4.2 Decomposition
In 1632, Rembrandt painted "The Anatomy Lesson of Dr. Nicolaes Tulp."
The painting shows the dissection of a corpse's arm with a sharp knife.
This dissection gives us insights into how the human body is structured.
Unlike biological dissection, chemistry dissects substances to learn about their molecular structures.
However, sharp knives aren’t used in chemistry for this purpose.
Learning Objectives
Understand the definition and principles of decomposition reactions.
Identify various types of decomposition reactions and their energy effects.
Learn to write chemical equations for decomposition reactions.
Complete practice assignments (13 to 15, 17, 16 and 18, 19 to 22).
Decomposition Reactions
Definition and Characteristics
A decomposition reaction is when:
Only one reactant is involved.
This reactant is always a compound that breaks down into two or more products.
These substances are called decomposable substances.
Example: To extract pure magnesium (Mg) from magnesium chloride (MgCl2):
Magnesium chloride decomposes into chlorine (Cl2) and magnesium:
Reaction: MgCl2 → Mg + Cl2
Non-decomposable substances are made from a single type of atom and can’t be broken down further.
Comparison with Other Reactions
Decomposition reactions don’t need oxygen, unlike combustion reactions.
Example: Yeast cells process glucose (C6H12O6):
With oxygen: C6H12O6 + O2 → CO2 + H2O
Without oxygen:
Yeast breaks down glucose into carbon dioxide (CO2) and alcohol (C2H6O) while brewing beer.
Decomposition is different from separation:
Decomposition creates new substances from a compound (like yeast turning glucose into alcohol).
Separation is just pulling parts from a mixture (like chalk settling out of water).
Most decomposition reactions are endothermic, meaning they require energy.
Example: When we heat limestone (CaCO3) in lime kilns, it breaks down into quicklime (CaO) and carbon dioxide (CO2):
Reaction: CaCO3 → CaO + CO2
We need to keep supplying heat for this to happen.
Types of decomposition reactions based on energy sources:
Thermal Decomposition: uses heat.
Electrolysis: uses electricity to break down compounds.
Example: Making aluminum from bauxite (Al2O3).
Photolysis: uses light.
Sometimes causes unwanted reactions, like ruining flavors in beer if it’s exposed to light.
Some decomposition reactions are exothermic, which means they release energy.
Example: Sodium azide (NaN3) in airbags decomposes to form nitrogen gas that inflates the airbag in an accident:
Reaction: 2NaN3 → 3N2 + Na
It only needs a small amount of energy to start, like combustion.
Writing chemical equations for these reactions:
A decomposition reaction has one reactant on the left and two or more products on the right.
Example 1: Photolysis of Silver Chloride:
2AgCl → 2Ag + Cl2
Example 2: Decomposition of Smelling Salt (N2H2CO3):
Starting: N2H2CO3 → 2NH3 + CO2 + X
X is found to be water (H2O):
Final equation: N2H2CO3 → 2NH3 + CO2 + H2O
Historical example: Decomposition of Nitro-Glycerine
Nitro-glycerine breaks down to produce water vapor, nitrogen, carbon dioxide, and oxygen, creating a large pressure wave.
It needs just a little bit of energy to start breaking down (like being shaken lightly).
Invention of Dynamite:
In 1866, Alfred Nobel made dynamite by mixing nitro-glycerine with kieselguhr (a type of diatomaceous earth).
This mixture made it safe to handle as it prevented the nitro-glycerine from breaking down too easily.
Nobel profited greatly as it was used for construction and warfare.
Legacy of Alfred Nobel:
Later, Nobel felt guilty about the destructive uses of his invention.
He established the Nobel Prizes to ensure that his wealth would support good contributions to humanity.
Energy Effects in Decomposition Reactions
Endothermic Reactions
Most decomposition reactions are endothermic, meaning:
They need energy input to occur.
Example: Heating limestone (CaCO3) in lime kilns breaks it down into quicklime (CaO) and carbon dioxide (CO2):
Reaction: CaCO3 → CaO + CO2
Heat needs to be supplied continuously for this to happen.
Types of Decomposition Using Energy Sources
Thermal Decomposition: uses heat as energy.
Electrolysis: needs electrical current to break down compounds:
Example: Producing aluminum from bauxite (Al2O3).
Photolysis: uses light as energy.
Unwanted reactions can happen, like breaking down flavors in beer due to light exposure.
Exothermic Reactions
Some decomposition reactions are exothermic, releasing energy.
Example: Sodium azide (NaN3) decomposes in car airbags to produce nitrogen gas that inflates the airbag during a collision:
Reaction: 2NaN3 → 3N2 + Na
Needs a little energy to start, similar to combustion.
Writing Chemical Equations for Decomposition Reactions
General Approach
A decomposition reaction is marked by one reactant on the left side of the arrow and two or more products on the right-side, which can either be decomposable or non-decomposable substances.
Example 1: Photolysis of Silver Chloride
Write the equation for photolysis of silver chloride (AgCl):
Only the reactant formula goes on the left of the arrow:
2AgCl → 2Ag + Cl2
Example 2: Decomposition of Smelling Salt
Write the equation for the decomposition of smelling salt (N2H2CO3):
Starting position:
N2H2CO3 → 2NH3 + CO2 + X
Determine what substance X is:
Found to be water (H2O):
Final equation:
N2H2CO3 → 2NH3 + CO2 + H2O
Historical Example: Alfred Nobel and Dynamite
Decomposition of Nitro-Glycerine
Nitro-glycerine, an explosive, decomposes to form:
Water vapor, nitrogen, carbon dioxide, and oxygen, creating a huge pressure wave.
It required a small amount of energy to start the breakdown (like slight shaking).
Invention of Dynamite
In 1866, chemist Alfred Nobel invented dynamite by mixing nitro-glycerine with kieselguhr (a kind of diatomaceous earth):
This mix prevented nitro-glycerine from breaking down on its own, allowing for safe handling.
Nobel became very rich from this invention since it was used in construction (like tunneling) and wars.
Legacy of Alfred Nobel
Nobel felt guilty about how his invention was used destructively.
He set up the Nobel Prizes to ensure his money recognized good contributions to humanity.
Summary of Key Concepts
A decomposition reaction means a single compound reactant produces two or more products (either decomposable or non-decomposable).
Most decomposition reactions are endothermic, need energy inputs (thermal, electrical, or light).
Chemical equations for decomposition show one formula on the reactant side and several products on the product side.