Chemical Reactions and Chemical Equations – Complete Notes
(For CBSE Class 9/10 Students)
1. Chemical Reaction
Definition: A chemical reaction is a process in which one or more substances called reactants undergo a chemical change to form new substances called products with different chemical properties. During a reaction, old chemical bonds break and new bonds are formed.
Examples of Chemical Reactions
1. Rusting of Iron:
4Fe + 3O2 → 2Fe2O3
2. Combustion of Methane:
CH4 + 2O2 → CO2 + 2H2O
3. Decomposition of Calcium Carbonate:
CaCO3 → CaO + CO2
4. Neutralization Reaction:
HCl + NaOH → NaCl + H2O
5. Photosynthesis:
6CO2 + 6H2O → C6H12O6 + 6O2
2. Chemical Equation
Definition: A chemical equation is the symbolic representation of a chemical reaction using chemical formulas and symbols. A balanced chemical equation follows the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction.
3. Types of Chemical Reactions
1. Combination Reaction
Definition: A combination reaction is a type of chemical reaction in which two or more substances combine to form a single product. In this reaction, the number of reactants is more than the number of products.
2H2 + O2 → 2H2O
2Mg + O2 → 2MgO
2Na + Cl2 → 2NaCl
4Fe + 3O2 → 2Fe2O3
N2 + 3H2 → 2NH3
2. Decomposition Reaction
Definition: A decomposition reaction is a type of chemical reaction in which a single compound breaks down into two or more simpler substances. This reaction usually occurs due to heat, light, or electricity.
CaCO3 → CaO + CO2
2H2O → 2H2 + O2
2HgO → 2Hg + O2
(NH4)2Cr2O7 → N2 + Cr2O3 + 4H2O
2AgCl → 2Ag + Cl2
3. Displacement Reaction
Definition: A displacement reaction is a type of chemical reaction in which a more reactive element displaces a less reactive element from its compound. It is also called a single displacement reaction.
Zn + CuSO4 → ZnSO4 + Cu
Fe + CuSO4 → FeSO4 + Cu
Mg + 2HCl → MgCl2 + H2
Cl2 + 2KBr → 2KCl + Br2
2Al + 3CuSO4 → Al2(SO4)3 + 3Cu
4. Double Displacement Reaction
Definition: A double displacement reaction is a type of chemical reaction in which two compounds react by exchanging their ions to form two new compounds. This reaction often results in the formation of a precipitate, gas, or water.
AgNO3 + NaCl → AgCl + NaNO3
BaCl2 + H2SO4 → BaSO4 + 2HCl
HCl + NaOH → NaCl + H2O
Pb(NO3)2 + 2KI → PbI2 + 2KNO3
CaCO3 + 2HCl → CaCl2 + H2O + CO2
5. Redox Reaction
Definition: A redox reaction is a chemical reaction in which oxidation and reduction take place simultaneously. Oxidation is the loss of electrons, while reduction is the gain of electrons.
4Fe + 3O2 → 2Fe2O3
2H2 + O2 → 2H2O
Zn + Cu2+ → Zn2+ + Cu
Important Concepts
Limiting Reactant: The reactant that gets completely consumed first in a chemical reaction and limits the amount of product formed is called the limiting reactant.
Law of Conservation of Mass: According to this law, mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of reactants is equal to the total mass of products.
Corrosion
Definition: Corrosion is the slow and gradual destruction of metals due to chemical reactions with air, moisture, or other substances present in the environment. It is a natural process that converts pure metals into more stable compounds such as oxides, hydroxides, or sulphides.
Example (Rusting of Iron):
4Fe + 3O2 + H2O → 2Fe2O3
Conditions Necessary for Rusting:
- Presence of oxygen
- Presence of moisture (water)
Prevention of Corrosion
- Painting: A layer of paint prevents air and moisture from coming in contact with the metal surface.
- Oiling and Greasing: Oil or grease forms a protective layer that prevents exposure to air and water.
- Galvanization: Coating iron with a layer of zinc to prevent rusting.
- Electroplating: Depositing a thin layer of another metal like chromium or nickel over the surface.
- Alloy Formation: Mixing metal with other elements to improve resistance to corrosion (e.g., stainless steel).
Precipitation and Precipitation Reaction
Precipitate
Definition: A precipitate is an insoluble solid substance that forms when two aqueous solutions react with each other. It separates out from the solution.
Precipitation Reaction
Definition: A precipitation reaction is a type of double displacement reaction in which two soluble salts react in aqueous solution to form an insoluble solid called a precipitate.
Examples of Precipitation Reactions
1. Formation of Silver Chloride (White Precipitate):
AgNO3 + NaCl → AgCl ↓ + NaNO3
2. Formation of Barium Sulphate (White Precipitate):
BaCl2 + H2SO4 → BaSO4 ↓ + 2HCl
3. Formation of Lead Iodide (Yellow Precipitate):
Pb(NO3)2 + 2KI → PbI2 ↓ + 2KNO3
Note: The downward arrow (↓) indicates the formation of a precipitate.
Rancidity
Rancidity is the oxidation of fats and oils present in food, which leads to unpleasant smell and taste.
Frequently Asked Questions (FAQs)
1. What is corrosion?
Corrosion is the slow destruction of metals due to reaction with air and moisture, such as rusting of iron.
2. What is rusting of iron?
Rusting is the formation of iron oxide when iron reacts with oxygen and water. 4Fe + 3O2 + H2O → 2Fe2O3
3. What is a precipitate?
A precipitate is an insoluble solid formed when two aqueous solutions react.
4. What is a precipitation reaction?
A precipitation reaction is a double displacement reaction in which an insoluble solid is formed.
5. Why is corrosion prevention important?
Corrosion prevention increases the life of metals and reduces economic loss.
In this article, we will learn about corrosion and precipitation reaction in detail. This chapter is important for CBSE Class 9 and Class 10 Chemistry board exams. We will understand definitions, chemical equations, prevention methods, and examples in simple language.
Related Topics: Chemical Reaction Notes | CBSE Official Chemistry Resources
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