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Chapter 1: Chemical Reactions and Equations
1. Introduction
All around us, different changes keep taking place. Some of these are physical changes, like melting of ice or dissolving of sugar, where no new substance is formed. Others are chemical changes, in which one or more new substances with different properties are formed.
For example, when magnesium ribbon burns in air, it forms a new substance called magnesium oxide. This process is known as a chemical reaction.
2. What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances (called reactants) change to form new substances (called products).
Example:
Magnesium reacts with oxygen to form magnesium oxide.
Magnesium + Oxygen → Magnesium oxide
Mg + O2 → 2MgO
Here, magnesium and oxygen are reactants, and magnesium oxide is the product.
3. Chemical Equations
A chemical equation is a short, symbolic way of representing a chemical reaction using chemical formulae.
Example:
Zn + H2SO4 → ZnSO4 + H2
In this equation:
- Reactants → Zn and H2SO4
- Products → ZnSO4 and H2
This shows that zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.
4. Balanced and Unbalanced Chemical Equations
When we write a chemical equation, it must follow the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total number of atoms of each element should be the same on both sides.
Unbalanced Equation (Skeleton Equation)
Fe + H2O → Fe3O4 + H2
Here, the number of atoms of iron, hydrogen, and oxygen are not equal on both sides.
Balanced Equation
To balance, we adjust coefficients so that each element has equal atoms on both sides.
3Fe + 4H2O → Fe3O4 + 4H2
Now, the equation is balanced.
5. Steps to Balance a Chemical Equation
- Write the unbalanced equation.
- Count the number of atoms of each element on both sides.
- Start balancing with the element that occurs in the smallest number.
- Balance metals first, then non-metals, and finally hydrogen and oxygen.
- Use whole numbers as coefficients.
Check again that all elements are balanced
6. Types of Chemical Reactions
(i) Combination Reaction
Two or more reactants combine to form a single product.
General form:
A + B → AB
Example:
2H2 + O2 → 2H2O
Another example:
CaO + H2O → Ca(OH)2
This is also known as slaking of lime, which produces heat (an exothermic reaction).
(ii) Decomposition Reaction
A single reactant breaks down into two or more simpler products. Energy in the form of heat, light, or electricity is required.
General form:
AB → A + B
Examples:
Thermal decomposition (by heat):
2Pb(NO3)2 → 2PbO + 4NO2 + O2
Electrolytic decomposition (by electricity):
2H2O → 2H2 + O2
Photochemical decomposition (by light):
2AgCl → 2Ag + Cl2
(This reaction occurs in sunlight and is used in black-and-white photography.)
(iii) Displacement Reaction
A more reactive element displaces a less reactive element from its compound.
General form:
A + BC → AC + B
Example:
Zn + CuSO4 → ZnSO4 + Cu
Here, zinc displaces copper from copper sulphate solution because zinc is more reactive than copper.
(iv) Double Displacement Reaction
In this reaction, two compounds exchange ions to form two new compounds.
General form:
AB + CD → AD + CB
Example:
Na2SO4 + BaCl2 → BaSO4 + 2NaCl
In this reaction, a white insoluble solid (BaSO4) is formed, known as a precipitate. Hence, it is also called a precipitation reaction.
(v) Oxidation and Reduction Reactions (Redox Reactions)
Oxidation is the process in which a substance gains oxygen or loses hydrogen.
Reduction is the process in which a substance loses oxygen or gains hydrogen.
A reaction that involves both is called a Redox Reaction.Example:
CuO + H2 → Cu + H2O
Here:
- CuO is reduced to Cu (loss of oxygen).
- H2 is oxidized to H2O (gain of oxygen).
7. Effects of Oxidation in Daily Life
(i) Corrosion
Corrosion is the slow destruction of metals by reaction with air, moisture, or chemicals present in the environment.
Example:
Rusting of iron.4Fe + 3O2 + 6H2O → 4Fe(OH)3
Over time, hydrated ferric oxide (Fe2O3·xH2O) forms, which we see as rust.
Prevention of Corrosion:
- Painting or applying grease
- Galvanization (coating with zinc)
- Electroplating
Using corrosion-resistant materials
(ii) Rancidity
Rancidity is the process in which oils and fats get oxidized, leading to an unpleasant smell and taste.
Prevention:
- Storing food in airtight containers
- Adding antioxidants (like BHA, BHT)
- Refrigeration
- Using nitrogen gas in food packages
8. Exothermic and Endothermic Reactions
- (Burning of fuels)
Exothermic Reaction: Heat energy is released during the reaction.
Example:
C + O2 → CO2 + heat Endothermic Reaction: Heat energy is absorbed from the surroundings.
Example:
CaCO3 → CaO + CO2- (Burning of fuels)
9. Importance of Chemical Reactions
- Formation of new materials like plastics, medicines, fertilizers, etc.
- Energy production through combustion.
- Biological processes like respiration and digestion are chemical reactions.
- Industrial manufacturing of acids, bases, and salts.
10. Summary
- Chemical reactions change reactants into products.
- Chemical equations symbolically represent these reactions.
- They must be balanced to follow the Law of Conservation of Mass.
- Main types of reactions: Combination, Decomposition, Displacement, Double Displacement, and Redox.
- Oxidation plays a major role in corrosion and rancidity.
- Chemical reactions can be exothermic or endothermic.
✅ Oswaal Class 10 SST — Topper's Choice
✅ Together With SST — Best for Practice
✅ Oswaal Science Class 10 — Chapter Wise
✅ RD Sharma Maths — Most Popular
✅ Oswaal Class 10 SST — Topper's Choice
✅ Together With SST — Best for Practice
✅ Oswaal Science Class 10 — Chapter Wise
✅ RD Sharma Maths — Most Popular