Video Transcript
In this video, we will learn how to
outline chemical reactions and describe direct combination reactions.
Chemical reactions are happening
all the time in our everyday lives, whether we notice them or not. A chemical reaction is the process
where one or more substances are changed into one or more new substances. The starting substances in a
chemical reaction are called reactants. The new substances that are formed
during a chemical reaction are called the products.
Chemical reactions are very
useful. Chemical reactions help organisms
grow and help sustain life on Earth. For example, cells in our bodies
use the starting substances oxygen and sugar to produce energy in a chemical process
called respiration. The ending substances are carbon
dioxide and water. Plants carry out the chemical
process of photosynthesis. This process uses light energy, and
the reactants are carbon dioxide and water. The products of photosynthesis are
sugars for food and growth and oxygen gas.
Scientists and engineers use
chemical reactions to convert less useful substances into more useful
substances. For example, crude oil, or
petroleum, is taken out of the Earth and changed with chemical reactions to make
useful fuels like gasoline. Gasoline is a fuel for cars and
other vehicles. In the engine of a car, gasoline
undergoes a chemical reaction called combustion. This reaction produces carbon
dioxide and water. Unfortunately, chemical reactions
may also lead to negative effects on the environment and our health. The carbon dioxide that is formed
when humans burn gasoline and other fossil fuels is a greenhouse gas, and greenhouse
gases make the planet hotter.
Now that we know what chemical
reactions are, let’s explain how they occur. There are two main steps in a
chemical reaction. First, bonds within the reactants
are broken. Next, the products form as new
bonds are created. Let’s look at the burning of
magnesium to help us understand this process. We can heat a piece of magnesium
metal with a Bunsen burner by holding the metal in the flame with tongs. When the magnesium ignites, it
burns with a very bright white light. A whitish-gray powder forms and can
be collected with a watch glass. The reactants are magnesium metal
and oxygen gas from the air. The product formed in this reaction
is the compound magnesium oxide. This is the powder that we
collected on the watch glass.
Now, let’s clear some space to find
out how the reaction occurs. First of all, the double bond in
the oxygen molecule must break. Heat energy is needed to break this
strong bond. Once the bond is broken, each
oxygen atom will combine with a magnesium atom, and new chemical bonds form. The new substance formed is
magnesium oxide. We just represented the burning of
magnesium using diagrams and words. But there is a more useful way to
represent chemical reactions. We could write a chemical
equation.
A chemical equation is a set of
chemical formulas which represent the reactant and product substances in a chemical
reaction. A chemical equation may also
contain information about the conditions under which the reaction takes place. Examples of conditions include the
temperature or pressure at which the reaction is happening or if heating is
required. Here is a word equation for the
burning of magnesium again. It gives the names of the reactants
and product.
To write the chemical formulas of
each substance, we need to use the chemical symbol of each element. The chemical symbol of magnesium is
Mg, and the chemical symbol of oxygen is O. The balanced chemical equation for
this reaction is two Mg plus O2 reacts to form two MgO. Magnesium and oxygen are both
elements, so their formulas have only one chemical symbol. Oxygen gas naturally exists as
elemental molecules of O2. Magnesium oxide is a compound. Its chemical formula contains the
chemical symbols of magnesium and oxygen.
Now let’s discuss the other types
of symbols that are used in chemical equations. A plus sign is always used between
different reactants or products. In our equation, we used a plus
sign between the two reactants magnesium and oxygen. We did not use a plus sign on the
product side of the equation because magnesium oxide is one substance. The reaction arrow points from the
reactants to the products. The Δ symbol is used to tell that
heat is being added to the reaction.
There are also two types of numbers
used in a chemical equation. A number may be part of a chemical
formula. This number is called a
subscript. For example, the subscript of two
in the formula for oxygen tells us that an oxygen molecule contains two oxygen
atoms. The second type of number used in a
chemical equation is written in front of a chemical formula and is called a
coefficient. For example, the two in front of
the chemical formula MgO means that there are two units of magnesium oxide. This type of number is used to
balance a chemical equation.
According to the law of
conservation of mass, mass is neither created nor destroyed during a chemical
reaction.
Let’s look at our equation
again. Before the reaction happens, we
start with two magnesium atoms and two oxygen atoms. After the reaction, we still have
two magnesium atoms and two oxygen atoms. But they have combined together to
form the compound magnesium oxide. So, the number of reactant atoms of
an element before a reaction happens is always equal to the number of product atoms
of an element after a reaction happens.
There are many different types of
chemical reactions. We are going to focus on direct
combination reactions. A direct combination reaction is a
type of chemical reaction where one product is formed from two or more
reactants. The reactants may be elements or
compounds. But the product is always a
compound.
There are three types of direct
combination reactions. The first is the simplest. It happens when one element reacts
with another element to form a compound. The burning of magnesium is this
type of direct combination reaction. The element magnesium reacts with
the element oxygen to form the compound magnesium oxide. Another example is when the element
carbon reacts with the element oxygen to form the compound carbon dioxide.
The second type of direct
combination reaction is when an element reacts with a compound to produce a new
compound. An example of this type of reaction
is when the element oxygen reacts with the compound carbon monoxide to form the
compound carbon dioxide.
Finally, the third type of direct
combination reaction is when a compound reacts with another compound to produce a
new compound. An example of this type of reaction
is when the compound ammonia reacts with the compound hydrogen chloride to produce
the compound ammonium chloride.
Now, let’s discuss how this last
reaction can be carried out in the laboratory. First, we soak a piece of cotton
wool with concentrated hydrochloric acid. Then, we place this piece of cotton
wool inside one end of a glass tube. Next, we soak another piece of
cotton wool with concentrated ammonia solution and place it inside the other end of
the tube. Then, we close up both ends of the
tube with rubber stoppers. The hydrochloric acid in the cotton
wool releases colorless hydrogen chloride gas, or HCl. The other piece of cotton wool
releases colorless ammonia gas, or NH3. The two colorless gases move
through the tube and react with each other. A white smoky substance appears and
forms a ring inside the tube. This ring contains solid ammonium
chloride, or NH4Cl.
We’ve already discussed how the
combustion of fossil fuels is both useful and harmful. Now let’s review some of the air
pollutants these reactions produce.
The four main types of pollutants
produced from burning fossil fuels are carbon dioxide, carbon monoxide, sulfur
oxides, and nitrogen oxides. We know that having large amounts
of carbon dioxide in the atmosphere leads to global warming. Breathing in carbon monoxide can
cause headaches, fainting, or even death. Sulfur oxides, such as sulfur
dioxide and sulfur trioxide, can cause damage to the lungs. Sulfur oxides also react with water
droplets in the atmosphere to form acid rain. Acid rain causes damage to the
surfaces of buildings through a type of chemical reaction called corrosion. Nitrogen oxides can damage our
eyes, nervous system, and lungs. Nitrogen oxides also react with
water in the atmosphere to form acid rain.
Before we summarize our learning,
let’s practice with a few questions.
What does a chemical equation
represent through symbols and formulae?
First of all, a chemical equation
is a set of chemical formulas which represent the reactant and product substances in
a chemical reaction. In a chemical equation, the
reactants appear on the left side. Then, the reaction arrow is
written, and the products appear on the right side. Here is an example of a chemical
equation. The chemical symbol C stands for
the element carbon. Carbon is a reactant in this
chemical reaction. The chemical formula O2 stands for
oxygen. Oxygen is another reactant in this
reaction. The chemical formula CO2 stands for
carbon dioxide. Carbon dioxide is the product of
the reaction.
Many chemical equations also
include the conditions of the reaction. We may write the temperature or
pressure at which the reaction is happening or if heat is required. A Δ symbol above the reaction arrow
is used to tell that heat is required for a reaction. So, what does a chemical equation
represent through symbols and formulae? It represents the reactants,
products, and conditions of reaction.
Which of the following chemical
reactions is a direct combination reaction? (A) Mg plus two H2O react to form
MgOH2 plus H2. (B) Two H2O reacts to form two H2
plus O2. (C) PbNO32 plus two KI react to
form PbI2 plus two KNO3. (D) CH4 plus two O2 react to form
CO2 plus two H2O. (E) Eight Fe plus S8 react to form
eight FeS.
A direct combination reaction is a
type of chemical reaction where one product is formed from two or more
reactants. In a chemical equation of a direct
combination reaction, we will see two or more reactant substances on the left side
of the reaction arrow. But on the right side, we will see
only one product substance. The reactants may be elements or
compounds, but the product is always one compound. As we go through the answer
choices, let’s use the letter E to represent an element and the letter C to
represent a compound.
In choice (A), the chemical symbol
Mg represents the element magnesium and the chemical formula H2O represents the
compound water. The products are the compound
magnesium hydroxide, or MgOH2, and the element hydrogen. This chemical reaction has two
products, not one. So, it is not a direct combination
reaction.
In choice (B), the compound water
reacts to form two products, which are both elements. The reaction has only one reactant
and two products. So, it is not a direct combination
reaction.
In choice (C), two compounds react
to form two new compounds. Because this reaction has two
products, it is not a direct combination reaction.
In choice (D), a compound reacts
with an element to form two new compounds. A direct combination reaction does
not have two products. So choice (D) is also
incorrect.
This leaves only choice (E), which
must be the correct answer. In this reaction, two elements
react to form one compound. The chemical reaction that is a
direct combination reaction is choice (E): eight Fe plus S8 react to form eight
FeS.
Now, let’s summarize our learning
with some key points. A chemical reaction is a process
where one or more substances are changed into one or more new substances. Chemical reactions may be helpful
or harmful. For example, burning fossil fuels
produces carbon dioxide. Having too much carbon dioxide in
the atmosphere leads to global warming. Burning fossil fuels also produces
sulfur oxides and nitrogen oxides. These gases are harmful to our
health and react with water in the atmosphere to make acid rain.
During a chemical reaction, bonds
in the reactants break, and products then form as new bonds are created. A chemical equation is a set of
chemical formulas which represents the reactants and products of a chemical
reaction. Chemical equations may also include
the conditions of the reaction. A direct combination reaction is a
type of reaction where one product is formed from two or more reactants.
