Synthesis Reaction: Definition, Formula & Examples

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  • 0:00 What Is a Synthesis Reaction?
  • 2:21 Examples
  • 5:13 Lesson Summary
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Lesson Transcript
Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Synthesis reactions, the act of combining two or more substances together to make a product, occur all around us, from the kitchen to our chemical laboratories. In this lesson, learn more about this amazing reaction, its formula, and examples.

What is a Synthesis Reaction?

A cronut and crookie, two very hilarious food terms, they surely took the world by storm when making their grand debut. Did you know those are great examples of products made from a synthesis reaction? It may seem odd to mix a food term with a science term but it is definitely true. Before we discuss how this is so, let's go over what a synthesis reaction is.

A synthesis reaction is the joining together of two reactants, or compounds, to produce a complex product, also called a compound. Sometimes synthesis reactions can result in the formation of more than one product, as we'll see shortly with the process of photosynthesis.

Whether it is one complex product or multiple products, a great way to remember what a synthesis reaction is, is to think of the word 'combination.' Essentially, you are combining reactants to make a desired product. The formula for a synthesis reaction, involving the formation of a complex product, is shown here in Diagram 1. As you can see, substance D combines with substance R to produce a more complex compound, labeled DR.

Diagram 1: Formula for a synthesis reaction

As you can gather from the equation shown in Diagram 1, a synthesis reaction is not a multi-step process. That is, there is no need to be concerned with learning multiple steps for this reaction. In fact, a synthesis reaction is perhaps one of the most widely used reactions; not just in chemistry, but also in unconventional places, such as our kitchens.

Before we practice a few examples, there are two concepts to keep in mind regarding synthesis reactions. First, binary compounds can be produced from a synthesis reaction. A binary compound is a compound that contains only two different elements. For example, the combination of two reactants, iron and oxygen, will produce the binary compound iron oxide, commonly known as rust.

Second, always remember when working with synthesis reactions to balance your equation. As you will see shortly, coefficients, for example numbers, are sometimes required to ensure what is present on the left side of a reaction is equal in amount to what is present on the right side of a reaction. Now that we have addressed these two fundamental concepts, let's apply what we've learned to a few examples.


Example 1: You are working diligently in a scientific laboratory and decide to mix a solution containing lithium with another solution containing chlorine. What do you predict will be produced from the mixture of these solutions?

Elements under question: lithium (Li) and chlorine (Cl2)

Synthesis equation:

licl unbalanced

Balanced synthesis equation:


From this synthesis reaction, we can see that 2 molecules of lithium chloride (LiCl2) are produced. Because two molecules of chlorine (Cl2) are present as a reactant, this needs to be balanced for the product LiCl (where only one chlorine atom is present). The addition of a coefficient of 2 (in front of LiCl) will ensure all chlorine atoms, on both the reactant and product side, are balanced. However, we cannot stop right there with balancing. A coefficient of 2 must be added to the lithium atom on the reactant side, to ensure all lithium atoms (on both sides of the reaction) are balanced.

Example 2: The process of photosynthesis involves the use of a synthesis reaction. What is produced when carbon dioxide reacts with water during photosynthesis?

Elements under question: carbon dioxide (CO2) and water (H2O)

Synthesis equation (balanced):


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