Cheat Sheet: Chemical Reactions and Equations | Science Class 10 PDF Download

Chemical Reactions

A process where substances (reactants) undergo a chemical change to form new substances (products) is called chemical reaction.

Indicators of chemical reaction:

• Change in state

• Change in color

• Evolution of a gas

• Change in temperature

Examples:

• Milk turning sour (fermentation)

• Rusting of iron

• Cooking of food

• Digestion and respiration

Chemical Equations

• Word Equation: Describes reactants and products in words, e.g., Magnesium + Oxygen → Magnesium oxide.

• Chemical Equation: Uses chemical formulae, e.g., Mg + O₂ → MgO (skeletal, unbalanced).

• Balanced Chemical Equation: Ensures the number of atoms of each element is equal on both sides, following the Law of Conservation of Mass. 
  Example: 2Mg + O₂ → 2MgO

• Balancing Steps (Hit-and-Trial Method):

  1. Write the skeletal equation.

  2. List the number of atoms for each element on both sides.

  3. Balance the element with the maximum atoms first (e.g., in Fe + H₂O → Fe₃O₄ + H₂, start with Fe₃O₄).

  4. Use coefficients to equalize atoms (e.g., 3Fe + 4H₂O → Fe₃O₄ + 4H₂).

  5. Verify all atoms are balanced.

  6. Add physical states: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous.

• Balanced Chemical Equation with States: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

Types of Chemical Reactions

1. Combination Reaction

Two or more reactants combine to form a single product.

Characteristics: Often exothermic (releases heat).

Examples:

• CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (Quick lime + Water → Slaked lime)

• C(s) + O₂(g) → CO₂(g) (Burning of coal)

• 2H₂(g) + O₂(g) → 2H₂O(l) (Formation of water)

2. Decomposition Reaction

A single reactant breaks down into two or more products.

Characteristics: Often endothermic (requires energy: heat, light, or electricity).

Examples:

• Thermal Decomposition: CaCO₃(s) → CaO(s) + CO₂(g) (Heat)

• Photolytic Decomposition: 2AgCl(s) → 2Ag(s) + Cl₂(g) (Sunlight)

• Electrolytic Decomposition: 2H₂O(l) → 2H₂(g) + O₂(g) (Electricity)

3. Displacement Reaction

A more reactive element displaces a less reactive element from its compound.

Example: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) (Iron displaces copper)

Other Examples:

• Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

• Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)

4. Double Displacement Reaction

Exchange of ions between two reactants, often forming a precipitate.

• Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (Precipitation reaction)

• Characteristics: Produces an insoluble product (precipitate).

5. Oxidation and Reduction (Redox Reactions)

Oxidation: Gain of oxygen or loss of hydrogen.

• Example: 2Cu + O₂ → 2CuO (Copper oxidized to copper oxide)

• Example: 4Na(s) + O₂(g) → 2Na₂O(s) (Sodium oxidized)

Reduction: Loss of oxygen or gain of hydrogen.

• Example: CuO + H₂ → Cu + H₂O (Copper oxide reduced to copper)

• Example: ZnO + C → Zn + CO (Zinc oxide reduced to zinc)

Redox Reaction: Involves both oxidation and reduction.

• Example: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ (HCl oxidized, MnO₂ reduced)

Effects of Oxidation in Everyday Life

Corrosion

When metals are attacked by substances like moisture or acids, forming oxides or other compounds its called corrosion.

Examples:

• Rusting of iron: Fe forms reddish-brown Fe₂O₃·nH₂O.

• Green coating on copper: Cu forms CuCO₃·Cu(OH)₂.

• Black coating on silver: Ag forms Ag₂S.

Impact: Damages iron structures (e.g., bridges, ships). Prevented by painting or galvanizing.

Rancidity

Rancidity is oxidation of fats/oils in food, causing bad smell/taste.

Prevention

• Use antioxidants 
• Store in air-tight containers flushed with nitrogen (e.g., in chip bags).

Key Activities and Observations

• Activity (Magnesium Burning): Mg burns with a dazzling white flame, forms white MgO powder (exothermic, combination).

• Activity (Zinc + Acid): Zn + H₂SO₄ → ZnSO₄ + H₂ (gas evolution, temperature increase).

• Activity (CaO + Water): Exothermic, forms Ca(OH)₂, used in whitewashing.

• Activity (Ferrous Sulphate): Green FeSO₄·7H₂O decomposes to brown Fe₂O₃, SO₂, SO₃ (thermal decomposition).

• Activity (Lead Nitrate): Decomposes to PbO, NO₂ (brown fumes), O₂ (thermal decomposition).

• Activity (Electrolysis of Water): Produces H₂ (double volume) and O₂.

• Activity (Silver Chloride): White AgCl turns grey Ag in sunlight (photolytic decomposition).

• Activity (Iron + CuSO₄): Ironystyrene nail turns brownish, blue CuSO₄ fades (displacement).

• Activity (Na₂SO₄ + BaCl₂): Forms white BaSO₄ precipitate (double displacement).

• Activity (Copper Oxidation): Cu turns black CuO on heating (oxidation).