Copper has two oxidation states +1& +2. For more stability copper choses to be in +1 state. But copper also have +2 state. What are the factors which helps copper to remain in +2 state?
The state of a substance is determined by its Gibbs energy: the lower the Gibbs energy, the more stable the state. On the other hand, Gibbs energy is dependent on temperature and other parameters. In the case of copper oxides, the second parameter is an oxygen partial pressure in the surrounding atmosphere. Thus CuO is a more stable oxide than Cu2O at low temperatures and high oxygen partial pressures. When the temperature increases and the oxygen content in the atmosphere decreases, Cu2O becomes a more stable oxide. However, you need a certain minimum temperature (activation energy) to enable a transition between two oxides.
Dear Unniyarcha K.K thank you for posting this interesting and fundamental question on RG. When you say that "For more stability copper choses to be in +1 state" this statement is only partially correct. In copper chemistry the stability of the oxidation states +1 and +2 depends very much on whether we have a solution or a solid-state material. Copper(I) is only stable in the solid state (e.g. Cu2O, CuI, CuCl), whereas copper(II) is more stable in solution or when water is present e.g. in hydrated copper(II) salts. Higher oxidation states (+3 and +4) are can be stabilized in combination with oxygen or fluorine.
Good luck with your work!
In addition to Frank and Vadim, I want to point on the reasons for the stability:
- Cu(I) is a d10 system and thus gains stability from the closed and filled 3d shell, Cu(II) is d9.
- Cu(II) is thus less stable and suffers additionally from the fact that +II is a relatively high oxidation state for a noble metal such as copper. Thus, the redox potential Cu(I)/Cu(II) is usually positive, meaning that Cu(II) is an oxidant.
- A closer look shows that hydration energies play an important role in that efficient hydration stabilises the (high) Cu(II) oxidation state and can lower thus the oxidation potential. This is very extreme for water as solvent where the hydration energy of Cu(II) is 2100 kJ/mol compared with Cu(I) of 582 kJ/mol. Only this makes Cu(II) stable in aqueous solutions. The effect is thus simply due to the 2+ charged ion Cu2+ interacting stronger with the H2O dipoles than the 1+ charged ion Cu+.
- copper enzymes make use of this by varying the ligands to Cu. They can thus vary the redox potential and drive reaction either from Cu(II) to Cu(I) or vice versa.
For further reading see: Kaim W., Schwederski B., Klein A., Bioinorganic Chemistry, Wiley, 2nd edition, 2013, ISBN 9780470975237
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