TBR exam 4, question 23

[Pamplemousse123]

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The question is:

The aluminum plate in the aluminum-air battery must be changed periodically, because:

A.aluminum hydroxide buildup hinders the conductivity of the current that powers the car.
B.all of the aluminum in the battery eventually oxidizes away and must be replenished.

Just to give you background information, the passage states that the following half reactions occuR:
o2 +2h20 + 4e- --> 4oh-
al --> al+3 + 3e-

I chose A because I remember reading that batteries stop working when the reaction approaches equilibrium and the best answer along those lines is A. Not all the aluminum should be oxidized as there will always be an equilibrium and no reaction goes to completion. yes, the equilibrium can lie on the side of products, but still (b) seemed to extreme for me. The correct answer is B. Can someone explain why?

 

[Rabolisk]

The better answer is B, because you wouldn't necessarily change the aluminum plate because of aluminum hydroxide buildup. On the other hand, it is reasonable to believe that as all the aluminum is depleted, you would have to use new aluminum plate. Logically, it makes sense that as you run out of Al, you put in more Al. As far as equilibrium goes, an Al-air battery presumably has access to unlimited quantities of oxygen, so aluminum is the limiting reactant.