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.
