Sciencemadness Discussion Board - Gravimetric evidence of copper(II) oxide hemihydrate

Gravimetric evidence of copper(II) oxide hemihydrate

SnailsAttack - 9-6-2023 at 19:05

A gravimetric study I did on copper(II) oxide turned up evidence for the existence of a stoichiometric hemihydrate, whose formula could be written as CuO · 0.5H₂O, or perhaps Cu₂(OH)₂O or (equivalently) CuO · Cu(OH)₂.

I haven't found any sources that describe this compound, possibly because it's visually indistinguishable from copper(II) oxide (CuO).

In my test I produced around a gram of copper(II) oxide by pyrolyzing some copper hydroxycarbonate (Cu₂(OH)₂CO₃) that I made from copper acetate (from vinegar and scrap copper) and sodium carbonate (from baking soda), which were recrystallized several times to ensure purity.

The copper(II) oxide was left in an open dish in my bedroom, increasing in mass by 10% (ostensibly due to absorption of water from the air) in less than 24 hours. When the mass remained steady for at least 3 days I pyrolyzed it again to double-check against the original mass.

Code:

Original mass of CuO (immediately after pyrolyzing at ~300°C): 1.156 g

= Date/time, mass of CuO
May 31st, ? am   1.156 g
May 31st, ? am   1.198 g
May 31st, 10 pm  1.266 g
June 1st, 5 pm   1.278 g
June 3rd, 10 am  1.289 g
June 3rd, 10 pm  1.285 g
June 5th, 2 pm   1.286 g
June 6th, ? am   1.286 g

Mass of CuO after pyrolyzing again: 1.156 g

The 11.25% change in mass suggests that the molar mass increased by 8.95 grams per mole ((79.546 * 1.1125) - 79.546), which is 99.4% similar to half the mass of a water molecule (9.01 g/mol), which I believe is very strong evidence of a stoichiometric 'hemihydrate' of copper(II) oxide.

I'm not sure whether this compound is best described as a hydrate (CuO · 0.5H₂O) or if the water is present as hydroxide (Cu₂(OH)₂O or CuO · Cu(OH)₂), making it an oxyhydroxide.

SnailsAttack - 9-6-2023 at 19:06

Interestingly the mass of the copper(II) oxide hemihydrate remained stable for at least 3 days in presence of atmospheric CO₂, which suggests that the formation of copper hydroxycarbonate (which is what I was originally testing for) occurs much more slowly.