Video Transcript
In this video, we will learn how to
describe and explain single- and double-substitution reactions. We’ll compare the chemical activity
of different elements and use the series of chemical activity to predict if a
chemical reaction will occur.
A substitution reaction is a type
of chemical reaction in which one element replaces another element and new
substances are formed. The word substitution means trading
places or swapping out. Substitution reactions are also
called displacement reactions. There are two types of substitution
reactions: single-substitution reactions, which are sometimes called simple
substitution reactions, and double-substitution reactions.
In a single-substitution reaction,
one more active element replaces a less active element in a compound. There are two products in the
single-substitution reaction: one is a compound and the other is the less active
element. In a double-substitution reaction,
two compounds react. A more active element in one of the
compounds replaces a less active element in the other compound. The products of the
double-substitution reaction are two new compounds. By definition, in a
single-substitution reaction, a more active metal element will replace a less active
metal element in a chemical compound.
Elements differ in their chemical
activity. The series of chemical activity is
a list that puts elements in order based on their activity. The higher an element is on the
list, the more active it is. For example, sodium metal is higher
on the series of chemical activity than zinc metal. This means that sodium is a more
active metal than zinc. This also means that sodium can
replace zinc in a substitution reaction.
When observing reactions in the
laboratory, we will see signs that tell us an element is more active. If two similar reactions were
carried out, a reaction with sodium would be much more vigorous than a reaction with
zinc. Even though hydrogen is a nonmetal
element, it can be replaced by more active metals in a single-substitution
reaction. Metals in group one on the periodic
table, which are also known as alkali metals, are more active than hydrogen.
On the series of chemical activity
shown, potassium and sodium are group one metals. When a group one metal reacts with
water, a single-substitution reaction occurs. And the products of the reaction
are a group one hydroxide and hydrogen gas. For example, when sodium metal
reacts with water, the products are sodium hydroxide and hydrogen gas. Let’s take a closer look at this
reaction. When a piece of solid sodium metal
is placed in a container of water, a very vigorous and violent reaction occurs. Hydrogen gas bubbles through the
solution, is released, and can ignite, producing flames. The reaction releases a large
amount of heat, and the solution that remains in the container is an aqueous
solution of sodium hydroxide. During the reaction, sodium metal
atoms replace hydrogen atoms in the water molecules. These hydrogen atoms then bind to
form hydrogen gas, which is released from the sodium hydroxide solution.
Metals can also have
single-substitution reactions with dilute mineral acids, such as sulfuric acid and
hydrochloric acid. When a metal that is higher on the
activity series than hydrogen reacts with the acid, a salt and hydrogen gas are
produced. Metals that are higher on the
activity series will have stronger reactions with the acid. And metals which are lower than
hydrogen on the activity series will not react with the acid. For example, the reaction of
magnesium with hydrochloric acid will be more vigorous than the reaction of iron
with hydrochloric acid. This is because magnesium metal is
higher on the activity series than iron metal.
If we added a piece of magnesium
metal to a test tube containing hydrochloric acid and added a piece of iron to
another test tube with the acid, we’d see that the magnesium metal reacts with the
acid very quickly, producing bubbles, releasing hydrogen gas and heat. On the other hand, the iron metal
reacts with the acid more slowly. Hydrogen gas is produced, a little
bit less heat is released, and the solution slowly turns green.
In the reaction with magnesium,
magnesium atoms replace hydrogen atoms in the hydrochloric acid molecules, releasing
hydrogen gas and leaving behind a solution of magnesium chloride salt. In the reaction with iron, iron
atoms replace hydrogen atoms in the hydrochloric acid molecules, releasing hydrogen
gas and leaving behind a solution of iron(II) chloride salt.
Another type of single-substitution
reaction is the reaction between a highly active metal and a salt which contains a
less active metal. The products of this reaction are a
new salt and the less active metal that was replaced. We can use the series of chemical
activity to predict whether a metal will react with a specific salt solution.
Let’s say we added a piece of zinc
to a solution of copper(II) sulfate salt, which has a blue color. During the reaction, zinc takes the
place of copper in the salt solution. And over time, solid orangish-brown
copper accumulates at the bottom of the beaker. As zinc continues to replace copper
in the solution, the solution turns colorless. Zinc was able to react and replace
copper in the solution because zinc is more active and higher on the activity series
than copper. If instead we placed zinc into a
magnesium sulfate solution, no reaction would occur. This is because zinc is less active
than magnesium. In order for a single-substitution
reaction to happen between a metal and a salt, the metal must be more active than
the metal found in the salt.
Now, we’re going to discuss several
double-substitution reactions. In the general pattern of a
double-substitution reaction, two compounds react to produce two new compounds. A neutralization reaction is an
example of a double-substitution reaction. In a neutralization reaction, the
two compounds that react are an acid and an alkali. And the two compounds that are
produced are a salt and water. For example, when hydrochloric acid
reacts with potassium hydroxide, which is an alkali, the products are a potassium
chloride salt solution and water.
When we look at the series of
chemical activity, we see that potassium is higher than hydrogen, which means that
potassium is more active than hydrogen. Therefore, potassium will take the
place of hydrogen in the hydrochloric acid molecules, forming the potassium chloride
salt. The remaining hydrogen ions will
combine with the hydroxide ions in the solution to form water molecules.
Another example of a
double-substitution reaction is a precipitation reaction. In a precipitation reaction, the
two reactants are two different salt solutions. Mixing together two salt solutions
does not always result in a reaction. However, if one of the combinations
of ions in the mixture of salt solutions forms an insoluble salt, then a reaction
will occur. The insoluble salt that forms is
called a precipitate. So the reaction is called a
precipitation reaction.
An example of a precipitation
reaction is the reaction between potassium chloride solution and silver nitrate
solution. When we look at the series of
chemical activity, we notice that potassium is much more active than silver. During the reaction, the more
active potassium takes the place of silver in the solution. And the silver ions react and
combine with the chloride ions to form a precipitate of silver chloride. Silver chloride is a white
precipitate, and it will make the solution appear cloudy as it forms.
Our final example of a
double-substitution reaction is the reaction between a compound called a metal
carbonate and an acid. Unlike the other
double-substitution reactions we’ve looked at, this reaction produces three
products: a salt, carbon dioxide gas, and water. Let’s look at a specific example to
help us understand this type of reaction.
When sodium carbonate reacts with
hydrochloric acid, sodium chloride salt, carbon dioxide gas, and water are
produced. Let’s use a balanced chemical
equation to help us understand what happens during the reaction. We can see on the series of
chemical activity that sodium is quite a bit more active than hydrogen. Therefore, sodium will replace
hydrogen in the hydrochloric acid molecules, forming sodium chloride salt. The hydrogen ions that were
replaced will react with the carbonate ion to form carbon dioxide gas and water. When performing this reaction in
the lab, we’ll know that carbon dioxide was made when we see bubbles.
Before we summarize what we’ve
learned about substitution reactions, let’s take a look at a question.
When a piece of sodium metal is
placed into a large trough of cold water, a chemical reaction takes place. Which of the following statements
best explains why this is the case? (A) Sodium is more active than
hydrogen. (B) Sodium is less active than
hydrogen. (C) Sodium is more active than
water. (D) Sodium is less active than
water.
In this question, we are told that
a chemical reaction occurs when sodium metal is placed into cold water. Sodium metal is a metal found in
group one on the periodic table. Group one metals are very active
metals. Sodium and potassium metal, which
are both group one metals, appear at the very top of the series of chemical
activity. Metals found at the top of the
series are the most active, and metals found toward the bottom of the series are the
least active.
Group one metals can replace
hydrogen in substitution reactions because they are more active than hydrogen. In a single-substitution reaction,
a more active metal element will replace a less active metal element in a chemical
compound. It is also possible for an active
metal to replace hydrogen in a compound as long as hydrogen is less active.
In this question, we know that the
reactants of the single-substitution reaction are sodium, the group one metal, and
water. The products of this type of
single-substitution reaction are a group one hydroxide and hydrogen gas. Let’s write a word equation to
represent the reaction between sodium and water. We can write sodium plus water
before the reaction arrow and sodium hydroxide plus hydrogen gas to represent the
products. Now, let’s write a chemical
equation for the reaction. The chemical formulas of the
reactants are Na followed by the state symbol “s” for sodium solid and H2O followed
by the state symbol “l” for liquid water. The product sodium hydroxide is a
solution. The chemical formula is NaOH
followed by “aq” to represent an aqueous solution. And the chemical formula for
hydrogen gas is H2 followed by the state symbol “g.”
Now that we have this basic
unbalanced equation, let’s take a closer look at what’s going on in this
reaction. We know from the series of chemical
activity that sodium is more active than hydrogen. During the reaction, the more
active sodium atoms replace the less active hydrogen atoms in the water
molecules. And the hydrogen atoms that got
replaced combine to form hydrogen gas. We’ve confirmed that a
single-substitution reaction takes place because sodium is a more active element
than hydrogen. The statement which best explains
why sodium metal reacts with water is answer choice (A). Sodium is more active than
hydrogen.
Now, let’s summarize what we
learned in this video about substitution reactions. There are two types of substitution
reactions: single or simple substitution and double substitution. The series of chemical activity
lists elements in order by their activity and can be used to predict if a reaction
will occur or how vigorous a reaction might be. In a single-substitution reaction,
a more active metal replaces a less active metal in a chemical compound. An active metal can also replace
hydrogen in a compound as long as hydrogen is less active than the metal.
Examples of single-substitution
reactions include the reaction between a group one metal and water, which produces a
group one hydroxide and hydrogen gas; the reaction between a metal and an acid,
which produces a salt and hydrogen gas; and the reaction between a metal and a salt
solution. In this type of reaction, a more
active metal replaces a less active metal in a solution. And the less active metal forms a
solid precipitate. Finally, neutralization,
precipitation, and reactions of metal carbonates with acids are all types of
double-substitution reactions.
