COPPER NITRATE (CU (NO3) 2): STRUCTURE, PROPERTIES, USES - CHEMISTRY - 2025

The copper nitrate (II) or cupric nitrate, the chemical formula Cu (NO ₃) ₂, is a bright and attractive colors blue-green inorganic salt. It is synthesized on an industrial scale from the decomposition of copper minerals, including the minerals gerhardite and rouaite.

Other more feasible methods, in terms of raw material and desired amounts of the salt, consist of direct reactions with metallic copper and its derivative compounds. When copper is in contact with a concentrated solution of nitric acid (HNO ₃), a redox reaction occurs.

In this reaction, copper is oxidized and nitrogen is reduced according to the following chemical equation:

Cu (s) + 4HNO ₃ (conc) => Cu (NO ₃) ₂ (aq) + 2H ₂ O (l) + 2NO ₂ (g)

Nitrogen dioxide (NO ₂) is a harmful brown gas; the resulting aqueous solution is bluish. Copper can form the cuprous ion (Cu ⁺), the cupric ion (Cu ²⁺), or the less common ion Cu ³⁺; however, the cuprous ion is not favored in aqueous media by many electronic, energetic, and geometric factors.

The standard reduction potential for Cu ⁺ (0.52V) is greater than for Cu ²⁺ (0.34V), which means that Cu ⁺ is more unstable and tends to gain an electron to become Cu (s). This electrochemical measurement explains why CuNO ₃ does not exist as a reaction product, or at least in water.

Physical and chemical properties

Copper nitrate is anhydrous (dry) or hydrated with different proportions of water. Anhydride is a blue liquid, but after coordinating with water molecules - capable of forming hydrogen bonds - it crystallizes as Cu (NO ₃) ₂ · 3H ₂ O or Cu (NO ₃) ₂ · 6H ₂ O. These are the three forms of salt most available on the market.

The molecular weight for dry salt is 187.6 g / mol, adding to this value 18 g / mol for each molecule of water incorporated into the salt. Its density is equal to 3.05 g / mL, and it decreases for each molecule of water incorporated: 2.32 g / mL for the tri-hydrated salt, and 2.07 g / mL for the hexa-hydrated salt. It does not have a boiling point, but rather sublimates.

All three forms of copper nitrate are highly soluble in water, ammonia, dioxane, and ethanol. Their melting points drop as another molecule is added to the outer coordination sphere of copper; fusion is followed by thermal decomposition of copper nitrate, producing the noxious gases of NO ₂:

2 Cu (NO ₃) ₂ (s) => 2 CuO (s) + 4 NO ₂ (g) + O ₂ (g)

The chemical equation above is for the anhydrous salt; for hydrated salts, water vapor will also be produced on the right hand side of the equation.

Electronic configuration

The electron configuration for the Cu ²⁺ ion is 3d ⁹, displaying paramagnetism (the electron in the 3d ⁹ orbital is unpaired).

Since copper is a transition metal of the fourth period of the periodic table, and having lost two of its valence electrons due to the action of HNO ₃, it still has the 4s and 4p orbitals available to form covalent bonds. Furthermore, Cu ²⁺ can make use of two of its outermost 4d orbitals to coordinate with up to six molecules.

NO ₃ ^- anions are flat, and for Cu ^{2+ to be} able to coordinate with them it must have an sp ³ d ² hybridization that allows it to adopt an octahedral geometry; this prevents the NO ₃ ^- anions from "hitting" each other.

This is achieved by Cu ²⁺, placing them in a square plane around each other. The resulting configuration for the Cu atom within the salt is: 3d ⁹ 4s ² 4p ⁶.

Chemical structure

In the upper image, an isolated molecule of Cu (NO ₃) _{2 is represented} in the gas phase. The oxygen atoms of the nitrate anion coordinate directly with the copper center (internal coordination sphere), forming four Cu – O bonds.

It has a square plane molecular geometry. The plane is drawn by the red spheres at the vertices and the copper sphere in the center. The interactions in the gas phase are very weak due to the electrostatic repulsions between the NO ₃ ^- groups.

However, in the solid phase the copper centers form metallic bonds –Cu – Cu–, creating polymeric copper chains.

Water molecules can form hydrogen bonds with NO ₃ ^- groups, and these will offer hydrogen bonds for other water molecules, and so on until creating a water sphere around Cu (NO ₃) _2.

In this sphere you can have from 1 to 6 external neighbors; hence the salt is easily hydrated to generate the hydrated tri and hexa salts.

The salt is formed from one Cu ²⁺ ion and two NO ₃ ^- ions, giving it a characteristic crystallinity of ionic compounds (orthorhombic for anhydrous salt, rhombohedral for hydrated salts). However, the bonds are more covalent in nature.

Applications

Due to the fascinating colors of copper nitrate, this salt finds use as an additive in ceramics, on metal surfaces, in some fireworks and also in the textile industry as a mordant.

It is a good source of ionic copper for many reactions, especially those in which it catalyzes organic reactions. It also finds uses similar to other nitrates, either as a fungicide, herbicide or as a wood preservative.

Another of its main and newest uses is in the synthesis of CuO catalysts, or of materials with photosensitive qualities.

It is also used as a classic reagent in teaching laboratories to show the reactions within voltaic cells.

Risks

- It is a strongly oxidizing agent, harmful to the marine ecosystem, irritant, toxic and corrosive. It is important to avoid all physical contact directly with the reagent.

- It is not flammable.

- It decomposes at high temperatures releasing irritating gases, including NO ₂.

- In the human body it can cause chronic damage to the cardiovascular and central nervous systems.

- May cause irritation to the gastrointestinal tract.

- Being a nitrate, within the body it becomes nitrite. Nitrite wreaks havoc on blood oxygen levels and the cardiovascular system.

References

1. Day, R., & Underwood, A. Quantitative Analytical Chemistry (5th ed.). PEARSON Prentice Hall, p-810.
2. MEL Science. (2015-2017). MEL Science. Retrieved on March 23, 2018, from MEL Science: melscience.com
3. ResearchGate GmbH. (2008-2018). ResearchGate. Retrieved on March 23, 2018, from ResearchGate: researchgate.net
4. Science Lab. Science Lab. Retrieved on March 23, 2018, from Science Lab: sciencelab.com
5. Whitten, Davis, Peck, & Stanley. (2008). Chemistry (eighth ed.). p-321. CENGAGE Learning.
6. Wikipedia. Wikipedia. Retrieved on March 22, 2018, from Wikipedia: en.wikipedia.org
7. Aguirre, Jhon Mauricio, Gutiérrez, Adamo, & Giraldo, Oscar. (2011). Simple route for the synthesis of copper hydroxy salts. Journal of the Brazilian Chemical Society, 22 (3), 546-551