Grade 8
  1. Introduction to Chemistry  1.1. Nature and scope of chemistry  1.2. Importance of chemistry in daily life  1.3. Chemistry and its relation with other sciences  1.4. The role of chemistry in industry, medicine and the environment  1.5. Scientific investigation and experimental methods in chemistry
  2. Matter and its properties  2.1. Definition and classification of matter  2.2. Physical and chemical properties of matter  2.3. Law of conservation of matter  2.4. Extensive and Intensive Properties of Matter  2.5. Kinetic molecular theory of matter  2.6. States of matter: solids, liquids, gases, plasmas, and Bose-Einstein condensates  2.7. Changes in state and energy are involved  2.8. Diffusion and Brownian motion
  3. Elements, Compounds, and Mixtures  3.1. Definition and differences between elements, compounds, and mixtures  3.2. Atomic Structure and Atomic Number  3.3. Chemical symbols and formulas  3.4. Trends in the Periodic Table (Groups and Periods)  3.5. Classification of Elements: Metals, Nonmetals and Metalloids  3.6. Rare Earth Elements and Transition Metals  3.7. Types of mixtures: homogeneous and heterogeneous  3.8. Separation of Mixtures Using Physical and Chemical Methods
  4. Separation Techniques  4.1. Filtration, Sedimentation and Decantation  4.2. Evaporation and crystallization  4.3. Distillation and Fractional Distillation  4.4. Chromatography and its applications  4.5. Sublimation in the purification of compounds  4.6. The role of centrifugation in biochemical processes
  5. Atomic Structure  5.1. Dalton's atomic theory  5.2. Structure of the atom: protons, neutrons and electrons  5.3. Bohr's atomic model  5.4. Introduction to Quantum Mechanical Model  5.5. Electron Configuration and Energy Levels  5.6. Excitation and emission spectra  5.7. Isotopes and their applications
  6. Periodic Table and Chemical Trends  6.1. History and Development of the Periodic Table  6.2. Modern periodic law  6.3. Periodicity and trends in atomic and ionic sizes  6.4. Ionization Energy, Electron Affinity and Electronegativity  6.5. Introduction to Lanthanides and Actinides  6.6. Application of periodic trends in chemistry
  7. Chemical Bonding and Molecular Structure  7.1. Types of chemical bonds: ionic, covalent and metallic bonds  7.3. Polar and Nonpolar Molecules  7.4. Hydrogen bonding and its applications  7.5. Intermolecular forces: dipole-dipole, London dispersion force
  8. Chemical Reactions and Stoichiometry  8.1. Chemical Equations and Balance  8.2. Types of Chemical Reactions: Combination, Decomposition, Displacement and Redox  8.3. Introduction to stoichiometry and the mole concept  8.4. Limiting reactant in chemical equations  8.5. Reaction rate, catalyst, and factors affecting reaction rate
  9. Acids, Bases and Salts  9.1. Definition and Properties of Acids and Bases  9.2. Strong vs. Weak Acids and Bases  9.3. pH Scale and pH Calculation  9.4. Neutralization reactions  9.5. Buffer solutions and their importance  9.6. Applications of Acids, Bases and Salts in Daily Life
  10. Solutions and Solubility  10.1. Definition of Solution, Solute, and Solvent  10.2. Factors Affecting Solubility  10.3. Concentration units: molarity and molality  10.4. Saturated, unsaturated and supersaturated solutions  10.5. Concept of osmosis and reverse osmosis  10.6. Concentrative properties of solutions
  11. Gases and Gas Laws  11.1. Properties of gases  11.2. Introduction to Ideal and Real Gases  11.3. Boyle's Law, Charles' Law and Avogadro's Law  11.4. Ideal Gas Equation and Its Applications  11.5. Application of Gas Laws in Real Life
  12. Thermochemistry and energy transformation  12.1. Energy in Chemical Reactions  12.2. Exothermic vs. Endothermic Reactions  12.3. Heat and Enthalpy Changes  12.4. Calorimetry and heat transfer in reactions
  13. Metals and Nonmetals  13.1. Physical and Chemical Properties of Metals and Nonmetals  13.2. Reactivity Series of Metals  13.3. Corrosion and its prevention  13.4. Extraction of metals from ores  13.5. Uses of metals and non-metals in daily life
  14. Introduction to Organic Chemistry  14.1. Characteristics of carbon and organic compounds  14.2. Functional groups and their importance  14.3. Simple Hydrocarbons: Alkenes, Alkenes and Alkynes  14.4. Isomerism in organic compounds  14.5. Introduction to polymers and their uses
  15. Environmental Chemistry and Sustainability  15.1. Green Chemistry Principles  15.2. Effects of Chemical Pollution on Ecosystems  15.3. Acid rain and its effects  15.4. Ozone layer depletion and global warming  15.5. Waste Management and Recycling in Chemistry

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

CuO Cu H2

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.


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Types of Chemical Reactions: Combination, Decomposition, Displacement and Redox

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