Lesson 8 — Activity 1: Composition and Decomposition Reactions
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Lesson 8 — Activity 1:
Composition and Decomposition Reactions
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Scientists like order and predictability. They like to know where things are, where they belong, and how they relate to each other. It helps scientists see patterns and similarities among seemingly different things. You have learned that there are millions of chemical reactions happening every day, everywhere, all around you. To make understanding all those different types of reactions easier, they are classified into several types so predictions can be made about them. In this activity, you will learn about composition and decomposition reactions.
Composition and decomposition are two more common types of chemical reactions. They are the opposite of each other.
In a composition reaction, two or more reactants (a substance that takes part in and undergoes change during a reaction) combine to create one
product. Composition reactions are also sometimes called synthesis
reactions. Both
names mean the same thing: the elements or compounds that react create
one substance together.
The chemical equation for a composition (synthesis) reaction looks something like this if A and B are elements (reactants).
A composition, or synthesis reaction, is a type of chemical reaction in which two or more simple substances combine to form a more complex product. The reactants may be elements or compounds. The product is always a compound.
For example, the combination of iron (Fe) and sulfur (S) to form iron sulfide (also known as fool's gold because it is a substance that looks like gold but is not) is an example of a synthesis reaction.
Synthesis reactions are simple to identify because the number of
products (A + B) will always be less than the number of reactants (AB). A synthesis
reaction can be a step in a larger chain of reactions.
Examples of
synthesis reactions include magnesium and oxygen combine to form magnesium oxide. Hydrogen and oxygen combine to form dihydrogen monoxide (water).
Another common example of a composition, or synthesis reaction, is the rusting or tarnishing of silver. This occurs when silver reacts with sulfur to produce the material that tarnishes silver.
Decomposition reactions are
the direct opposite of synthesis reactions. In a decomposition reaction,
one reactant creates two or more products. That is, the compound breaks
down, or decomposes, into the various elements or compounds it was made
of.
The chemical equation for a decomposition is the reverse of the example above.
The chemical equation for a decomposition is the reverse of the example above.
In some cases, the reactant breaks into its component elements, but a decomposition may involve a breakdown into any smaller molecules. The process may occur in a single step or multiple ones.
Because chemical bonds are broken, a decomposition reaction requires the addition of energy to begin. Usually the energy is supplied as heat, but sometimes simply an electric shock, radiation, or change in humidity or acidity initiates the process.
The
electrolysis of water into oxygen and hydrogen gas is an example of a
decomposition reaction. Electrolysis is a process in which a chemical change, especially
decomposition, is brought about by passing an electric current through a
solution of electrolytes so that the electrolyte's ions move toward the
negative and positive electrodes and react with them. Let's take a look at what this type of decomposition reaction looks like below.
Here is another example of decomposition in our everyday lives.
Click on the Play button below to watch a video on composition (synthesis) and decomposition reactions.
Click on the Play button below to watch a video on composition (synthesis) and decomposition reactions.
Self-Check
Try This!
Look at the examples below of different reactions.
Decide if they are composition (synthesis) or decomposition reactions.
