Princeton Review Hyperlearning: Dry Cell Q

[4563728]

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In passage 91 for Gen Chem, number 2: "The pH of a fully charged alkaline dry cell must be..."
a. less than the pH of a drained one
b. more than the pH of a drained one
c. the same as the pH of a drained one
d. acidic

the overall equation is MnO2 (s) +4H+ +Zn(s) --> Mn2+(aq) +2H2O + Zn2+(aq)

In the passage it says: "All alkaline dry cells are powered by a Zn/MnO2 reaction which occurs under basic conditions."

The correct answer was A but doesn't that conflict with the information in the passage which says it it occurs under basic conditions?

 

[inasensegone]

In passage 91 for Gen Chem, number 2: "The pH of a fully charged alkaline dry cell must be..."
a. less than the pH of a drained one
b. more than the pH of a drained one
c. the same as the pH of a drained one
d. acidic

the overall equation is MnO2 (s) +4H+ +Zn(s) --> Mn2+(aq) +2H2O + Zn2+(aq)

In the passage it says: "All alkaline dry cells are powered by a Zn/MnO2 reaction which occurs under basic conditions."

The correct answer was A but doesn't that conflict with the information in the passage which says it it occurs under basic conditions?

No... just because something occurs under basic conditions, doesn't mean that it doesn't have any [H3O+]. The pH of A can still be lower than the pH of B, while both are still basic. For example, a pH of 9 vs a pH of 11. Both are basic, but the pH of 9 is lower than the pH of 11.

If you look at the balanced reaction, protons are part of the reactants. Once these become products (which happens as the cell is being used), Manganese is reduced from +4 to +2 and Oxygen accepts the protons and there are no free protons on the product side. In the end, the result is that the cell has a lower [H3O+]... AKA a higher pH than the fully charged cell.

Again, the key point is that just because something has a lower pH than something else, doesn't automatically make it acidic. It just has a lower pH.

 

[4563728]

That's a great way to explain it. Thanks so much, I get it now!!