Chemical reactions and equations
Domain 3 - keemilised reaktsioonid ja võrrandid - tests whether you can turn a described change into a balanced equation, name the reaction type, and say whether energy is released or absorbed.
Balancing
- Atoms are conserved: the number of each element must be equal on both sides. Change only coefficients (2H₂ + O₂ → 2H₂O), never subscripts.
- Balance by trial: start with the most complex formula, leave O and H until last, then multiply through to clear fractions.
- Remember the diatomic elements: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
- Write state symbols (s), (l), (g), (aq) when a complete equation is demanded, and ionic equations for reactions in solution.
Reaction types
- Synthesis: A + B → AB (2Na + Cl₂ → 2NaCl). Decomposition: AB → A + B (CaCO₃ → CaO + CO₂ on heating).
- Displacement: a more active metal pushes a less active one out of its salt (Zn + CuSO₄ → ZnSO₄ + Cu).
- Double displacement: two solutions swap ions, often giving a precipitate (AgNO₃ + NaCl → AgCl↓ + NaNO₃) or a neutralisation.
- Combustion: fuel + O₂ → CO₂ + H₂O when complete; incomplete burning gives CO, soot and less energy.
Energy
- Exothermic reactions release heat (ΔH < 0): combustion, neutralisation, CaO + water. Endothermic ones absorb it (ΔH > 0): thermal decomposition of limestone, dissolving ammonium nitrate.
- A catalyst lowers activation energy and speeds the reaction without being consumed.
Worked example
Balance the combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O. Carbon: 3 CO₂. Hydrogen: 8 H → 4 H₂O. Oxygen on the right = 6 + 4 = 10, so 5 O₂. Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Check: C 3 = 3, H 8 = 8, O 10 = 10.
Traps that cost marks
- Balancing by changing subscripts (H₂O → H₂O₂) - that makes a different substance.
- Forgetting that oxygen and hydrogen travel as molecules.
- Leaving the equation unbalanced after adding products.
- Calling a reaction endothermic because it needs heating to start - the test is whether the products hold more energy, and many reactions need a spark yet are strongly exothermic.