Why The Statue Of Liberty Is Green

Chemical reactions can be divided into two classes: redox (reduction-oxidation) reactions
and nonredox reactions based on whether electron transfer process is involved or not. A redox
reactionconsists of two half reactions: a reductive half-reaction in which a reactant accepts
electrons and an oxidative half-reaction in which a reactant donates electrons. The nature of a
redox reactionis that one reactant donates its electrons to the other reagent. For example, in the
oxidation of copper by oxygen (equation 1), copper atoms donate electrons to an oxygen
molecule so copper isoxidized while oxygen is reduced .
2Cu + O2 → Cu2O equation 1

The Statue of Liberty gets its blue-green color from patina formed on its copper surface
mainly through oxidation along withseveral other chemical reactions. The main constituent of
patina contains a mixture of 3 compounds: Cu4SO4(OH)6 in green; Cu2CO3(OH)2 in green; and
Cu3(CO3)2(OH)2 in blue. The following reactionsare involved.
The oxidation starts with the formation of copper (I) oxide (Cu2O), which is red or pink in color
(equation 1), when copper atoms initially react with oxygen molecules in the air.Copper (I)
oxide is further oxidized to copper (II) oxide (CuO), which is black in color (equation 2). In the
19th and early 20th century, coal was the major fuel source for American industry and itusually
contains sulfur. Thus, the black copper (II) sulfide (CuS) also forms (equation 3). Over the years,
CuO
2Cu2O + O2 → 4CuO equation 2
Cu + S → 4CuS equation 3
and CuS slowlyreacts with carbon dioxide (CO2) and hydroxide ions (OH-) in water from the air
to eventually form Cu2CO3(OH)2 (equation 4) , Cu3(CO3)2(OH)2 (equation 5) and Cu4SO4(OH)6
(equation 6), which constitutethe patina. The extent of humidity and the level of sulfur-related
air pollution have a significant impact on how fast the patina develops, as well as the relative
ratio of the three components....