MCAT General Chemistry Question 180: Answer and Explanation

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Question: 180

15. Which of the following best describes why over-charging a Ni–Cd battery is not detrimental?

  • A. The energy density of a Ni–Cd battery is high, so it can store more charge than other batteries per its mass.
  • B. The electrodes of a Ni–Cd battery can discharge through the circuit when they are fully charged.
  • C. The Ni–Cd battery will stop accepting electrons from an outside source when its electrodes are recharged.
  • D. Ni–Cd batteries have a high surge current and can dissipate the overcharge before damage can occur to electrodes.

Correct Answer: C

Explanation:

During the recharge cycle, Ni-Cd cells will accept current from an outside source until the Cd and NiO(OH) electrodes are pure; at this point, the reaction will stop because Cd(OH)2 runs out and no more electrons can be accepted. Choices (A) and (B) are both true statements, but they fail to explain why overcharging the battery (continuing to try to run current into the battery even when the electrodes are reverted to their original state) is not a problem with Ni-Cd batteries. Finally, surge current refers to the initial burst of current seen in some batteries; once charged, the surge current will not increase even if the power source continues to be run because no additional charge will be stored on the electrodes, eliminating choice (D).

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