Why metal(eg. iron)placed at aqueous solution will be more easily occur rusting?Is it due to the mobile ions in the aqueous solution ?Please give me an detail explanation ,thank you.
See "oxidation of iron metal" at
http://en.wikipedia.org/wiki/Rust
Rust results in the formation of iron hydroxide at the surface, under the combined action of water and oxygen. The iron oxide does not form a protective layer, but instead flakes off.
Yes, the presence of mobile ions in the aqueous solution plays a crucial role in causing the rusting of metals like iron. Rusting is an electrochemical process that involves the oxidation of iron in the presence of water and oxygen.
When a metal like iron is exposed to an aqueous solution, such as water, it forms an electrolyte solution. In this solution, water dissociates to produce hydrogen ions (H+) and hydroxide ions (OH-). These ions can participate in the electrochemical reactions that lead to rusting.
Rusting occurs when both oxygen and water are present. The oxygen molecules from the air dissolve in the water, and the dissolved oxygen combines with the metal surface to form a layer of iron oxide (rust). This process involves the transfer of electrons between the metal and the oxygen.
The mobile ions present in the aqueous solution enhance the electrochemical reactions that occur during rusting. They act as catalysts, accelerating the oxidation process. Specifically, the mobile hydroxide ions (OH-) in the solution can react with the iron metal to form iron hydroxide (Fe(OH)2). This reaction produces two free electrons:
Fe + 2OH- → Fe(OH)2 + 2e-
The hydroxide ions replenish their supply by accepting electrons from the water molecule, forming more hydroxide ions and hydrogen gas:
2H2O + 2e- → H2 + 2OH-
This helps to maintain the concentration of hydroxide ions in the solution, allowing the cycle to continue.
Furthermore, the presence of mobile ions in the solution increases the conductivity of the electrolyte. This conductivity facilitates the flow of electrons between the metal and the dissolved oxygen, making the rusting process more efficient.
In summary, the mobile ions in the aqueous solution promote the rusting of metals like iron by acting as catalysts, accelerating the electrochemical reactions. The presence of these ions facilitates the transfer of electrons, leading to the formation of iron hydroxide (Fe(OH)2) and the subsequent oxidation of the metal surface to form rust.