Iron is plated from a bath with 0.50 mol/L iron(II) ions…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Iron is plated from a bath with iron(II) ions. For hydrogen on iron the overpotential is at the working current. The standard potential of is . Take . (a) At what potential does iron deposit? (b) At pH , at what potential does hydrogen form? Which goes first? (c) Below what pH does hydrogen go first? (d) If the overpotential were zero, as on platinum, which would go first at pH ?
Need a reminder? The key ideaShow
At the cathode the ion with the highest deposition potential goes first, and at the anode the lowest goes first. . Hydrogen evolves at . Current efficiency is the mass deposited divided by the mass expected from , times 100. Energy is , and .
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Iron. .
Step 2. Hydrogen at pH 2.0. . Iron has the higher potential, , so iron goes first.
Step 3. The limit. Set . Then . Below pH the bath is too acid, and hydrogen goes first.
Step 4. No overpotential. With , , which is well above . Hydrogen would go first, and no iron could be plated.
Answer: iron at , hydrogen at so iron goes first, and the limit is pH .
Check. The gap in Step 2 is . That is a small margin, so the bath must be kept above the limiting pH.
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