Video: Identifying the Role of Mg(s) in the Reaction of HCl(aq) and Mg(s) in a Set of Roles

In the given reaction, which term best describes the role of Mg (s)? 2HCl (aq) + Mg (s) ⟶ MgCl₂ (aq) + H₂ (g). [A] Brønsted acid [B] oxidizing agent [C] reducing agent [D] precipitate [E] cathode.


Video Transcript

In the given reaction, which term best describes the role of Mg solid? 2HCl aqueous plus Mg solid react to form MgCl₂ aqueous plus H₂ gas. A) Brønsted acid, B) oxidizing agent, C) reducing agent, D) precipitate, or E) cathode.

Mg is the symbol for magnesium, an element to the far left of the periodic table in group two. Here, we can see it reacting with HCl, otherwise known as hydrochloric acid, reacting to form the salt, magnesium chloride, and hydrogen gas. This is an example of the classic metal plus acid reaction. Now that we’ve looked to the reaction itself, let’s have a look at the options. A Brønsted acid is a substance that donates hydrogen ions to another substance. An oxidizing agent is a substance that oxidizes another substance. When an oxidizing agent oxidizes another substance, it will in turn be reduced, while a reducing agent is the opposite. A reducing agent reduces another substance and is, in turn, oxidized.

We commonly see oxidation as a loss of electrons or an increase in oxidation number, while we generally see reduction as the gaining of electrons or a decrease in the oxidation number. Meanwhile, a precipitate is a solid produced from a solution. And lastly, cathode is the name for the electrode in an electrochemical cell, where reduction takes place. So as a quick recap, we have the options of Brønsted acid like hydrochloric acid, sulfuric acid, nitric acid, and so forth of oxidizing agent or reducing agent. And here, I’ve got the example of iodide ions reacting with bromine providing electrons, turning them into bromide ions and iodine.

The iodide ions are giving up their electrons to the bromine. So they’re reducing, bromine, they are reducing agents, and they are oxidized themselves. And at the same time, the bromine is accepting those electrons from iodine becoming reduced itself and causing the iodide to be oxidized to iodine. And then we have precipitate, for example, the reaction between sodium carbonate and calcium chloride, producing a precipitate of calcium carbonate and soluble sodium chloride. And finally, cathode, which is where we’d see reduction take place in electrochemical cell, for instance, in the electrolysis of water where hydrogen ions would be reduced to hydrogen.

We can easily dismiss Brønsted acid because magnesium has no hydrogen to donate in the first place. I’ll deal with oxidizing and reducing agent together. To do that, we need to figure out the oxidation number for magnesium in the reactants and the products. The oxidation state for an element not bonded to any other element is defined as zero. But what about magnesium in MgCl₂? The chloride ion has a single negative charge. And we have two of them. So the charge of the magnesium ion in magnesium chloride is two plus. For a simple ion, the oxidation number is equal to the magnitude of its charge. So a charge of two plus means an oxidation number of plus two.

This means that because of this reaction, the oxidation number of our magnesium increases. This is an oxidation. If magnesium is being oxidized, then something else must be reduced. So magnesium is acting as a reducing agent. Now, it’s not actually necessary, but I can show you what’s being reduced. The hydrogen in hydrochloric acid has an oxidation number of plus one. But in hydrogen gas, it has an oxidation number of zero. So it’s the hydrogen in hydrochloric acid that’s being reduced in this process. But before we draw a line under reducing agent, let’s just check the last two options.

Now, let’s consider precipitate. Firstly, magnesium is a reactant, not a product. When we talk about producing a precipitate, we’re talking about making a product. As we can see from the example of sodium carbonate and calcium chloride, we produced a solid product. Because it’s being produced all the way through the solution, we get that fine powder characteristic of a precipitate. So since magnesium here is not being produced from a solution phase reaction and it’s not a product, we can’t consider it a precipitate.

Cathode is a little bit more subtle. Ideally, electrodes shouldn’t participate directly in the reaction. Of course, in some processes, they do get worn away. But it’s not a desirable process. In this process, magnesium is a major feature in the reaction. So it’s unlikely that magnesium would be used as the electrode material. It would, of course, be possible to separate this reaction into two half cells, which don’t directly chemically interact. But that’s not the equation we’ve been given. So we can be confident that the term that best describes the role of magnesium in this particular reaction is reducing agent.

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