In a concentration cell corrosion, what gas is produced?

• A. hydrogen
• B. oxygen
• C. carbon dioxide
• D. ammonia

Answer: (A)

Hydrogen gas is produced.

In a concentration cell, two identical electrodes are in solutions with different ion concentrations of the same electrolyte. The electrode in the more dilute solution becomes the anode and dissolves, while the electrode in the more concentrated solution becomes the cathode where reduction occurs. If the electrolyte contains hydrogen ions (acidic conditions), the common cathodic reaction is the reduction of H+ to H2: 2H+ + 2e- → H2. As a result, hydrogen gas bubbles form at the cathode. Oxygen gas would require reduction of dissolved oxygen or water oxidation under different conditions, which isn’t the typical outcome for this setup. Carbon dioxide and ammonia gases aren’t produced from these electrochemical half-reactions in a standard concentration-cell corrosion scenario.