Lesson Video: Displacement Reactions Chemistry

In this video, we will learn how to describe displacement reactions, exploring their relation to the reactivity series.

17:29

Video Transcript

Displacement Reactions

Chemical reactions occur when bonds are formed or broken between atoms. When a reaction occurs where one atom takes the place of another in a compound or solution, we call that a displacement reaction. In this lesson, we will learn how to describe displacement reactions and explore their relation to the reactivity series.

In chemistry, to displace something means to replace that thing in a compound or a solution. So, a displacement reaction is a reaction where one atom or ion replaces another, usually a metal replacing another metal. We also sometimes call these reactions substitution reactions or replacement reactions. One example of a displacement reaction is the reaction of magnesium dipped in a zinc chloride solution. The chemical equation looks like this: magnesium plus zinc chloride produces zinc plus magnesium chloride.

Because one element displaces another in the chloride solution, we call this reaction a single displacement reaction. Single displacement reactions have the general form A plus BC produces B plus AC. In reactions like these, the pure elemental metal and the metal in solution or in the compound trade places. There are double displacement reactions where both metals are involved in a compound or solution. We will learn about double displacement reactions later on in the video. In this reaction, we start with a strip of magnesium dipped in a solution with zinc ions and chloride ions. At the end of the reaction, magnesium ions have entered the solution. In addition, the zinc that was previously in solution has coated on the outside of the magnesium strip.

For a single displacement reaction like this, there are multiple possible signs that a reaction is taking place. The most obvious sign that we can see in the example here is the coating of the metal. In this reaction, zinc will plate onto the outside of the magnesium strip. That coating will be visible as a different color. We may also see bubbles forming around the metal strip. We could also measure a temperature change. For reactions like this, the temperature change is usually an increase in temperature.

Lastly, we might also see a color change of the aqueous solution. While many aqueous solutions are colorless, if our reaction starts or ends with a colored aqueous solution, the change of the metal ion in the solution will likely change the color of the solution. We’ve learned a bit about what happens when you dip magnesium in zinc chloride. But what happens when you try the reverse reaction, dipping zinc in a solution of magnesium chloride? Well, in that case, no reaction will occur. We will not see any of the signs that a reaction is taking place. Why does magnesium react in a solution of zinc, but not the other way around? Well, it has something to do with the reactivities of these two metals.

This is the reactivity series. It’s a list of metals in order, from most reactive at the top to least reactive at the bottom. Magnesium is higher up on the reactivity series than zinc, meaning it’s more reactive. More reactive metals can displace less reactive metals. When we dip magnesium in a zinc solution, the more reactive solid magnesium displaces the zinc entering solution. The zinc is displaced and then plates on the strip as a solid. For any pair of metals, the more reactive one will be able to displace the less reactive one from solution. This means that the more reactive metals tend to be in solution, whether that means they start as a solid and enter solution or they start in solution and remain in solution.

At the other end of the reactivity series, the less reactive metals tend toward the solid state. That could mean starting as a solid and remaining a solid or starting as an ion in solution and being displaced to become a solid. This means that a reaction will only occur if the more reactive metal begins as a solid. From there, it will be able to displace the less reactive metal from solution. If the less reactive metal begins as the solid, then no displacement and no reaction will occur.

Interestingly, there are two nonmetals included in the reactivity series. Why are these two nonmetals included? Let’s start with hydrogen. Hydrogen is included in the reactivity series because the metals above it will displace a hydrogen in a single displacement reaction with an acid. For example, combining iron and hydrochloric acid produces iron chloride and hydrogen gas. If we combine a less reactive metal, like gold, with an acid, no reaction will occur. So, while hydrogen itself isn’t a metal, it’s still a useful bookmark on the reactivity series, telling us which metals react with acid, the ones above it, and which metals do not react with acid, the ones below it.

Carbon serves a similar purpose on the reactivity series. Its placement tells us which metals can and can’t be extracted using carbon. Due to construction and manufacturing, there’s a strong worldwide need for pure metals. We can take metals from the ground, but they aren’t usually found in their pure form. They’re mixed with other minerals in deposits called ores. To extract the pure metal from the ore, we need to process it.

One way to isolate the pure metal from a compound is by heating it alongside carbon. Chemically, that looks like displacing the metal with carbon in a single displacement reaction. For example, we can extract zinc from zinc oxide with the following reaction: zinc oxide plus carbon produces zinc plus carbon monoxide. However, this kind of reaction will only occur if carbon is more reactive than the metal it’s displacing. For the metals higher up on the reactivity series, we have to extract them using electrolysis, a very energy-intensive process.

The chemical equation for electrolysis involves the metal oxide breaking down into the metal and oxygen gas. This type of reaction where a compound breaks down into its constituent parts is called a decomposition reaction. However, in this video, we’re more concerned with the reaction that uses carbon to extract the metal, a single displacement reaction. Carbon’s presence on the reactivity series tells us which metals can and can’t be extracted using this single displacement reaction involving carbon.

But single displacement reactions are not the only kind of displacement reaction. A double displacement reaction is a reaction between two compounds where the ions trade places to form new products. In a generalized form, this looks like AB plus CD produces AD plus BC, where each reactant breaks up. The ions find new partners and then recombine to form the product. One example of a double displacement reaction is the reaction of silver nitrate and sodium chloride to form silver chloride and sodium nitrate.

In this reaction, silver and sodium swap places. The two cations are now paired with the opposite anion from the one they started with resulting in new products. For double displacement reactions, the two reactants are typically aqueous solutions. And the most common way that we can tell that a reaction has occurred is if a solid precipitate forms. A precipitate means a solid product that forms out of solution. It is worth noting that there some double displacement reactions that produce a gas or water as the product.

When we first combine the two reactants, for a very brief moment, the four ions exist together in solution. If a reaction is to occur and a precipitate is to form, it will be because of the strong attraction between two of the ions in the solution. In this reaction, the silver ion and the chloride ion have a stronger attraction to one another than either one does to the surrounding water molecules. As a result, one of the products is silver chloride, visible as a white milky cloud. The sodium nitrate remains as an aqueous solution.

In fact, we commonly write double displacement reactions, including the state symbols. That way, we can readily identify the solid precipitate that forms. If we don’t know the products of a double displacement reaction, we can use solubility rules or other references like tables or charts to know what precipitate will form. The product that is soluble in water will end up as an aqueous solution as the water molecules will be able to pull apart the two ions. The product that is insoluble in water will be the solid precipitate.

If we looked up these two compounds on a solubility chart, it would tell us that sodium nitrate is soluble and silver chloride is insoluble. Solubility rules would give us the same answer. Essentially, all nitrates are soluble. And while many chlorides are soluble, silver chloride as an exception is insoluble. The insoluble silver chloride is the precipitate for this reaction. Silver chloride forms a white milky cloud. Other solid precipitates may have different colors or different consistencies, but they’re almost always visible to the naked eye.

Overall, for double displacement reactions, the two compounds will swap partners to create two new compounds. We can use solubility rules or references to determine which of the new arrangement of ions will be insoluble as a solid precipitate and which will be soluble as an aqueous solution. Now that we’ve learned a bit about displacement reactions, let’s do some practice problems to apply what we’ve learned.

Which of the following metals would react with acid and has an oxide that can be reduced by carbon? (A) Magnesium, (B) iron, (C) copper, (D) silver, or (E) platinum.

This question is asking us to find the metal that both would react with acid and has an oxide that can be reduced by carbon. Where can we find this information? Well, to answer this question, we need to look at the reactivity series. The reactivity series is a list of metals with the most reactive metals at the top and the least reactive metals at the bottom. The reactivity series tells us which metals are more or less reactive than other metals. And we should also know that more reactive metals can displace less reactive metals from a compound or solution. Displace simply means to replace in the compound or solution.

Let’s take a look at the reactions described in the question and see what information we can glean about them from the reactivity series. One example of a reaction between a metal and an acid is the reaction of iron and hydrochloric acid to produce iron chloride and hydrogen gas. If we look at how the chloride compound changes from the beginning to the end of the reaction, we can see that the hydrogen is displaced by the metal ion to form iron chloride. In order for a medal to react with an acid, the metal must displace the hydrogen from the acid.

Following the rule that more reactive metals can displace less reactive metals, it follows that the metals that can displace hydrogen are the ones above it on the reactivity series. Only these metals will be able to displace hydrogen and react with acids. Copper, silver, and platinum are all below hydrogen on the reactivity series, so they will not be able to displace hydrogen to react with acid.

Now let’s take a look at the other reaction described in the question. One example of an oxide being reduced by carbon is the combination of zinc oxide and carbon to form zinc and carbon monoxide. Being reduced is the opposite of being oxidized. So, in this reaction, zinc oxide being reduced simply means that it loses its oxygen, although in other reactions without oxygen, being reduced simply means gaining electrons from the beginning to the end of the reaction. If we look at what happens to the oxide in this reaction, we can see that the carbon displaces the zinc from the oxide to form carbon monoxide. If we again follow our rule that more reactive metals can displace less reactive metals, we can look at the reactivity series to see which metals carbon will be able to displace.

Carbon can displace metals that are less reactive than it. So, it’s the metals below carbon on the reactivity series that can be reduced by carbon. Carbon is not able to displace the more reactive metals above it on the reactivity series. So, those metal oxides will not be able to be reduced by carbon. Now that we know when these reactions can and can’t occur, we can ask this question in a simpler way. We can ask, which of the following metals is more reactive than hydrogen and is less reactive than carbon? Magnesium is more reactive than carbon, so the second half of the question is not true for magnesium.

Iron is the only one of the five choices in the sweet spot, more reactive than hydrogen and less reactive than carbon. Choice (B) iron is the correct answer. Both of the reactions we’ve looked at here, the reaction between a metal and an acid and a metal oxide and carbon, belonged to a group of reactions, called single displacement reactions. Single displacement reactions occur when a lone element, like iron, swaps places with one of the elements from a compound or solution, like the hydrogen from hydrogen chloride. The second reaction here where carbon displaces a metal from a metal oxide is a commonly used method to isolate pure metals. In the example, we could use this process to isolate pure zinc.

As we determined while answering the question, a similar reaction can reduce iron three oxide to extract pure iron. For metals too reactive to be reduced by carbon, electrolysis must be used to extract them instead. So, which of the following metals would react with acid and has an oxide that can be reduced by carbon? That’s choice (B) iron.

Now that we’ve done some practice, let’s review the key points of the video. A displacement reaction is a reaction where one element takes the place of another in a compound or solution. The reactivity series is a list of metals in order of reactivity from most reactive at the top to least reactive at the bottom. Remember that the nonmetals, carbon and hydrogen, are included in the reactivity series for comparison. The reactivity series is relevant to displacement reactions specifically because more reactive metals can displace less reactive metals during displacement reactions.

A single displacement reaction is when an element and an ion in a compound or solution trade places. It has the general form A plus BC produces B plus AC. Signs that a single displacement reaction has occurred include a metal coating forming on the metal, bubbles forming on the metal, a temperature change of the solution, and a possible color change of the solution.

Another type of displacement reaction, a double displacement reaction, involves two pairs of ions in compounds or often in aqueous solutions, swapping partners to create two new products. Often, the products of a double displacement reaction are an insoluble solid precipitate and an aqueous solution. However, it’s worth mentioning that there are some double displacement reactions that produce water or a gas as a product instead.

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