Common Chemical Reactions and Energy Change

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  • 1:09 Combination Reaction
  • 1:50 Decomposition Reaction
  • 2:20 Single-Replacement Reaction
  • 2:52 Double-Placement Reaction
  • 3:29 Combustion Reaction
  • 4:27 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.

This lesson covers the five common types of chemical reactions: combination, decomposition, single-replacement, double-replacement, and combustion. You will learn how to predict what kind of chemical reaction will occur. You'll also explore how matter is conserved, but energy can change.

Types of Chemical Reactions

Chemical reactions are happening all around you: bread rising and baking in the oven, the leaves changing color in the fall, even the digestion of food in your own body. All are chemical reactions. In a chemical reaction, one or more things, called reactants, change into one or more new things, called products.

Chemical reactions can be written as a word equation or in shorthand. A word equation would look like oxygen + hydrogen = water. Shorthand, this would look like O2 + H = H2O. The things reacting are on the left side of the equation, and the products are on the right. Reactions can require energy or give off energy, but matter is always conserved. What that means is that however much you start with in reactants, you end with the exact same amount in products.

In chemical equations, the reactants are written on the left side and products are on the right.
Chemical Equations

There are five types of chemical reactions: combination, decomposition, single-replacement, double-replacement and combustion.


A combination reaction is when two or more substances react to form one new substance; essentially, when A + B = AB, as in the example of hydrogen and oxygen combining to make water. The reactants can be either two elements, as in the water example, or two compounds AB + CD = ABCD, such as when magnesium oxide combines with carbon dioxide to make magnesium carbonate (MgO + CO2 = MgCO3). Or, even one element and one compound: A + BC = ABC.


A decomposition reaction, as you can guess, is the opposite of a combination reaction. This is a chemical change in which a single compound breaks down into two or more products (AB = A + B). Most decomposition reactions require energy in the form of heat or light. An example is when mercury oxide, with the addition of heat, breaks down into mercury and oxygen (2HgO + heat = 2Hg + O2).

In a decomposition reaction, a compound breaks down into two or more products.
Decomposition Reaction Example


Single-replacement reactions start getting a bit trickier. This is a chemical reaction in which one reactant replaces a second one (AB + C = AC + B). In this type of reaction, both the reactant and product consist of one element and one compound. One example is when you put potassium into water to form potassium hydroxide plus hydrogen gas. Very explosive! (2K + 2H2O = 2KOH + H2).

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