH₃·H₂ONH₄⁺ + CO ₃^2－ + 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＝CO₃^2-+ NH₄⁺&hpyridinium dichromate ellip;…(1)

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

H₂CO₃+2NH₃＝CO₃^2- +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.