Chemical Equations and Their Types
Complete Notes for Competitive Examinations
By Home Academy
1. Introduction to Chemical Equation
A chemical equation is the symbolic representation of a chemical reaction using chemical formulas and symbols.
It shows:
Reactants
ProductsPhysical states
Conditions of reaction
General Representation
Reactants → Products
Hydrogen reacts with oxygen to form water.
2. Components of Chemical Equation
| Component | Meaning |
|---|---|
| Reactants | Substances taking part |
| Products | Substances formed |
| Arrow (→) | Direction of reaction |
| + Sign | Separates substances |
| Coefficient | Number of molecules |
| State Symbols | Physical state |
3. Physical State Symbols
| Symbol | Meaning |
|---|---|
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Aqueous solution |
Example with States
CaCO_3(s) \rightarrow CaO(s) + CO_2(g)
4. Characteristics of Chemical Equation
Must obey law of conservation of mass
Number of atoms remains equal on both sidesShould be balanced
Represents chemical change
5. Balanced and Unbalanced Equations
Unbalanced Equation
H_2 + O_2 \rightarrow H_2O
Atoms unequal.
Balanced Equation
2H_2 + O_2 \rightarrow 2H_2O
Atoms equal on both sides.
6. Types of Chemical Reactions
| Type | Description |
|---|---|
| Combination Reaction | Two or more substances combine |
| Decomposition Reaction | Compound breaks down |
| Displacement Reaction | One element replaces another |
| Double Displacement Reaction | Exchange of ions |
| Redox Reaction | Oxidation and reduction together |
| Neutralization Reaction | Acid + Base reaction |
| Combustion Reaction | Burning in oxygen |
7. Combination Reaction
Definition
Two or more reactants combine to form a single product.
Important Equations
Formation of Water
2H_2 + O_2 \rightarrow 2H_2O
Formation of Calcium Oxide
2Ca + O_2 \rightarrow 2CaO
Important Points
Single product formed
Usually exothermic
8. Decomposition Reaction
Definition
A single compound breaks into simpler substances.
Types of Decomposition
| Type | Cause |
|---|---|
| Thermal | Heat |
| Electrolytic | Electricity |
| Photolytic | Light |
Important Equations
Thermal Decomposition
CaCO_3 \xrightarrow{heat} CaO + CO_2
Electrolysis of Water
2H_2O \xrightarrow{electricity} 2H_2 + O_2
Photolytic Decomposition
2AgCl \xrightarrow{sunlight} 2Ag + Cl_2
9. Displacement Reaction
Definition
A more reactive element displaces a less reactive element.
Important Equation
Zn + CuSO_4 \rightarrow ZnSO_4 + Cu
Important Points
Based on reactivity series
More reactive metal displaces less reactive metal
10. Double Displacement Reaction
Definition
Exchange of ions between compounds.
Important Equation
AgNO_3 + NaCl \rightarrow AgCl + NaNO_3
Types
| Type | Example |
|---|---|
| Precipitation | Formation of insoluble salt |
| Neutralization | Acid + Base |
11. Neutralization Reaction
Definition
Reaction between acid and base producing salt and water.
Important Equation
HCl + NaOH \rightarrow NaCl + H_2O
Important Points
Produces salt and water
Usually exothermic
12. Combustion Reaction
Definition
Substance burns in oxygen producing heat and light.
Important Equation
CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O
Important Points
Oxygen required
Produces heat energy
13. Redox Reaction
Definition
Oxidation and reduction occur simultaneously.
Oxidation
Addition of oxygen
Removal of hydrogen
Loss of electrons
Reduction
Removal of oxygen
Addition of hydrogen
Gain of electrons
Important Equation
CuO + H_2 \rightarrow Cu + H_2O
14. Exothermic and Endothermic Reactions
| Exothermic | Endothermic |
|---|---|
| Heat released | Heat absorbed |
| Respiration | Photosynthesis |
Respiration
C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + Energy
Photosynthesis
6CO_2 + 6H_2O \xrightarrow{sunlight} C_6H_{12}O_6 + 6O_2
15. Corrosion and Rancidity
Corrosion
Slow eating away of metals due to air and moisture.
Example
Rusting of iron
4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O
Rancidity
Oxidation of fats and oils causing bad smell and taste.
Prevention of Rancidity
Refrigeration
Airtight containers
Antioxidants
16. Important Equations for Exams
Water Formation
2H_2 + O_2 \rightarrow 2H_2O
Ammonia Formation
N_2 + 3H_2 \rightarrow 2NH_3
Rusting of Iron
4Fe + 3O_2 \rightarrow 2Fe_2O_3
Electrolysis of Water
2H_2O \rightarrow 2H_2 + O_2
Burning of Magnesium
2Mg + O_2 \rightarrow 2MgO
17. Important Exam Points
Chemical equation must be balanced.
Reactants are written left side.
Products are written right side.
Combination reaction forms one product.
Decomposition breaks compounds.
Oxidation = gain of oxygen.
Reduction = loss of oxygen.
Rusting is oxidation.
Respiration is exothermic.
Photosynthesis is endothermic.
18. Frequently Asked MCQs
1. Reaction in which two substances combine:
A. Decomposition
B. Combination
C. Displacement
D. Neutralization
✅ Answer: B
2. Which reaction involves exchange of ions?
A. Combination
B. Double displacement
C. Combustion
D. Decomposition
✅ Answer: B
3. Rusting of iron is:
A. Reduction
B. Neutralization
C. Oxidation
D. Combustion
✅ Answer: C
4. Which gas is produced during electrolysis of water?
A. Nitrogen
B. Oxygen and hydrogen
C. Chlorine
D. Carbon dioxide
✅ Answer: B
5. Photosynthesis is:
A. Exothermic
B. Endothermic
C. Neutralization
D. Redox only
✅ Answer: B
6. Which reaction produces salt and water?
A. Combustion
B. Neutralization
C. Combination
D. Decomposition
✅ Answer: B
19. Fact-Based Questions
Question 1
Consider the following statements:
Balanced equations obey conservation of mass.
Rusting is a chemical change.
Photosynthesis releases heat.
A. Only 1 and 2
B. Only 2 and 3
C. Only 1 and 3
D. 1, 2 and 3
✅ Answer: A
Question 2
Which of the following are decomposition reactions?
Electrolysis of water
Heating of calcium carbonate
Formation of ammonia
A. Only 1 and 2
B. Only 2 and 3
C. Only 1 and 3
D. 1, 2 and 3
✅ Answer: A
Question 3
Consider the following:
Oxidation involves gain of oxygen.
Reduction involves gain of hydrogen.
Neutralization forms salt and water.
A. Only 1 and 2
B. Only 2 and 3
C. 1, 2 and 3
D. Only 1 and 3
✅ Answer: C
20. Previous Year Type Questions (PYQs)
PYQ 1
Which type of reaction is represented by:
Zn + CuSO_4 \rightarrow ZnSO_4 + Cu
A. Combination
B. Decomposition
C. Displacement
D. Neutralization
✅ Answer: C
PYQ 2
The reaction between acid and base is called:
A. Combustion
B. Neutralization
C. Redox
D. Displacement
✅ Answer: B
PYQ 3
Which reaction is used in photography?
A. Thermal decomposition
B. Photolytic decomposition
C. Neutralization
D. Combination
✅ Answer: B
21. Assertion–Reason Questions
Assertion (A)
Rusting of iron is oxidation.
Reason (R)
Iron combines with oxygen and moisture.
A. Both A and R are true and R explains A
B. Both true but R not explanation
C. A true, R false
D. A false, R true
✅ Answer: A
Assertion (A)
Photosynthesis is an endothermic reaction.
Reason (R)
It absorbs sunlight energy.
A. Both A and R are true and R explains A
B. Both true but R not explanation
C. A true, R false
D. A false, R true
✅ Answer: A
22. Quick Revision Table
| Reaction Type | Example |
|---|---|
| Combination | H₂ + O₂ → H₂O |
| Decomposition | CaCO₃ → CaO + CO₂ |
| Displacement | Zn + CuSO₄ → ZnSO₄ + Cu |
| Neutralization | HCl + NaOH → NaCl + H₂O |
| Combustion | CH₄ + O₂ → CO₂ + H₂O |
Conclusion
Chemical equations and their types form the foundation of chemistry. Questions from balancing equations, reaction types, oxidation-reduction, corrosion, and important reactions are repeatedly asked in competitive examinations. Understanding important equations and concepts is essential for scoring high marks.
Prepared By Home Academy