Grade 9

Intermediate

6 min

Lesson 5 of 12

Synthesis and Decomposition

Not Started

Learn to identify and predict the outcomes of reactions where substances combine or break apart.

What if you could predict exactly when a substance will stay stable and when it will burst apart into its hidden components? Just like building and breaking Lego sets, chemicals follow strict rules for joining and separating.

Learning Objectives

1.Identify synthesis reactions where two or more reactants form a single product.
2.Identify decomposition reactions where one reactant breaks down into multiple products.
3.Predict the products of simple binary synthesis and decomposition reactions.

Synthesis: The Great Combination

A synthesis reaction (also known as a combination reaction) occurs when two or more simple substances combine to form a single, more complex product. Think of it like a marriage: two individuals become one couple. The general formula is

A + B \rightarrow AB

. In most cases, these reactions involve two elements reacting to form a binary compound. For example, when a metal reacts with a non-metal, they transfer electrons to create an ionic bond. These reactions are often exothermic, meaning they release energy in the form of heat or light, such as when magnesium ribbon burns brightly in oxygen.

Let's look at how sodium and chlorine combine. 1. Identify the reactants: Sodium (

Na

) and Chlorine gas (

Cl_2

). 2. Combine them into one product: Sodium Chloride (

NaCl

). 3. The unbalanced equation:

Na + Cl_2 \rightarrow NaCl

. 4. The balanced equation:

2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)

Quick Check

In a synthesis reaction, how many products are formed?

Look at the right side of the arrow in the general formula

A + B \rightarrow AB

Answer

Exactly one product is formed.

Decomposition: The Big Breakup

A decomposition reaction is the exact opposite of synthesis. It occurs when a single compound breaks down into two or more simpler substances. The general formula is

AB \rightarrow A + B

. Unlike synthesis, decomposition usually requires an input of energy—such as heat, light, or electricity—to break the chemical bonds holding the compound together. This is why many chemicals are stored in dark bottles; light alone can provide enough energy to trigger a decomposition reaction!

Using electricity to split water into its elements: 1. Start with the single reactant: Water (

H_2O

). 2. Identify the elements it is made of: Hydrogen (

H_2

) and Oxygen (

O_2

). 3. Write the equation:

2H_2O(l) \xrightarrow{electricity} 2H_2(g) + O_2(g)

4. Note that the product side has two distinct substances.

Quick Check

What is usually required to make a decomposition reaction happen?

Think about what is needed to break a strong bond.

Answer

An input of energy (such as heat, light, or electricity).

Predicting the Outcome

To predict the products of these reactions, you must look at the reactants. If you see two elements alone, they will likely undergo synthesis to form a compound. If you see one compound alone, it will likely undergo decomposition into its constituent elements. Remember that certain elements, like Oxygen ( $O_2$ ), Hydrogen ( $H_2$ ), and Chlorine ( $Cl_2$ ), are diatomic, meaning they always travel in pairs when they are alone as pure elements.

Predict the products for the following:

Al(s) + O_2(g) \rightarrow ?

1. Recognize the pattern: Two elements (

A + B

) means synthesis. 2. Determine the product: Aluminum (

Al^{3+}

) and Oxygen (

O^{2-}

) form

Al_2O_3

. 3. Write the skeleton:

Al + O_2 \rightarrow Al_2O_3

. 4. Balance the equation:

4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)

Key Takeaways

1Synthesis reactions follow the pattern $A + B \rightarrow AB$ , resulting in a single product.
2Decomposition reactions follow the pattern $AB \rightarrow A + B$ , starting with a single reactant.
3Decomposition usually requires energy (heat, light, or electricity) to break bonds.
4Always remember diatomic elements ( $H_2, N_2, O_2, F_2, Cl_2, Br_2, I_2$ ) when they are products of decomposition.

Quick Check

Multiple Choice

Which of the following represents a synthesis reaction?

Multiple Choice

If Mercury(II) oxide ( $HgO$ ) is heated, it undergoes decomposition. What are the products?

True/False

The reaction $ $2KClO_3 \rightarrow 2KCl + 3O_2$ $ is a synthesis reaction.

What's Next?

Review Tomorrow

In 24 hours, try to write down the general formulas for synthesis and decomposition from memory and explain the difference to a friend.

Practice Activity

Look at the ingredients on a food label. While those are complex, imagine the 'synthesis' required to bake a cake versus the 'decomposition' that happens when you digest it!

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Synthesis and Decomposition

Learning Objectives

Synthesis: The Great Combination

Decomposition: The Big Breakup

Predicting the Outcome

Key Takeaways

Quick Check

What's Next?

Synthesis and Decomposition

Learning Objectives

Synthesis: The Great Combination

Decomposition: The Big Breakup

Predicting the Outcome

Key Takeaways

Quick Check

What's Next?