4.04 Chemical Reactions: Synthesis and Decomposition

4.04 Chemical Reactions: Synthesis and Decomposition

My Goals for this Lesson:

• Identify synthesis and decomposition reactions from examples.
• Identify the properties of synthesis and decomposition reactions.
• Predict the products produced by synthesis and decomposition reactions.

I’m preparing to understand the chemical reactions called synthesis and decomposition.

Synthesis Reactions

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Synthesis reactions, sometimes called reactions, involve two or more reactants combining to form one new substance.

This means that in a synthesis equation, you will see two or more on the left side of the equation, and only one formed on the right side of the equation. A general equation for synthesis reactions can be represented by the following: A + B → AB

In this equation, A and B can represent OR , and AB represents a made of two or more elements.

Types of Synthesis Reactions

1) Metal with a nonmetal

1. What kind of compound do a metal and non-metal make?

1. Give an equation for this type of reaction.

Be sure that the formula of the product is a neutral compound, and remember to balance the equation.

2) Two nonmetals

1. What kind of compound do two non-metals make?

1. Give an equation for this type of reaction.

The products of some of these reactions can be challenging to predict at this point, because some nonmetals can combine in more than one ratio, such as NO2 and N2O5. As you continue through this course, you will learn more to help you predict the products of these synthesis reactions.

3) Metal oxide with water

1. What kind of compound does a metal oxide and water make?

1. Describe the charge of the metal during this reaction.

1. Describe the charges of the oxygen and the hydroxide ion.

1. Give an equation for this type of reaction.

1. What is a common name for calcium oxide?

1. What is a common place this compound is found?

4) Nonmetal oxide with water

1. What kind of compound do a nonmetal oxide and water make?

1. Give an equation for this type of reaction.

Real World Examples

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In one of the previous examples, you saw that sulfur trioxide (SO3) reacts with water to produce . Sulfur trioxide is a common air pollutant, which combines with rain to produce a solution (acid rain) that falls to the earth below.

Buildings and statues made of marble are very susceptible to damage from acid rain because the carbonate in the marble reacts easily with , causing the deterioration of the marble.

Decomposition Reactions

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As the word implies, a decomposition reaction breaks apart a substance into two or more simpler substances. A decomposition reaction is the opposite of a synthesis reaction; it starts with only one reactant but ends with more than one product. Most decomposition reactions require energy, usually in the form of heat or electricity, to be added in order for the reaction to occur.

In a decomposition reaction, a single compound undergoes a reaction that produces more than one product. The following general equation illustrates decomposition reactions:

AB → A + B

AB is a compound made of two or more elements. A and B are individual elements or compounds.

Types of Decomposition Reactions

1) Decomposition of a binary compound

1. What are the products from decomposition of a binary compound?

1. What is usually required for this reaction to occur?

1. Give an equation for this type of reaction.

Remember that each of the elements produced should be written in their most common form, which includes the nonmetals that occur as diatomic molecules—N2, H2, O2—and the halogens of Group 17.

2) Decomposition of a metal carbonate

1. What are the products from decomposition of a metal carbonate?

1. What is usually required for this reaction to occur?

1. Give an equation for this type of reaction.

3) Decomposition of a metal chlorate

1. What are the products from decomposition of a metal chlorate?

1. What is usually required for this reaction to occur?

1. Give an equation for this type of reaction.

Remember that oxygen gas is diatomic, has a subscript of 2, and chlorine has a -1 charge as an ion when you are writing out the products for these reactions.

4) Decomposition of a metal hydroxide

1. What are the products from decomposition of a metal hydroxide?

1. What is usually required for this reaction to occur?

1. Give an equation for this type of reaction.

Remember that the hydroxide ion has a -1 charge, but the oxygen ion has a -2 charge, which is useful information when writing neutral formulas.

5) Decomposition of an oxyacid

1. What are the products from decomposition of an oxyacid?

1. Give an equation for this type of reaction.

Notice that the nonmetal oxide that is produced is the polyatomic ion from the oxyacid, with one of its oxygen atoms removed. In the case of the decomposition of carbonic acid (H2CO3), one oxygen and two hydrogen atoms are removed from the oxyacid to form water. This leaves behind the nonmetal oxide carbon dioxide (CO2). H2CO3 → CO2 + H2O

Real World Example

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Most decomposition reactions require in the form of or in order for the reaction to occur. The decomposition of a substance by an current is called electrolysis, in which the root electro- means electricity and the root -lysis means to break. A common example of electrolysis is when an electrical current is passed through water, causing it to decompose into hydrogen gas and oxygen gas. 2H2O → 2H2 + O2

Please view the short video to help understand the decomposition of water using electrolysis.

Please do the “Let’s Review” at the end of the lesson to see if you can predict the products of the reactions.