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.
