Question Video: Explaining Why No Salt Bridge, Porous Separator, Or an Equivalent Is Necessary for Lead–Acid Batteries Chemistry

Why is no salt bridge, porous separator, or an equivalent necessary for lead–acid batteries?


Video Transcript

Why is no salt bridge, porous separator, or an equivalent necessary for lead–acid batteries? (A) The battery casing acts as a salt bridge, completing the circuit. (B) Secondary galvanic cells do not need salt bridges. (C) Both half-cells use the same electrolyte. (D) A porous separator would prevent the recharging of the battery. Or (E) the lead atoms and ions are too large to desorb from the electrodes.

Our question is asking about the structure of lead–acid batteries. Unlike primary galvanic cells or other secondary galvanic cells, lead–acid batteries do not require salt bridges or porous separators. To answer this question, let’s touch briefly on how a lead–acid battery functions and then examine each available answer choice and determine the best option.

A lead–acid battery is a secondary galvanic cell that undergoes two different phases: a discharging phase and a charging phase. The discharge phase is when a galvanic cell or battery powers an external device as it generates electrons through a redox reaction, such as when a starter motor or electrical system is used with a lead–acid battery.

The recharge phase is when an external current is applied to reverse discharging and convert electrical energy into chemical energy, such as when an alternator is used to recharge a car battery. Answer choice (A) states that the battery casing itself acts as a salt bridge that completes the circuit. However, since these casings are made from synthetic insulating materials, such as rubber or plastic, they are inherently unable to conduct electricity and complete a circuit. This means that answer choice (A) is incorrect.

Option (B) states that secondary galvanic cells do not need salt bridges. Many secondary galvanic cells will have a different electrolyte for different electrodes, in which case, a salt bridge is needed. So, since we cannot state that no secondary galvanic cells need a salt bridge, option (B) is incorrect.

Answer (C) states that both half-cells use the same electrolyte. This is true for some secondary galvanic cells, meaning that a salt bridge would be unnecessary in this case. This seems like a potentially correct answer choice. However, we will need to confirm this by looking at the remaining answer choices.

The porous separator that answer choice (D) mentions allows ions to pass through, completing a circuit. While porous separators are frequently used in electrochemical cells, these structures make recharging possible, which is not characteristic of a lead–acid battery. We can see in the diagram that there are no separators between the anode and cathode in this diagram of a lead–acid battery. So, answer choice (D) is incorrect.

Answer choice (E) states that lead atoms and ions are too large for them to desorb from the electrodes. This is untrue, because ion desorption is what allows current to flow and the battery cell to function. Since we know that lead–acid batteries do, in fact, function, then we can say that this answer choice is incorrect. Therefore, salt bridges and porous separators are not necessary because lead–acid batteries use the same electrolyte for each electrode.

The correct answer is (C). Both half-cells use the same electrolyte.

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