Grade 8 Chapter 13: Chemical Change and Chemical Bond

Introduction

In our daily lives, we observe various changes around us. Some changes are temporary and do not alter the fundamental nature of substances (physical changes), while others are permanent and result in the formation of new substances (chemical changes). This chapter delves deeper into chemical changes, exploring how atoms rearrange to form new compounds through the formation of chemical bonds. We will learn about different types of chemical bonds, the concept of valency, and how to represent chemical changes using chemical equations.

Chemical Change and Chemical Reactions

A chemical change (or chemical reaction) is a permanent change in which the chemical composition of a substance changes, resulting in the formation of one or more new substances with entirely different properties. This process involves the breaking of existing chemical bonds and the formation of new ones.

• Examples: Burning of paper, rusting of iron, cooking food, digestion.

Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction, using chemical formulas to show the reactants and products.

• Reactants: Substances that undergo chemical change (written on the left side of the arrow).
• Products: New substances formed as a result of the chemical change (written on the right side of the arrow).
• The arrow (→) indicates the direction of the reaction.
• Example: 2H₂ (g) + O₂ (g) → 2H₂O (l)
  (Hydrogen gas + Oxygen gas → Water)

Chemical Bonds

A chemical bond is a force that holds atoms together in molecules or compounds. Atoms form chemical bonds to achieve a stable electronic configuration, typically by having 8 electrons in their outermost shell (octet rule) or 2 electrons for the first shell (duplet rule, for hydrogen and helium).

1. Ionic Bond (Electrovalent Bond)

An ionic bond is formed by the complete transfer of one or more electrons from a metal atom to a nonmetal atom. This transfer results in the formation of oppositely charged ions (cations and anions), which are then held together by strong electrostatic forces of attraction.

• Formation:
  • Metal atoms tend to lose electrons to form positively charged ions called cations.
  • Nonmetal atoms tend to gain electrons to form negatively charged ions called anions.
• Example: Formation of Sodium Chloride (NaCl)
  • Sodium (Na) (Electronic configuration: 2, 8, 1) loses 1 electron to form Na⁺ (2, 8).
  • Chlorine (Cl) (Electronic configuration: 2, 8, 7) gains 1 electron to form Cl⁻ (2, 8, 8).
  • Na⁺ and Cl⁻ ions are attracted to each other, forming an ionic bond.

Properties of Ionic Compounds

• Physical State: Generally solid and hard due to strong electrostatic forces.
• Melting and Boiling Points: High melting and boiling points.
• Solubility: Generally soluble in water.
• Conductivity: Conduct electricity in molten state or in aqueous solution (due to free-moving ions), but not in solid state.

2. Covalent Bond

A covalent bond is formed by the mutual sharing of electrons between two nonmetal atoms. Atoms share electrons to achieve a stable electronic configuration (octet or duplet rule).

• Formation: Occurs between two nonmetal atoms. Each atom contributes one or more electrons to form a shared pair, and this shared pair is attracted by both nuclei.
• Types of Covalent Bonds:
  • Single Covalent Bond: Sharing of one pair of electrons (e.g., H₂: H-H).
  • Double Covalent Bond: Sharing of two pairs of electrons (e.g., O₂: O=O).
  • Triple Covalent Bond: Sharing of three pairs of electrons (e.g., N₂: N≡N).
• Example: Formation of Hydrogen Molecule (H₂)
  • Each Hydrogen atom (Electronic configuration: 1) needs 1 electron to complete its duplet.
  • They share one pair of electrons, forming a single covalent bond.
• Example: Formation of Methane (CH₄)
  • Carbon (2, 4) needs 4 electrons. Each Hydrogen (1) needs 1 electron.
  • Carbon shares one electron with each of the four Hydrogen atoms, forming four single covalent bonds.

Properties of Covalent Compounds

• Physical State: Usually liquids or gases, or soft solids.
• Melting and Boiling Points: Generally low melting and boiling points (due to weaker intermolecular forces).
• Solubility: Generally insoluble in water but soluble in organic solvents.
• Conductivity: Do not conduct electricity (as they do not form ions).

Valency and Chemical Formulas

Valency: The combining capacity of an element. It is the number of electrons an atom gains, loses, or shares to achieve a stable outermost electron shell (octet or duplet).

• Metals typically have valency equal to the number of electrons they lose (1, 2, or 3).
• Nonmetals typically have valency equal to the number of electrons they gain or share to complete an octet.

Writing Chemical Formulas

Chemical formulas represent the elements present in a compound and their respective proportions. The criss-cross method is often used:

1. Write the symbols of the elements.
2. Write their valencies below the symbols.
3. Criss-cross the valencies (interchange them) to form subscripts.
4. Simplify the ratio if possible.

• Example: Forming Sodium Chloride (NaCl)
  Na (Valency 1), Cl (Valency 1) → NaCl
• Example: Forming Magnesium Chloride (MgCl₂)
  Mg (Valency 2), Cl (Valency 1) → MgCl₂
• Example: Forming Aluminium Oxide (Al₂O₃)
  Al (Valency 3), O (Valency 2) → Al₂O₃

Balancing Chemical Equations

Balancing a chemical equation ensures that the Law of Conservation of Mass is obeyed, meaning the number of atoms of each element on the reactant side is equal to the number of atoms of that element on the product side.

• This is done by placing appropriate coefficients (numbers) in front of the chemical formulas.
• Example: Unbalanced: H₂ + O₂ → H₂O
  Balanced: 2H₂ (g) + O₂ (g) → 2H₂O (l)

Summary

• Chemical Change: Permanent change forming new substances by breaking and forming chemical bonds.
• Chemical Equation: Symbolic representation of a chemical reaction (Reactants → Products). Must be balanced to obey Law of Conservation of Mass.
• Chemical Bond: Force holding atoms together, formed to achieve stable electron configuration (octet/duplet).
• Ionic Bond: Formed by complete transfer of electrons between metal (forms cation) and nonmetal (forms anion). Ionic compounds are solids, high melting/boiling points, conduct electricity in molten/aqueous state.
• Covalent Bond: Formed by mutual sharing of electrons between nonmetal atoms. Covalent compounds are usually liquids/gases/soft solids, low melting/boiling points, do not conduct electricity.
• Valency: Combining capacity of an element.
• Chemical Formulas: Represent composition of compounds, derived using valencies.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 13: Chemical Change and Chemical Bond.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.