Question

It is my understanding that copper forms an oxide layer upon heating. The colour of the copper depends on the thickness of this layer and can range from reds to purple to green etc. I have confirmed this by heating a sample and noticed that the oxide layer is robust is 'adhered' to the bulk copper

Separately, I have attempted to form an oxide layer through electrolysis, passing a current from one copper electrode to another via a CuSO4 solution. A black coating (assumed to be CuO) formed around one electrode. However, the CuO does not adhere at all to the bulk material, rubbing away extremely easily, leaving only shiny metal.
Assuming I am indeed forming CuO, why does this not adhere to the bulk in the same way CuO produced by heating does?

Answer 1

When copper is heated, it forms a robust oxide layer that adheres to the bulk copper. However, when copper oxide is formed through electrolysis, it does not adhere to the bulk material and can be easily rubbed away. The difference in adhesion is attributed to the different mechanisms of oxide formation.

Step-by-step explanation:

When copper is heated, it forms an oxide layer on its surface. The color of this oxide layer can vary depending on its thickness, appearing as red, purple, or green.

This oxide layer is robust and tightly adheres to the bulk copper, giving it a protective coating. However, when copper oxide (CuO) is formed through electrolysis, it does not adhere to the bulk material and can be easily rubbed away, leaving only shiny metal.

The reason for this difference in adhesion lies in the different mechanisms of oxide formation. When copper is heated, oxygen from the air reacts with copper sulfide (Cu2S), forming copper(I) oxide (Cu2O). This oxide layer then reacts with any remaining copper sulfide, reducing it back to metallic copper. The resulting oxide layer is tightly bound to the copper surface.

In contrast, electrolytic oxidation involves the direct application of an electric current to convert copper metal into copper ions, which then react with oxygen in the solution to form copper oxide (CuO). This type of oxide does not form a strong bond with the copper surface, leading to its easy removal.