Chemical Reactions and Equations
Chemical Reactions and Equations carries the balancing question almost every year in the Karnataka SSLC paper: you must write a correct equation, name its type and explain the observation (gas, precipitate, colour change, temperature change).
Writing an equation
Balance by changing coefficients only, never subscripts. Add state symbols (s), (l), (g), (aq) and write the condition (heat, sunlight, electricity) above the arrow. Diatomic elements are written H2, O2, N2, Cl2.
The reaction types
- Combination: CaO(s) + H2O(l) gives Ca(OH)2(aq), exothermic.
- Decomposition: CaCO3 on heating gives CaO + CO2; water on electrolysis; AgCl in sunlight (photolytic).
- Displacement: Zn + CuSO4 gives ZnSO4 + Cu.
- Double displacement: Na2SO4 + BaCl2 gives BaSO4(s) + 2NaCl.
- Redox: oxidation is gain of oxygen or loss of hydrogen; the substance oxidised is the reducing agent.
Worked example
Balance Fe + H2O gives Fe3O4 + H2. Fe3O4 fixes 3 Fe, so write 3Fe. It fixes 4 O, so write 4H2O, which gives 8 H, so write 4H2. Answer: 3Fe + 4H2O gives Fe3O4 + 4H2. Iron displaces hydrogen from steam, so the type is displacement, not combination.
Traps
Writing FeO or Fe2O3 for the oxide; leaving hydrogen unbalanced because it appears in two places; omitting heat above the arrow in decomposition; calling corrosion or rancidity a reduction, when both are oxidation.