Writing Chemical Equations

Writing Chemical Equations

• Chemical equations can be concise and suggestive by using symbols for reactants and products.
• A word equation represents a chemical reaction, e.g., Iron + Sulfur → Iron Sulphide and Mg + O2 → MgO.

Balanced Chemical Equations

• The law of conservation of mass states that mass cannot be created nor destroyed in a chemical reaction.
• The total mass of reactants must be equal to the total mass of products formed after the reaction.

Balancing Chemical Equations

• A chemical equation must be balanced to obey the law of conservation of mass.

• To balance an equation, follow these steps:

  Step 1

  • Draw boxes around each formula and write down the number of atoms of each element present.

  Step II

  • List the number of atoms of different elements present in the equation.

  Step III

  • Start balancing with a compound that contains the maximum number of atoms.
  • Balance elements one by one, e.g., iron, oxygen, hydrogen, etc.

  Step IV

  • Examine the equation and pick up the next element that is not balanced.
  • Multiply formulas by coefficients to equalize the number of atoms on both sides of the equation, e.g., 3Fe + 4H2O → Fe3O4 + 2H2.

Types of Chemical Reactions

• Calcium hydroxide reacts slowly with carbon dioxide in air to form a thin layer of calcium carbonate on walls.
• Calcium carbonate is also responsible for white deposits in kettles and boilers.

Combination Reaction

• Calcium oxide reacts with water to produce calcium hydroxide: CaO + H₂O → Ca(OH)₂
• Magnesium wire combines with oxygen to form magnesium oxide: 2Mg + O₂ → 2MgO
• In combination reactions, two or more reactants combine to form a single product.

Decomposition Reaction

• Thermal decomposition of copper sulfate crystals results in anhydrous copper sulfate and water vapors: CuSO₄.5H₂O (s) → CuSO₄ (s) + 5H₂O (g)
• Thermal decomposition of calcium carbonate results in calcium oxide and carbon dioxide: CaCO₃ (s) → CaO (s) + CO₂ (g)
• In decomposition reactions, one reactant breaks down into two or more products.

Displacement Reaction

• Iron displaces copper from copper sulfate solution: Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)
• Zinc displaces copper from copper sulfate solution: Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)
• In displacement reactions, a more reactive element displaces a less reactive element from a compound.

Double Displacement Reaction

• Mixing barium chloride solution and sodium sulphate solution forms a white precipitate of barium sulphate: BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2NaCl (aq)
• Such reactions are called precipitation reactions, where there is an exchange of ions between two reactants to form at least one new insoluble compound.

Oxidation and Reduction

• No specific examples or reactions mentioned in this section.