Question

When acidic water from the Rio Tinto (pH ~ 2.5 – 3.5) containing dissolved iron enters the sea, the pH rises to about 7.5 and the small amounts of acid in the Rio Tinto water are completely neutralized. The Fe precipitates as ferric hydroxide as follows: Fe (H2O)6 3+ (aq) Fe(OH)3 (s) + 3 H3O+ (aq) Explain these observations in the context of LeChatelier’s Principle.

Answer 1

the equilibrium is

[Fe(H₂O)₆]³⁺_(aq) Fe(OH)_3(s) + 3 H₃O⁺_(aq)

the ph of the water is changes from 2.5 to 7.5

i..e, there is decrease of H⁺ concentration when it enters the sea.

so according to lechatliers principle,

concentration of products decreases, then the equilibrium moves from left to right.

similarly here the concentration of H⁺ ions decreases , so the equilibrium moves from left to right and leads to more amount of Fe(OH)₃. this ferric hydroxide react with an acid in water and neutralize the reaction.