Lesson Video: Reactivity Series Chemistry

In this video, we will learn how to use the reactions with water, acids, oxygen, hydrogen, and metal oxides, in order to determine the metals’ order of reactivity.


Video Transcript

The Reactivity Series

Some metals, like sodium, react simply by being placed in water. Other metals, like copper, resist reacting to even corrosive acids. The reactivity series is a list of metals that gives us their relative reactivity in order. In this lesson, we will learn how to use and observe the reactions of metals to determine the order of their reactivity.

The reactivity series looks like this. It’s a top-to-bottom list of various metals. Sometimes, we will see more or fewer metals included in the list, but they will always appear in the same relative order. The elements at the top of the list, like potassium and sodium, are more reactive. In their elemental form, they will more easily combine with other substances to form new compounds. Conversely, the metals at the bottom of the list, like gold and platinum, will tend to remain in their solid elemental form, even when added to solutions of other reactive compounds. The list gradually becomes less and less reactive from top to bottom, so metals in the middle of the table will react with some substances and not react with others.

We can group the metals together based on the substances that they react with. The most reactive metals react with water. For example, sodium displaces a hydrogen from water to form sodium hydroxide and hydrogen gas. The next most reactive group of metals on the list don’t react with water, but they do react with acids, even in dilute form. Note that the metals that react with water also react with acids. One example of a reaction between a metal and an acid is the reaction between iron and hydrochloric acid. Iron displaces the hydrogen from the hydrochloric acid, forming iron chloride and hydrogen gas.

Going farther down the reactivity series, we can find metals that react with oxygen. These metals will combine with oxygen to form oxides, such as when copper combines with oxygen to form copper oxide. The lowest metals on the reactivity series are described as highly unreactive because they do not react with any of the above named substances.

It is also worth noting that reactivity and stability are inversely related. The most reactive metals, toward the top, are the least stable. And the least reactive metal, toward the bottom, are the most stable. Unreactive metals, like silver, require a lot of energy to form compounds. Therefore, they are much more likely to remain in their elemental states. On the other hand, a more reactive metal, like sodium, easily forms different compounds, making it very likely to change and very unstable. We might also notice two nonmetals included in this list of metals, carbon and hydrogen. Why are these two nonmetals included?

Well, hydrogen is included because when we react a metal with water or acid, it involves displacing a hydrogen. The hydrogen in the water or the acid is replaced by the metal and forms hydrogen gas as a product. The hydrogen from the metal is particularly easy to displace, so all metals above hydrogen on the reactivity series will react with acids by displacing that hydrogen. So, hydrogen is included in the series as a reference point separating the metals above it that will react with acids from the metals below it that will not. While the reaction between water and a metal also displaces a hydrogen, the hydrogen of water is a bit harder to displace. It requires more energy and a metal much more reactive than hydrogen.

The other nonmetal carbon is also included as a reference point. There are some specialized reactions that use carbon to extract pure metals from compounds by displacing it from the compound. While the specifics of these reactions are beyond the scope of this video, we sometimes include carbon in the reactivity series to know which metals, the ones below carbon, can be extracted in this way.

We’ve learned a bit about the reactivity series as it relates to reactions between metals and water, acids, and oxygen. Now, let’s take a look at reactions between two different metals.

Another way the reactivity series is informative is to test the reactivity of two metals relative to one another. Let’s consider the relative reactivity of magnesium and zinc. Based on the reactivity series, magnesium is higher up, so it is more reactive, while zinc is lower on the reactivity series, making it less reactive.

But what does this difference in reactivity look like in reactions we carry out in the laboratory? We can observe this difference in reactivity if we dip the solid form of one of the metals into an aqueous solution that contains ions of the other metal. We have already established that when a metal reacts with acid, it will displace hydrogen to form new products. Well, a similar thing happens here. Since magnesium is more reactive than zinc, it will displace the zinc ions in solution, forming a magnesium chloride solution as solid zinc forms. What does this look like in the laboratory? Well, the magnesium ions will displace the zinc ions, meaning that the zinc chloride solution will eventually become a solution of magnesium chloride. When the zinc ions are displaced from the solution, they will form solid zinc and plate on the surface of the magnesium strip.

Because one metal displaces another, these types of reactions are known as single displacement reactions. We also sometimes call them single replacement reactions. If we look at the opposite reaction, where we dip zinc metal into magnesium chloride solution, no reaction will occur, as abbreviated by NR. The less reactive zinc will be unable to displace the magnesium ions from the solution.

We can extend this relationship to compare any two metals on the reactivity series. In reactions like this, the more reactive metal, toward the top of the reactivity series, will tend toward a solution. Meaning if it begins in solution, it will remain in solution when the less reactive metal is introduced. If it begins as the solid metal, it will displace the existing metal from solution and enter solution itself. Conversely, the metal lower on the reactivity series will tend toward the solid state. If it starts as a solid, it will remain the solid. If it starts in solution, it will be displaced and plate as a solid on the existing metal.

All of this means that a reaction can only occur if the more reactive metal starts as the solid. For some questions, we may not know the identity or reactivity of the metals involved, so we can use the presence or absence of a reaction to determine which metal is more reactive. Signs that a reaction is indeed occurring includes seeing the plated metal in the form of a different colored coating on the original metal. You may also see bubbles forming on the surface of the metal. And while many aqueous solutions are colorless, if one of the solutions in the reaction does have a color like, say, the blue color of copper chloride, you may see the solution change color as well.

Occasionally, we will not just test one metal against another, but we’ll test a series of metals to determine an order of reactivity. In addition to testing each pair of metals to determine the order of reactivity, we can also notice that the more reactive metals will react more vigorously in the form of a thicker coating or more bubbles. Now that we’ve looked at single displacement reactions involving two metals, let’s look at one more type of reaction as well.

Another type of reaction that shows off the relative reactivity of different metals are the reactions between metals and oxygen, for example, the reaction between iron and oxygen to form iron oxide. Iron oxide is also known as rust. It is the product of a slow and gradual reaction between a piece of iron and the air around it. We can produce iron oxide with the same reaction more quickly by burning steel wool, which, despite its name, is actually mostly iron.

If we burn magnesium in the presence of oxygen in the air, we will see a bright white light like a sustained camera flash as magnesium oxide is produced. Magnesium is a more reactive metal than iron, so in both cases — the case where we ignite the metal and the case where we expose the metal to air — the magnesium oxide will be produced more quickly than the iron oxide. A less reactive metal, like copper, does not readily form an oxide. In order to produce copper oxide, the copper needs to be heated to between 300 and 800 degrees Celsius. A lot of energy is required to take this unreactive metal and form a compound from it.

As we can see here, more reactive metals will easily combine to form new compounds, while less reactive metals require more energy to form compounds. Now that we’ve learned about the reactivity series, let’s do some practice problems.

The following is a reactivity series for a number of metals, potassium, magnesium, zinc, tin, copper. Part (a) which metal will react the most vigorously with hydrochloric acid?

This question gives us a reactivity series, or a list of metals in order of reactivity. A reactivity series goes from most reactive to least reactive. While many reactivity series are oriented vertically, they can be oriented horizontally as well, in this case, going from most reactive on the left to least reactive on the right. If it were oriented top to bottom, the most reactive elements would be at the top and the least reactive at the bottom.

Part (a) of this question is asking which metal will react the most vigorously with hydrochloric acid. In other words, which metal will have the quickest, most intense reaction? That is going to be the most reactive metal. Less reactive metals will react less vigorously or not at all. In this case, the most reactive metal furthest to the left is potassium. We can observe the reactivity of potassium when it’s dipped in hydrochloric acid because it produces lots of bubbles very quickly when hydrogen gas is released. So, the metal that reacts most vigorously with hydrochloric acid is potassium.

Part (b) asks, which metal will not react at all with water or steam?

The key words here are “not react at all.” We wanna find a very unreactive metal. In fact, since we know there’s only one metal that won’t react with water or steam, we can simply pick the least reactive metal. In this question, the least reactive metal listed is copper. The most reactive metals, like potassium and magnesium, will react to both water and steam. Metals with moderate reactivity, like zinc and tin, won’t react to cold water but will react to the more energetic steam. The least reactive metals, like copper, won’t react at all with water or steam.

Finally, part (c) which metal will only be displaced by one other metal?

The key word in this part of the question is “displaced,” which means replaced in a compound or solution. Displacement sometimes occurs when we dip one metal into a solution containing another metal ion. For example, dipping magnesium into a solution of zinc chloride causes the magnesium to displace the zinc ion from solution. The result is that the solution fills with magnesium ions, and the zinc ions become solid zinc plated on the original magnesium strip.

One important thing to know about displacement is that it will only occur when the less reactive metal is being displaced. If we tried to carry out the reverse reaction, where we dip solid zinc into a magnesium chloride solution, the less reactive zinc would be unable to displace the more reactive magnesium from solution. So, as a rule, we can write out that more reactive metals displace less reactive metals.

Another way of phrasing this question is, which metal has only one metal that is more reactive than it? Looking at the reactivity series, we can see that magnesium has one metal, potassium, that is more reactive than it. So, it has one metal that it will be displaced by. The metals to the right of magnesium — zinc, tin, and copper — will not displace magnesium. So, our answer for part (c) is magnesium.

To recap all of our answers, in this reactivity series, the metal that will react most vigorously with hydrochloric acid is potassium, the metal that will not react at all with water or steam is copper, and the metal that will only be displaced by one other metal is magnesium.

Now that we’ve done some practice problems, let’s review the key points of the video. A reactivity series is a list of metals in order of reactivity. We can group metals based on whether or not they react to water, acid, or oxygen. Reactions may involve the more reactive metal displacing the less reactive metal. One example of such a reaction is the reaction between magnesium and zinc chloride. The more reactive magnesium displaces the zinc to form a magnesium chloride solution, while the less reactive zinc forms a solid.

These types of reactions are called single displacement reactions or single replacement reactions. Some metals react with oxygen to form oxides. For example, magnesium combines with the oxygen in the air to form magnesium oxide. And finally, more reactive metals react more quickly and vigorously. And they don’t require as much energy to react.

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