Question

4. Given the following half reaction

Fe(OH)₃ (s) + e^- +3H⁺ çè Fe²⁺ + 3H₂O log K = 15.8

(6 pts) The Grinnell formation in Glacier N.P., formed from ocean sediments, is typified by contrasting gray and red colors, yet the total Fe (iron) concentration remains constant throughout the rock. Can you explain this color variation? (Hint: iron turns red as it “rusts”).

Answer 1

The given reaction is

Fe(OH)₃ (s) + e^- + 3 H⁺ ------> Fe²⁺ + 3 H₂O log K = 15.8

Also, the total iron concentration remains constant. Now,

[Fe]_total = [Fe²⁺] + [Fe³⁺]

Iron in rocks is usually oxidized to Fe³⁺ (since this is the more stable oxidation state of iron) and hence is easily converted to Fe(OH)₃ on reaction with water. In the atmosphere, this Fe(OH)₃ is easily converted to hydrated Fe₂O₃ which is rust. However, we are told that the total iron concentration remains constant. Therefore, we must have a significant conversion of Fe(OH)₃ to Fe²⁺ as per the given reaction. Moreover, the equilibrium constant for the said reaction is highly positive (since log K has a high positive value) meaning that the equilibrium favours the right hand side. Therefore, as more iron in rocks is oxidized to Fe³⁺ and consequently converts to Fe(OH)₃, the reduction of Fe(OH)₃ to Fe²⁺ takes place as shown above. Fe²⁺ cannot stay bare in the atmosphere and quickly combines with O₂ of air to form ferrous oxide (FeO), which is greyish-black in colour. This accounts for the grey and red colour of the rocks.

Since the total concentration of iron must stay constant, the Fe²⁺/Fe³⁺ oxides/hydroxides must stay in equilibrium and the two oxidation states of iron are easily interconvertible by loss or gain of an electron from the atmosphere.

The combination reaction is

2 Fe²⁺ + O₂ + 2 e^- -------> 2 FeO