Chemical Reactions and Equations
What is tested
This domain checks whether you can read a reaction, balance it, classify it and connect it to everyday effects such as corrosion and rancidity. UPMSP questions often ask for a balanced equation with state symbols, or ask why a reaction is oxidation or reduction.
Core rules
- A chemical equation is balanced when the number of atoms of each element is equal on both sides; subscripts cannot be changed, only coefficients.
- Combination: A + B → AB. Decomposition: AB → A + B, often needing heat, light or electricity. Displacement: a more reactive element displaces a less reactive one. Double displacement: ions exchange, often giving a precipitate.
- Oxidation is gain of oxygen or loss of hydrogen; reduction is the reverse. Both occur together in a redox reaction.
- Exothermic reactions release heat; endothermic reactions absorb it.
- Corrosion is attack on metals by moisture and air; rancidity is oxidation of fats and oils.
Worked example
Balance: Fe + H2O → Fe3O4 + H2. Start with Fe3O4: it needs 3 Fe and 4 O. Put 3 before Fe and 4 before H2O. The left now has 8 H, so put 4 before H2. Final: 3Fe + 4H2O → Fe3O4 + 4H2. Check: Fe 3 = 3, O 4 = 4, H 8 = 8.
Traps
Do not write H2O as H2O2; do not omit state symbols when asked; do not call a displacement reaction decomposition; remember that adding a catalyst does not change the balanced equation.