[morten925]

I am not completely certain nor educated yet, unfortunately. I want to know how lead carbonate is formed in calcerous water, and I think it goes like:

Pb(s) + HCO₃^-(aq) + O₂(aq)  PbCO₃(s) + H₂O(l)

[morten925]

Well I will come up with another guess that I find more proper (I noticed mistakes in the previous guess):

2Pb(s) + O₂+2H₂CO₃ :rarrow:2PbCO₃+2H₂O

[Borek]

In general you don't need oxygen, lead is reactive enough to react directly with any acid.

[morten925]

I don't think the second guess is appropriate, hence I will come up with a third suggestion:

2Pb(s) + O₂ + 2HCO₃^-  2OH^- + 2PbCO₃

[morten925]

In general you don't need oxygen, lead is reactive enough to react directly with any acid.

Thank you Borek. I will come up with a quote from the book, Environmental Chemistry 5th edition by Colin Baird and Michael Cann:

"The contamination of water by lead is less of a problem in areas of
calcareous water, since an insoluble layer containing compounds such as
PbCO₃ forms on the surface of the lead by reaction of the metal with dissolved
oxygen and the carbonate ion, CO₃^2-, in the water".

Oxygen will oxidize Pb(0) to Pb(II). If they included any kind of reaction I would not start seeking an online forum for help

[morten925]

Now following this regulatory, I get my final suggestion to be:
2Pb(s) + O₂(aq) + 2CO₃^2-(aq) + 2H₂O(l)  2PbCO₃(s) + 4OH^-(aq),

which should be balanced by charge, redox and mass

Can anyone agree on this?

[AWK]

Both reactions of Morten925 can be accepted and in fact are the same. In the first reaction the spectator cation exists (may be H+). Presence of oxygen accelerate reaction which is very slow. Usually a basic lead carbonate is formed but writing reaction with simple lead carbonate is easier. Some PbCO₃ is formed below 30 C.

[morten925]

Thank you very much AWK Yes, it is the assumption that water has not yet reacted with the carbonate anion.

[Borek]

What I was aiming at is the fact Pb reacts with acids in a direct reaction

Pb + 2H⁺ Pb²⁺ + H₂

so it can be dissolved in an acid without a need for the oxygen presence. However, for the above reaction to proceed, you need pH below 2.1 (easy to calculate from the standard potentials). Carbonic acid solutions never get that acidic, hence need for an additional oxidizing agent.

And every reaction that contains H⁺ and water or OH^- and water can be written in a different way, as H⁺, OH^- and H₂O are in the equilibrium in the solution. For this reason all three reactions you wrote are equivalent.