Chemical Reaction and Equation Class 10 || Science|| Chapter 1 Notes

Chemical Reaction and Equation Class 10 ||Science|| Chapter 1 Notes

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1. Introduction to Chemical Reactions

a) What is a Chemical Reaction?

• A chemical reaction is a process in which one or more substances, known as reactants, are transformed into new substances, called products.
• Indicators of a chemical reaction include:
  • Change in color
  • Evolution of gas
  • Formation of precipitate
  • Change in temperature
  • Change in state

b) Examples of Chemical Reactions

• Burning of coal: $\text{C (Coal)}+{\text{O}}_2\to {\text{CO}}_2$

• Rusting of iron: $\text{Fe}+{\text{O}}_2+{\text{H}}_2\text{O}\to {\text{Fe}}_2{\text{O}}_3\cdot {\text{xH}}_2\text{O}$

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2. Chemical Equations

a) Representation of Chemical Reactions

• A chemical equation represents a chemical reaction using symbols and formulas of the substances involved.

  • Reactants: Substances that undergo the reaction, written on the left side.
  • Products: New substances formed, written on the right side.

• Example: $\text{Zn}+{\text{H}}_2{\text{SO}}_4\to {\text{ZnSO}}_4+{\text{H}}_2$

  • Zinc reacts with sulfuric acid to form zinc sulfate and hydrogen gas.

b) Balanced Chemical Equations

• A chemical equation must be balanced, meaning the number of atoms of each element on the reactant side must equal the number on the product side.

• Example of a balanced equation: ${\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}$

  • Here, 2 molecules of hydrogen react with 1 molecule of oxygen to form 2 molecules of water.

c) Steps to Balance a Chemical Equation

1. Write the unbalanced equation.
2. List the number of atoms of each element on both sides.
3. Use coefficients to balance the atoms of each element, starting with the elements that appear the least.
4. Check to ensure all elements are balanced.
5. Rewrite the equation with correct coefficients.

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3. Types of Chemical Reactions

a) Combination Reaction

• Two or more reactants combine to form a single product.
  • Example: $\text{CaO}+{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2$
    • Quicklime reacts with water to form slaked lime.

b) Decomposition Reaction

• A single reactant breaks down into two or more products.

  • Example: $2{\text{H}}_2\text{O}\stackrel{\text{Electricity}}{\to }2{\text{H}}_2+{\text{O}}_2$
    • Water decomposes into hydrogen and oxygen gas.

  Types of decomposition:

  1. Thermal decomposition: Breakdown due to heat.
  2. Electrolytic decomposition: Breakdown using electricity.
  3. Photolytic decomposition: Breakdown using light.

c) Displacement Reaction

• A more reactive element displaces a less reactive element from its compound.
  • Example: $\text{Fe}+{\text{CuSO}}_4\to {\text{FeSO}}_4+\text{Cu}$
    • Iron displaces copper from copper sulfate to form iron sulfate and copper.

d) Double Displacement Reaction

• Two compounds exchange ions to form new compounds.
  • Example: ${\text{Na}}_2{\text{SO}}_4+{\text{BaCl}}_2\to {\text{BaSO}}_4+2\text{NaCl}$
    • Sodium sulfate reacts with barium chloride to form barium sulfate (a precipitate) and sodium chloride.

e) Oxidation and Reduction Reactions

• Oxidation: A reaction in which a substance gains oxygen or loses hydrogen.

• Reduction: A reaction in which a substance loses oxygen or gains hydrogen.

  • Example of oxidation: $\text{2Cu}+{\text{O}}_2\to 2\text{CuO}$

    • Copper is oxidized to copper oxide.

  • Example of reduction: $\text{CuO}+{\text{H}}_2\to \text{Cu}+{\text{H}}_2\text{O}$

    • Copper oxide is reduced to copper while hydrogen is oxidized to water.

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4. Effects of Oxidation in Everyday Life

a) Corrosion

• Corrosion is the process in which metals are gradually degraded due to reactions with environmental elements like oxygen, moisture, etc.

  • Example: Rusting of iron.

  $4\text{Fe}+3{\text{O}}_2+{\text{H}}_2\text{O}\to 2{\text{Fe}}_2{\text{O}}_3\cdot {\text{xH}}_2\text{O}$

  • Prevention of Corrosion:
    1. Painting or coating with another metal (galvanization).
    2. Using oil or grease.

b) Rancidity

• Rancidity occurs when fats and oils in food are oxidized, leading to unpleasant taste and smell.

  • Prevention of Rancidity:
    1. Adding antioxidants.
    2. Storing food in airtight containers.
    3. Refrigeration.

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5. Importance of Chemical Reactions

• Energy Production: Chemical reactions release energy, such as in combustion or respiration.
• Industrial Applications: Reactions are essential for producing everyday materials like cement, plastics, and medicines.
• Environmental Impact: Understanding reactions helps manage issues like pollution and waste treatment.

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6. Summary

• Chemical reactions involve the transformation of reactants into products, with various indicators like color change and gas evolution.
• Chemical equations represent these reactions and must be balanced to obey the law of conservation of mass.
• There are several types of reactions, including combination, decomposition, displacement, double displacement, and redox reactions.
• The concepts of corrosion and rancidity demonstrate the impact of oxidation in daily life and how it can be controlled.

This chapter lays the foundation for understanding how substances interact and change in both natural and industrial processes.