Chapter 5 - Decomposition Reactions

• Scheikunde
• Middelbare school
• havo
• Leerjaar 3

Decomposition reactions

Chapter 5

What are we going to do this week?

During class

Lesson 1:   Demo pH of cleaning agents

Lesson 2:  Start chapter 5: decomposition reactions

Homework (for the entire week)

Read 5.1 and 5.2 and make the exercises

After this lesson you can...

• Explain what a decomposition reaction is
• Read reaction profile diagrams
  
• Name the different types of decomposition reactions

How do you recognise a decomposition reaction?

A decomposition reaction is a chemical reaction in which two or more products are formed from one reactant.

The reaction products are often elements, but can sometimes be compounds!

Is the following a decomposition reaction?

limestone (s) quicklime (s) + carbon dioxide (g)

1. Yes
2. No

Is the following a decomposition reaction?

limestone (s) + quicklime (s) carbon dioxide (g)

1. Yes
2. No

Write down the decomposition reaction of water.

Reaction profiles

Reactions need a energy in order for them to start: activation energy

Is the amount of activation energy higher than the energy that is released         endothermic

Is the amount of activation energy lower than the energy that is released         exothermic

Choose the exothermic reaction profile.

1. 
2. 

Types of decomposition reactions

• Thermal decomposition (thermolysis)

• Electrical decomposition (electrolysis)

• Decomposition by light (photolysis)
  

Thermal decomposition

An example of thermal decomposition is the production of iron.

2 Fe₂O₃(s)            4 Fe(s) + 3 O₂ (g)          when heated to 2500°C

Another example is the cracking of the naptha fraction after the destillation of crude oil. Heating the naptha fraction

Thermal decomposition

Another example is the cracking of the naphtha fraction after the destillation of crude oil.

Heating the naphtha fraction to 800-900°C, smaller molecules are formed.

Electrical decomposition

An example of electrolysis is the decomposition

of water.

Hydrogen has a positive charge and will go to

the negative pole (cathode).

Oxygen has a negative charge and will go to the

positive pole (anode).

Decomposition by light

Dyes in curtains can be decomposed by sunlight, causing them to fade. This is an example of photolysis.

Is electrolysis an endothermic or exothermic reaction? Explain your answer.

0

Ammonium dichromate --> chromium(III)oxide + nitrogen + water

 (NH4)2Cr2O7 → Cr2O3 + N2 + 4 H2O

Was this decomposition reaction endothermic or exothermic?

1. Endothermic
2. Exothermic

After this lesson you can...

• Explain what a decomposition reaction is
• Read reaction profile diagrams
  
• Name the different types of decomposition reactions

Homework

You can now start on your homework for this week:

Read 5.1 and 5.2 and make the exercises