Grade 8 → Chemical Reactions and Stoichiometry ↓
Types of Chemical Reactions: Combination, Decomposition, Displacement and Redox
Introduction to chemical reactions
Chemical reactions are processes where substances, called reactants, change into different substances, called products. These processes occur all around us and are essential to life. Understanding the different types of chemical reactions helps us understand how substances change and interact with each other.
Combination reactions
Combination reactions, also called synthesis reactions, occur when two or more reactants combine to form a single product. These reactions may involve elements or compounds.
General formula
A + B → AB
Here A
and B
are reactants and AB
is the product.
Example
When hydrogen gas reacts with oxygen gas, water is formed:
2H 2 + O 2 → 2H 2 O
In this reaction, two molecules of hydrogen (H2) combine with one molecule of oxygen (O2) to form two molecules of water (H2O).
In the SVG visualization above, the red and blue circles represent different reactants that combine to form a product.
Decomposition reactions
Decomposition reactions are the opposite of combination reactions. In these reactions, a single compound breaks down into two or more simpler substances.
General formula
AB → A + B
Here AB
is the reactant which breaks down into A
and B
Example
Water can be decomposed into hydrogen and oxygen gases through electrolysis:
2H 2 O → 2H 2 + O 2
Here, water (H2O) breaks down into hydrogen gas (H2) and oxygen gas (O2).
In this SVG illustration, the rectangle represents a compound being decomposed into its individual components, represented by the circle.
Displacement reactions
Displacement reactions, also called substitution reactions, occur when an element reacts with a compound and replaces another element in that compound. There are mainly two types of displacement reactions: single displacement and double displacement.
Single displacement reaction
In single displacement reactions, a free element replaces another element in a compound.
General formula
A + BC → AC + B
Here, in the compound BC
, A
displaces B
to form AC
and releases B
Example
When zinc reacts with hydrochloric acid, zinc chloride and hydrogen gas are produced:
Zn + 2HCl → ZnCl 2 + H 2
Zinc (Zn) displaces hydrogen (H) in hydrochloric acid (HCl) to form zinc chloride (ZnCl2) and hydrogen gas (H2).
Double displacement reaction
In double displacement reactions, the ions of two compounds exchange places in aqueous solution to form two new compounds.
General formula
AB + CD → AD + CB
The ions of compounds AB
and CD
interchange places to form new compounds AD
and CB
.
Example
When solutions of sodium sulfate and barium chloride are mixed, barium sulfate and sodium chloride are formed:
Na 2 SO 4 + BaCl 2 → BaSO 4 + 2NaCl
The sodium ions (Na+) and barium ions (Ba2+) exchange places between their respective compounds.
This SVG shows an exchange between ions. The circles change position, representing the exchange, just as the ions do in double displacement.
Redox reactions
Redox reactions are chemical reactions that involve the transfer of electrons between two substances. The term 'redox' is a combination of 'reduction' and 'oxidation'
Oxidation
Oxidation is the loss of electrons by a molecule, atom, or ion during a reaction.
Reduction
Reduction is the gain of electrons by a molecule, atom, or ion during a reaction.
Example of a redox reaction
Consider the reaction of copper oxide and hydrogen:
CuO + H 2 → Cu + H 2 O
In this reaction, copper oxide (CuO) is reduced to copper (Cu) by losing oxygen, and hydrogen (H2) is oxidized to water (H2O) by gaining oxygen.
Visualization of redox reactions
This SVG diagram shows a redox reaction, in which substances are transformed, indicated by arrows showing the movement of electrons involved in the reaction.
Summary
Understanding the different types of chemical reactions – combination, decomposition, displacement, and redox – is fundamental in the study of chemistry. Each reaction type describes specific ways in which reactants are transformed into products. These concepts help balance chemical equations and apply stoichiometry to measure reactants and products in a reaction.