Types of Chemical Reactions

Chemical reactions can be broadly classified into four major categories based on how the atoms and molecules interact.

Part of: Class 10 Science Notes - Chemical Reactions and Equations

1. Combination Reaction (Synthesis)

A Combination Reaction, also known as a Synthesis Reaction, is a process where two or more simple substances (elements or compounds) combine to form a single, more complex compound.

General Formula: $A + B \to A B$
Energy Profile: These reactions often release energy, making them exothermic (though there are exceptions).
Etymology: The term "Synthesis" is used because it means "to build" or "to put together".
Examples:
Formation of Water: $2 H_{2} + O_{2} \to 2 H_{2} O$
Formation of Salt: $2 N a + C l_{2} \to 2 N a C l$

2. Decomposition Reaction

A Decomposition Reaction occurs when a single complex compound breaks down into two or more simpler substances.

General Formula: $A B \to A + B$
Energy Profile: These are mostly endothermic because energy is required to break the existing chemical bonds. Note that some exceptions exist, such as ozone decomposing into oxygen, which is exothermic.
Classification by Energy Source:

Type	Mechanism	Example
Thermal	Heat causes the breakdown.	$C a C O_{3} Δ C a O + C O_{2}$
Electrolytic	Electricity splits the compound.	$2 H_{2} O elec. 2 H_{2} + O_{2}$
Photolysis	Light (Solar) energy triggers decomposition.	$2 A g C l (s) h ν 2 A g (s) + C l_{2} (g)$

3. Displacement Reaction (Single Displacement)

A Displacement Reaction is a chemical process where a more reactive element replaces a less reactive element within a compound.

General Formula: $A + B C \to A C + B$
Underlying Reason: This occurs because the more reactive element has a stronger tendency to gain or lose electrons, leading to a more stable product.
Example: Iron reacting with Copper Sulfate:
$F e (s) + C u S O_{4} (a q) \to F e S O_{4} (a q) + C u (s)$
Common Applications: Metal extraction, thermite welding, water purification, and the production of antacids.

4. Double Displacement Reaction

A Double Displacement Reaction involves an exchange of ions between two reacting compounds to form two new compounds.

General Formula: $A B + C D \to A D + C B$
Characteristics: These reactions often result in the formation of a precipitate (an insoluble solid).
Example: Sodium Sulfate reacting with Barium Chloride:
$N a_{2} S O_{4} + B a C l_{2} \to B a S O_{4} (s) + 2 N a C l (a q)$
(Note: $B a S O_{4}$ is the precipitate in this reaction)

5. Important Note on Reactivity

All displacement reactions are governed by the Reactivity Series. A reaction will only occur if the standalone element is higher on the reactivity series than the element it is trying to replace.

Here are some frequently asked questions (FAQs) based on your notes, organized by the three specific sections you've created.

6. Frequently Asked Questions (FAQs)

Q: How can I tell the difference between a Combination and a Decomposition reaction?

Feature	Combination Reaction (Synthesis)	Decomposition Reaction
Basic Definition	A process where two or more substances combine to form a single compound.	A process where a single complex compound breaks down into two or more simpler substances.
General Formula	$A + B \to A B$	$A B \to A + B$
Number of Reactants	Multiple (two or more).	Single reactant.
Number of Products	Single product.	Multiple (two or more).
Energy Change	Often Exothermic (releases energy).	Mostly Endothermic (absorbs energy to break bonds).
Examples	$2 H_{2} + O_{2} \to 2 H_{2} O$	$C a C O_{3} Δ C a O + C O_{2}$

Q: Are all decomposition reactions the same?

No, they are classified by the energy source used to break the bonds:
Thermal: Uses heat.
Electrolytic: Uses electricity.
Photolysis: Uses light (solar) energy.

Q: What is the main difference between Single and Double Displacement?

In Single Displacement, a lone, more reactive element "kicks out" a less reactive one from a compound ( $A + B C \to A C + B$ ). In Double Displacement, two compounds "swap partners" to form two entirely new compounds ( $A B + C D \to A D + C B$ ), often forming a solid precipitate.

Types of Chemical Reactions Class 10 Notes CBSE Science