(BCR) Types of Chemical Reactions Lesson

Types of Chemical Reactions

Synthesis Reactions

A synthesis reaction occurs when two or more reactants combine to form a single product. A synthesis reaction can be represented by the general equation:

A + B → AB

In this general equation (and others like it in this lesson), the letters represent elements/compounds. The arrow shows the direction of the reaction. The letters on the left side of the arrow are the reactants that begin the chemical reaction. The letters on the right side of the arrow are the product of the reaction.

An example of a synthesis reaction is the combination of sodium (Na) and chlorine (Cl) to produce sodium chloride (NaCl). This reaction is represented by the chemical equation:

2 Na + Cl2 → 2 NaCl

Sodium is a highly reactive metal, and chlorine is a poisonous gas (see picture below). The compound they synthesize has very different properties. It is table salt, which is neither reactive nor poisonous. In fact, salt is a necessary component of the human diet.

Synthesis Reaction Example Sodium + Chlorine -> Sodium chloride

Another example of a synthesis reaction is seen in the picture below.   The brown haze in the air over the city of Los Angeles is smog. A major component of smog is nitrogen dioxide (NO2). It forms when nitric oxide (NO), from sources such as car exhaust, combines with oxygen (O2) in the air. The equation for this reaction is:

Synthesis Example; haze over city

2 NO + O2 → 2 NO2

Nitrogen dioxide is a toxic gas with a sharp odor. It can irritate the eyes and throat and trigger asthma attacks. It is a major air pollutant.

Decomposition Reactions

A decomposition reaction is the reverse of a synthesis reaction. In a decomposition reaction, one reactant breaks down into two or more products. This can be represented by the general equation:

AB → A + B

An example of a decomposition reaction is the breakdown of water as seen in the picture below. Water (H2O) decomposes to hydrogen (H2) and oxygen (O2) when an electric current passes through it. This reaction is represented by the equation:

Decomposition Example: oxygen gas rises in this test tube;hydrogen gas rises in this {another} test tube.

2 H2O → 2 H2 + O2

To review synthesis and decomposition reactions as well as see some demonstrations of these chemical reactions, please watch the video below.

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