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Question Video: Identifying the Device That Is Not Powered by Primary Cells Chemistry

Which of the following products is unlikely to be powered by a primary cell? [A] An electric car [B] An alarm clock [C] An SLR camera [D] A child’s toy [E] A hearing aid


Video Transcript

Which of the following products is unlikely to be powered by a primary cell? (A) An electric car, (B) an alarm clock, (C) an SLR camera, (D) a child’s toy, (E) a hearing aid.

Galvanic cells are types of electrochemical cells that produce electrical energy from chemical energy. One common example is a battery. Inside batteries, spontaneous chemical reactions occur, which produce electricity. These redox reactions cause electrons to flow through an external circuit and power our devices. There are two types of galvanic cells. Primary galvanic cells are only used one time before they must be discarded or recycled. Secondary galvanic cells can be reused. The chemical reactions inside them are reversible, and they can be recharged. We will look more closely at primary galvanic cells.

Let us look at a mercury battery as an example of a primary galvanic cell. The anode contains zinc, and this is where oxidation takes place. The cathode contains mercuric oxide, and this is where reduction takes place. Electrons will flow from the anode, or the negative electrode, through the device and back to the positive cathode. The potassium hydroxide solution between the anode and cathode functions as a salt bridge. It completes the circuit by allowing charged ions through but keeps the chemicals separated.

Let us look at the chemical reactions in the cell. We see zinc oxidized to zinc oxide in the anode half equation. And we see mercuric oxide reduced to liquid mercury in the cathode half equation. When writing the overall equation, the water electrons and hydroxide ions will cancel out and we can omit them. As zinc oxide and liquid mercury are produced, the battery becomes used up. And once there are no more reactants, it cannot be used any longer.

In secondary galvanic cells, an external current can be used to cause the redox reactions to run in reverse. This regenerates the reactants and allows the cell to be reused. However, in primary galvanic cells, the design of the cell and the chemicals involved do not allow an external current to reset the redox processes. Since the chemical reactions in primary galvanic cells are not reversible, primary galvanic cells are not rechargeable.

Therefore, if we look at the list of options from the question, we should be looking for the item that is most likely to need to be recharged often. Option (A) should stand out the most, as electric cars are famously rechargeable. The remaining answer choices all rely on primary galvanic cells. Since they do not need to be recharged, they are incorrect answers. The correct answer is an electric car.

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