electrolytic cell Q

[Deepa100]

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Passage Info:
Lead-acid cells contain a number of lead (II) sulfate covered grids or electrodes immersed in an aqueous solution of sulfuric acid. These grids have a large surface area so that a large current can be generated rather quickly, i.e., when the cells discharge and the car engine is started.

When the cell is charged, lead (II) sulfate undergoes the following two reactions: it is reduced to form lead at the cathode, and it is oxidized to form lead (IV) oxide at the anode. When the cell discharges, the reverse reactions occur; that is, lead is oxidized at the anode to form lead (II) sulfate (Reaction 1) and lead (IV) oxide is reduced at the cathode to form lead (II) sulfate (Reaction 2).
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Q: What happens if you add PbSO4 to the cell immediately after it is charged?
Ans:The discharge reactions for the lead-acid cell are shown in Reactions 1 and 2. After charging, both of these reactions favor the reactants; that is, lead is deposited at the cathode, and lead(IV) oxide is deposited at the anode. When the reaction moves in the forward direction, electrons are generated and the cell gives out a certain voltage. So, what would be the effect on this forward reaction and, hence, the output voltage if lead sulfate was added to the cell after charging? Since lead sulfate is a solid, it is not appreciably soluble in the electrolyte. As a result, the concentration of lead sulfate—and the position of the equilibrium—does not change. Since the value of the cell potential is affected by the equilibrium constant, it follows that no change in the position of the equilibrium results in no change in cell potential—choice A is the correct response.
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My Q: I don't quite understand the set up of the electrolytic cell here. Is sulfuric acid just providing the ions here? If so, why would adding PbSO4 not have any effect on the cell voltage?

 

[YA7ES]

that passage confused the *BEEP* out of me. that's from aamc 10 right?.. or 9?

 

[Deepa100]

No, it's actually from a kaplan test but I am sure it is patterned after AAMC 10 passage you are referring to.

 

[silverlion]

If this is how the real deal is...I'm gonna have to prepare for 6 more months before I give the exam instead of taking it in 2 weeks from now

 

[Phantastic]

Okay, so, don't worry so much about the apparatus here--it's an electrolytic cell (nonspontaenous), that is discharged (spontaneously--hence, the reverse reaction). Other than that, the underlying science concept being tested is solubility and the common ion effect.

Lead salts are insoluble, except for the Nitrate (NO3, 2-), Perchlorate (ClO4, 1-), and Acetate (C2H3O2, 1-) salts. Another way to express this is that Lead salts usually have very low K(sp) values. Solubility of common ions is dependent on the two solid's K(sp).

You didn't mention that the electrolyte in the cell was H2SO4, but if this is the case, HSO4, 1- is a weak acid, and likely has a larger K value than Lead Sulfate. Therefore, lead sulfate remains undissociated, and does not affect the equilibrium of the battery.

Hope that helps.