# Copper(II) oxide

Copper(II) oxide or cupric oxide is an inorganic compound with the formula CuO. A black solid, it is one of the two stable oxides of copper, the other being Cu$_2$O or copper(I) oxide (cuprous oxide). As a mineral, it is known as tenorite, or sometimes black copper. It is a product of copper mining and the precursor to many other copper-containing products and chemical compounds.

## Production

It is produced on a large scale by pyrometallurgy, as one stage in extracting copper from its ores. The ores are treated with an aqueous mixture of ammonium carbonate, ammonia, and oxygen to ultimately give copper(II) ammine complex carbonates, such as [Cu(NH$_3$)$_4$]CO$_3$. After extraction from the residues and after separation from iron, lead, etc. impurities, the carbonate salt is decomposed with steam to give CuO.
It can be formed by heating copper in air at around 300–800 °C:
    2 Cu + O$_2$ → 2 CuO
For laboratory uses, copper(II) oxide is conveniently prepared by pyrolysis of copper(II) nitrate or basic copper(II) carbonate:
    2 Cu(NO$_3$)$_2$ → 2 CuO + 4 NO$_2$ + O$_2$ (180°C)
    Cu$_2$(OH)$_2$CO$_3$ → 2 CuO + CO$_2$ + H$_2$O
Dehydration of cupric hydroxide has also been demonstrated:
    Cu(OH)$_2$ → CuO + H$_2$O

## Reactions

Copper(II) oxide reacts with mineral acids such as hydrochloric acid, sulfuric acid, and nitric acid to give the corresponding hydrated copper(II) salts:
    CuO + 2 HNO$_3$ → Cu(NO$_3$)$_2$ + H$_2$O
    CuO + 2 HCl → CuCl$_2$ + H$_2$O
    CuO + H$_2$SO$_4$ → CuSO$_4$ + H$_2$O
In presence of water it reacts with concentrated alkali to form the corresponding cuprate salts:
    2 NaOH + CuO + H$_2$O → Na$_2$[Cu(OH)$_4$]
It can also be reduced to copper metal using hydrogen, carbon monoxide, and carbon:
    CuO + H$_2$ → Cu + H$_2$O
    CuO + CO → Cu + CO$_2$
    2 CuO + C → 2Cu + CO$_2$
When cupric oxide is substituted for iron oxide in thermite the resulting mixture is a low explosive, not an incendiary.

## Structure and physical properties

Copper(II) oxide belongs to the monoclinic crystal system. The copper atom is coordinated by 4 oxygen atoms in an approximately square planar configuration.
The work function of bulk CuO is 5.3 eV.

## Uses

As a significant product of copper mining, copper(II) oxide is the starting point for the production of many other copper salts. For example, many wood preservatives are produced from copper oxide.
Cupric oxide is used as a pigment in ceramics to produce blue, red, and green, and sometimes gray, pink, or black glazes.
It is incorrectly used as a dietary supplement in animal feed. Due to low bioactivity, negligible copper is absorbed.
It is used when welding with copper alloys.
A copper oxide electrode formed part of the early battery type known as the Edison–Lalande cell. Copper oxide was also used in a lithium battery type (IEC 60086 code "G").

### Pyrotechnics and fireworks

Used as moderate blue coloring agent in blue flame compositions with additional chlorine donors and oxidizers such as chlorates and perchlorates. Providing oxygen it can be used as flash powder oxidizer with metal fuels such as magnesium, aluminium, or magnalium powder. Sometimes it is used in strobe effects and thermite compositions as crackling stars effect.

## Similar compounds

An example of natural copper(I,II) oxide is the mineral paramelaconite, Cu$^+$$_2$Cu$^{2+}$$_2$O$_3$.