Question 2

What makes this type of metals form coloured stains?

Answers:

1) The copper ring is actually a metal ring coated with copper (golden in colour). Reactions such as oxidation made the layer of copper fall off, exposing the metal beneath to chemical reactions that can happen in open air. Three possible metals were found to be able to produce a green stain. The metals are Nickel, Iron, and Vanadium. Since rings are jewellery, the most common metal used in the production of them is Nickel. The green stain therefore is [Ni(H2O)6­]2. 2) The brown spots in the kettle are insoluble iron (III) oxide. It is the product of the oxidation of iron (II) in water.

Chemical equation: 6CH3COOH +Fe2O3 2Fe(CH3COO)3 + 3H2O Ionic equation: 4Fe2+ (aq) + 8HCO3 - (aq) + O2 (g) 2Fe2O3(s) + 8CO2(g) + 4H2O(l)

When vinegar (aqueous solution of acetic acid) was added to the water in the kettle and left to boil, Fe3+ (aq) was formed directly. Fe(s) + NO­3-(aq) + 4H+(aq) Fe3+(aq) + NO(g) + 2H2O(l) When an acidic solution of iron (III) is made more basic, a gelatinous red-brown precipitate (a hydrous oxide, Fe2O3.nH2O) is formed.

Additional:

What made the green stain remain even after a few days?

Answer: The metal Nickel had already been oxidised to its stable state and therefore can remain as it is for a number of days on the skin until it its scrubbed off thoroughly. This is because going across the 1st transition series, the +2 oxidation state become more stable compared to the +3 oxidation state. This means that this process becomes increasingly difficult: M2+ (aq) M2+ + e- This is because the 3rd electron removed is from the inner 3d subshell which is closer to the nucleus. As the nuclear charge increases across the series, the ionization energy also increases. Fe3+ is expected to be more stable than Fe2+ because the removal of an electron from Fe2+ forms the more stable Fe3+, where the 5d orbitals are all half filled. Fe2+([Ar]3d6) Fe3+([Ar]3d5)