Reactions for the lead acid battery are:
OxidationPb(s)+HSOX4X−(l)⟶PbSOX4(s)+HX+(l)+2eX−ReductionPbOX2+HSOX4X−(l)+3HX+(l)+2eX−⟶PbSOX4(s)+2HX2OTotal reactionPb(s)+PbOX2(s)+2HSOX4X−+2HX+⟶PbSOX4+2HX2O
What will be the cell notation for this battery? My attempt:Pb(s),PbX2+(s)| HSOX4X−(l)| PbOX2(s),PbX2+(s),Pb(s)
The things that I have in mind,
- In both sides the electrode material is Pb(s)
- Separate every element that is in the same phase with a comma
- Separate every element with different phases with a single bar
- Separate the two half reactions with a single bar (as I find no salt bridge here)
I am having a tough time learning cell notation. Anyways this was a challenging one for me and I tried my best shot. I am really not sure whether everything is right or not. So please identify my mistakes and show me the right way to do it.
Answer
There are a couple of things wrong here. First off, your final reaction is unbalanced. Once you've fixed the balancing, read the other mistakes:
- The ions do not exist in the liquid state! They are solvated/hydrated by the solvent. Since the solvent is water here, we'll say that the ions are in the aqueous (aq) phase instead.
- While there is certainly no salt bridge here, there is still an electrolyte - an aqueous solution of sulphuric acid. Hence, you must mention it in your cell notation, between the anode and the cathode.
- The PbSOX4 formed at the anode is in solid state. Hence, writing it as PbX2+(s) is incorrect, as it is not dissociated into the ions PbX2+ and SOX4X2−.
With all these corrections, your final, correct cell representation should be:
Pb(s),HSOX4X−(aq) | PbSOX4(s),HX+ | HX2SOX4 (x M) | PbOX2(s),HSOX4X−(aq),HX+(aq) | PbSOX4(s)
Some websites (like KhanAcademy) and texts (NCERT 12), cite the lead-acid battery reaction as this instead:
OxidationPb(s)+SOX4X2−(aq)⟶PbSOX4(s)+2eX−ReductionPbOX2+SOX4X2−(aq)+4HX+(aq)+2eX−⟶PbSOX4(s)+2HX2O(l)Total reactionPb(s)+PbOX2(s)+2SOX4X2(aq)X−+4HX+(aq)⟶2PbSOX4(s)+2HX2O(l)
assuming the bisulphite ion to be further ionized. The cell notation in this case would then be:
Pb(s),SOX4X2−(aq) | PbSOX4(s) | HX2SOX4 (x M) | PbOX2(s),SOX4X2−(aq),HX+(aq) | PbSOX4(s)
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