Video: Identifying Which Substances Can Be Electrolyzed

Which of the following substances cannot be readily electrolyzed? [A] AlCl₃ (l) [B] NaCl (s) [C] BaBr₂ (aq) [D] MgF₂ (l) [E] LiCl (aq)

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Video Transcript

Which of the following substances cannot be readily electrolyzed? A) AlCl₃ liquid, B) NaCl solid, C) BaBr₂ aqueous D) MgF₂ liquid, or E) LiCl aqueous.

Electrolysis is the chemical decomposition of a substance through the passage of an electric current. “Electro” refers to electric and “lysis” to breakdown. This is what a basic electrochemical setup might look like. What the question is asking is what would happen for each of these substances if they are placed in the electrochemical setup. What we’re looking for is four samples that can be electrolyzed and one that can’t.

One of the basic principles of electrolysis is that we need a complete circuit. This means that we need something that will respond to the positive and negative charges of the electrode. This means we need some form of charged particle that is mobile.

So a simple test we can apply to all the candidates is to ask whether there are mobile ions present and what they are. All the substances are ionic. So they contain ions. But their state will dictate whether they are mobile or not.

AlCl₃ liquid is molten aluminum chloride. So the ions aluminum 3+ and chloride are mobile. If we did electrolyze aluminum chloride in its molten state, our products will be aluminum in its solid form and chlorine gas. This is, therefore, not the correct answer.

What about NaCl solid? NaCl or sodium chloride when solid does not contain mobile ions. The Na+ and Cl- ions of which it is composed are locked in place. Therefore, sodium chloride solid cannot be electrolyzed. This is, therefore, the correct answer. However, if it were sodium chloride liquid or sodium chloride aqueous, it would be electrolyzable.

Just to be safe, let’s have a look at options C, D, and E. In solution, barium bromide, BaBr₂, forms mobile ions Ba2+ and Br-. In solution, these ions are completely mobile and can be electrolyzed. The products from the electrolysis of barium are hydrogen gas, bromine gas, and barium hydroxide. Barium solid is not formed because barium is more reactive than hydrogen. So it is H+ that is reduced to the electrode and not Ba2+. Therefore, BaBr₂ is not the correct answer.

Molten magnesium fluoride MgF₂ will have mobile ions Mg2+ and F-. These mobile ions will be electrolyzed to form magnesium solid and fluorine gas. Therefore, magnesium fluoride liquid is not a correct answer.

Finally, we come to lithium chloride aqueous. The mobile ions in solution will be Li+ and Cl-. And as with barium, because we’re dealing with a more active metal than hydrogen we produce hydrogen gas, chlorine gas, and lithium hydroxide solution. So this too is not the correct answer.

So of the five substances given, the only one that cannot be readily electrolyzed is sodium chloride solid.

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