Understanding Chemical Reactions: Synthesis and Decomposition, Slides of Chemistry

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Types of Chemical

Reactions: Synthesis

& Decomposition

Reactions

Click on this icon for audio

There are millions of chemical reactions that are known to occur. Among these millions of reactions, certain types display similar characteristics. As a result, chemists are able to group reactions into 4 basic types to help organize these known reactions and to help chemists predict the products of unknown reactions. Click to learn about synthesis reactions Synthesis Decomposition Single Displacement Double Displacement Click on this icon for audio

In a synthesis reaction, two or more simpler reactants combine to form one larger product. The general formula for a synthesis reaction can be written as follows: Where A and B are the reactants, and they combine to form AB, the product. For a synthesis reaction to occur, the two reactant molecules must collide with each other, break any existing bonds between their atoms and form new bonds. Synthesis Reaction Example Synthesis Reactions^ Click on this icon for audio

A good example of a synthesis reaction occurs when diatomic molecules of hydrogen gas (H 2 (g) ) burn in the air, combining with diatomic molecules of oxygen gas (O 2 (g) ) to form water. Try to write out the balanced chemical equation for this reaction and you will see the general formula for a synthesis reaction represented. Balanced Synthesis Reaction Equation Synthesis Reactions^ Click on this icon for audio

Test your understanding: Which of the following reactions is a synthesis reaction? (Click the box with the correct answer.) 2HCl  H 2

• Cl 2 2Fe + 6NaBr  2FeBr 3
• 6 Na

C

2

H

8

+ O

2

 2CO

2

+ 4H

2

CO O

2

+ H

2

O  H

2

CO

3 Synthesis Reactions

Which of the following reactions is a decomposition

reaction? (Click the box with the correct answer.)

2HCl  H 2

• Cl 2 2Fe + 6NaBr  2FeBr 3
• 6 Na

C

2

H

8

+ O

2

 2CO

2

+ 4H

2

CO O

2

+ H

2

O  H

2

CO

3

Incorrect. Try again:

Synthesis Reactions

Decomposition Reactions

In a decomposition reaction, one large reactant compound breaks down (or decomposes) into two or more smaller substances. A decomposition reaction is the opposite of a synthesis reaction. Using this information, try to write a general formula for a decomposition reaction before clicking the button below. Click for the General Formula for a Decomposition Reaction Click on this icon for audio

In a decomposition reaction, one large reactant compound breaks down (or decomposes) into two or more smaller substances. The general formula for a decomposition reaction can be written as follows: Where the compound AB is the reactant which decomposes into the products A and B. More on Decomposition Reactions Decomposition Reactions Click on this icon for audio

Reactants in a decomposition reaction can decompose into simple elements, more complex compounds or combinations of the two. For example, potassium chloride (KCl) decomposes into solid potassium and chloride gas when electricity is used to provide energy. 2KCl (s)  2K (s)

• Cl 2 (g) Other decomposition reactions involve the breakdown of reactants into both elements and compounds, such as the decomposition of potassium chlorate (KClO 3 ) into potassium chloride (KCl (s) ) and oxygen gas (O 2 (g)

More Decomposition Reactions Decomposition Reactions Click on this icon for audio

Decomposition reactions are used in many types of explosives such as TNT. Typically, for a decomposition reaction to occur, energy is needed to get the reaction started, this is why, for example, for a stick of TNT to explode a fuse must be lit. This fuse provides the energy to allow the decomposition reaction to begin, resulting in a flaming explosion. Pharaoh’s Serpent – Decomposition Reaction Decomposition Reactions Click on this icon for audio

A good example of a decomposition reaction is commonly referred to as the pharaoh’s serpent reaction in which a white powder made up of mercury thiocyanate is ignited resulting in a visually stunning decomposition reaction. Click for the Reaction Equation Decomposition Reactions Click on this icon for audio

In the pharaoh’s serpent reaction the large, complex molecule mercury thiocyanate, (Hg(SCN) 2 ) decomposes into carbon nitride (C 3

N

4 ), mercury sulfide (HgS) and carbon disulfide (CS 2 ) when energy is provided in the form of a flame or spark. Try to write the balanced equation for this reaction on your own, then click the button below to view it. Pharaoh’s Serpent Reaction Equation Decomposition Reactions Click on this icon for audio

Test your understanding: Which of the following reactions is a decomposition reaction? (Click the box with the correct answer.) 2HCl  H 2

• Cl 2 2Al + 3Br 2  2AlBr 2 CaO + H 2 O  Ca(OH) 2

P

4

+ 5O

2

 2P

2

O

5 Decomposition Reactions

Which of the following reactions is a decomposition

reaction? (Click the box with the correct answer.)

2HCl  H 2

• Cl 2 2Al + 3Br 2  2AlBr 2 CaO + H 2 O  Ca(OH) 2

P

4

+ 5O

2

 2P

2

O

5

Incorrect. Try again:

Decomposition Reactions