Decomposition and Synthesis Reactions

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  • 0:05 Reactions
  • 0:53 Identify a Synthesis Reaction
  • 2:35 Predicting a Synthesis…
  • 3:26 Identifying a…
  • 4:10 Predicting a…
  • 5:11 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn how to write, identify and predict the products of simple synthesis and decomposition reactions. This includes the composition of reactions with oxygen, of two metals, and of metals with nonmetals, as well as the decomposition of metal carbonates, metal chlorates and metal hydroxides.


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.

Let's review a few vocabulary words. Remember that a reactant is a substance that participates in a chemical reaction. A product is a substance that forms in a chemical reaction. Now we can talk about a chemical reaction, which is when one or more substances change to produce a different substance.

Identifying a Synthesis Reaction

The first type of reaction we are going to talk about is the synthesis reaction. 'Synthesis' means 'to put together,' and that is what this reaction does - it takes two or more reactants and puts them together to form a single compound. A synthesis reaction is a reaction in which two or more substances combine to form a new compound.

When two elements combine in a synthesis reaction, the only product that can be made is a binary compound. A binary compound is a compound composed of only two elements: A + B = AB, or C + O2 = CO2. If a metal reacts with a nonmetal, an ionic compound forms because the electrons are transferred from one atom to the other. With this kind of reaction, it is easy to predict the product because the charges on the ions of each reactant help you determine how many of each atom there needs to be. When nonmetals react with each other, it is harder to predict the product. This is because they can combine into several different forms. For example, carbon and oxygen, C and O2, can combine to make CO2, or they can combine to make CO (C + O2 = CO2 or 2C + O2 = 2CO).

When a metal and non-metal react with each other, an ionic compound is created.
Metal Non-Metal Form Ionic Compound

If two compounds combine in a synthesis reaction, they can form a ternary compound, which is a compound made up of three elements. AB + BC = ABC or carbon dioxide combines with water to make carbonic acid: CO2 + H2O = H2CO3.

Predicting a Synthesis Reaction

You can predict what forms in a synthesis reaction by looking at what is reacting.

Reactions with Oxygen

When an element combines with oxygen, it creates an oxide of the element. Iron reacts with oxygen to make an iron oxide: 2Fe + O2 = 2FeO.

Reactions of Two Nonmetals

When two nonmetals react, they form a covalent compound. Water is an example. Hydrogen plus oxygen makes water: 2H2 + O2 = 2H2O.

When a non-metal element reacts with oxygen, a covalent compound forms.
Covalent Compound Example

Reactions of Metals with Nonmetals Other Than Oxygen

When a metal and nonmetal react, they often form an ionic compound, usually a salt. Sodium and chloride combine to form table salt: 2Na + Cl2 = 2NaCl.

Identifying a Decomposition Reaction

A decomposition reaction is the opposite of a synthesis reaction. In a decomposition reaction, a single compound breaks apart into two or more elements or compounds. Usually energy is required to make this happen.

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