Ammonium dichromate is a crystalline solid that is bright orange in color. It readily ignites and burns and produces a voluminous residue that has a green color. However, if we start heating it in a closed container, the container may rupture because of the decomposition of the material. It can also act as a strong oxidizing agent when mixed with or contaminated with the combustible material. It dissolves when put inside the water. Moreover, in this compound, as in all the chromates and the dichromates, chromium is in a +6 oxidation state that is commonly called hexavalent chromium. Let’s learn about Ammonium Dichromate Formula in detail.
It is a salt that comprises of the ammonium ions and dichromate ions both at the same time. The Ammonium dichromate is also popular by the name ‘vesuvian’ Fire sometimes, because of its application in the demonstrations of the tabletop ‘volcanoes’. However, this demonstration is not so popular in the schools due to the carcinogenic nature of the compound. In the early days of photography and some of the pyrotechnics use it.
Ammonium Dichromate Formula and Structure
The molecular formula of the ammonium dichromate is (NH4)2Cr2O7. And its structure id given in the image provided below.
At room temperature and pressure, the compound exists in orange color acidic crystals that are soluble in water and alcohol. Its production takes place by the action of the chromic acid on the ammonium hydroxide with subsequent crystallization. The (NH4)2Cr2O7 crystal (C2/c, z=4) consists of a single type of ammonium ion. Moreover, at the sites of symmetry C1 (2,3). Every single NH4+ center is irregularly surrounded by 8 oxygen atoms at N—O distances that range from ca. 2.83 – ca. 3.17 Å, typical of the bonds of hydrogen.
It has various applications like pyrotechnics and in the early time of photography as well as in the lithography, as a source of pure nitrogen in the chemistry labs, and also as a catalyst. In addition, industries also use it as a mordant in the process of dying in the dyeing pigments, in the manufacturing process of the alizarin, chrome alum, leather tanning and in oil purification as well.
Photosensitive films that contain PVA, ammonium dichromate and a phosphor are spin-coated as liquid slurries in the production of the phosphor raster of the TV screens and LCDs and many other different devices. On the other hand, the ammonium dichromate acts as the photoactive site as well.
The volcano demonstration includes the igniting of a pile of the salt and this initiates the following exothermic conversion:
(NH4)2Cr2O7 (s) → Cr2O3 (s) + N2 (g) + 4 H2O (g) (ΔH = −429.1 ± 3 kcal/mol)
Like the ammonium nitrate, it is also thermodynamically unstable. Its decomposition reaction proceeds to the process of completion once we initiate it, producing voluminous powder of chromium (III) oxide that is dark green in color.
Not all of the ammonium dichromate decomposes in this specific reaction. When the dark green color powder mixes up in the water then, as a result, a yellow or an orange color solution is the outcome we get from the leftover ammonium dichromate.
The Ammonium dichromate is similar to all the chromium (VI) compounds and it is highly toxic and also a proven carcinogen. Moreover, it is also a strong irritant at the same time.
In the airtight and sealed containers, ammonium dichromate may explode if provided excess heat. In the year 1986, 2 workers lost their lives and 14 others were injured at the Diamond Shamrock Chemicals in Ashtabula, Ohio when about 2,000 lbs. of ammonium dichromate exploded because it was being dried in a heater.
Solved Example on Ammonium Dichromate Formula
Question: What is the oxidation state for Cr in (NH4)2Cr2O7?
Solution: The oxidation number of the chromium i.e. Cr in (NH4)2Cr2O7 is +6.