Chemical Reactions and Equations
What the exam tests
Chemical Reactions and Equations tests whether you can read a chemical change, write a word equation as a formula equation, balance it, and name the reaction type. Bihar BSEB questions commonly ask for balanced equations with state symbols, identification of oxidation and reduction, and the difference between corrosion and rancidity.
Core rules
- Mass is conserved, so every element must have the same number of atoms on both sides.
- Write correct formulae first; balance by changing coefficients only, never subscripts.
- Add physical states and conditions: heat, light, electricity, catalyst.
- Combination: A + B → AB. Decomposition: AB → A + B; it may be thermal, electrolytic or photolytic.
- Displacement: a more reactive metal pushes a less reactive metal out of its salt.
- Double displacement: ions exchange, often forming a precipitate.
- Oxidation is gain of oxygen or loss of electrons; reduction is the reverse. Both occur in a redox reaction.
Worked example
Balance Fe + H2O → Fe3O4 + H2. Fe3O4 contains 3 Fe and 4 O, so write 3 before Fe and 4 before H2O. The left side now has 8 H, so write 4 before H2. Check: Fe 3 = 3, O 4 = 4, H 8 = 8. The balanced equation is 3Fe + 4H2O → Fe3O4 + 4H2. Iron is oxidised and hydrogen is reduced, so this is also a redox reaction.
Traps that cost marks
- Changing a formula, such as writing H2O2 for water, to make a balance work.
- Forgetting to balance hydrogen and oxygen in combustion equations.
- Mixing up roasting and calcination, or thermal and photolytic decomposition.
- Calling respiration a decomposition reaction; it is an oxidation reaction.