Types of Chemical Reactions

• Combination (synthesis) — A + B → AB.
• Decomposition — AB → A + B (thermal, electrolytic, photolytic).
• Displacement — more reactive element displaces less reactive from compound.
• Double displacement — exchange of ions; precipitate/gas/water formed.
• Exothermic vs endothermic — heat released or absorbed.

1. Combination — CaO + H₂O → Ca(OH)₂; C + O₂ → CO₂.
2. Decomposition — 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (heat); 2H₂O → 2H₂ + O₂ (electrolysis).
3. Displacement — Zn + CuSO₄ → ZnSO₄ + Cu (zinc more reactive).
4. Double displacement — Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl.
5. Precipitation — insoluble product (ppt) in double displacement.
6. Neutralisation — acid + base → salt + water (special double displacement).
7. Oxidation — gain of oxygen / loss of hydrogen (intro).
8. Redox — simultaneous oxidation and reduction (next topic).
9. Exothermic — respiration, combustion release heat.
10. Endothermic — decomposition of limestone needs heat.

Classify: Zn + 2HCl → ZnCl₂ + H₂

Step 1 — Single reactant Zn displaces H from HCl.
Step 2 — Zinc more reactive than hydrogen → **displacement** reaction.
Step 3 — Hydrogen gas evolved (test: burning splint pop).
Step 4 — Also redox: Zn oxidised, H⁺ reduced (detailed in redox topic).
Conclusion: single displacement reaction.

• Calling every reaction with two products decomposition (check one vs two reactants).
• Misconception: all reactions need heat (many proceed at room temperature).
• Confusing displacement with double displacement.
• Forgetting activity series for predicting displacement.
• Missing precipitate arrow (↓) in ionic equations.

• Give one example each of combination and decomposition.
• What type is AgNO₃ + NaCl → AgCl + NaNO₃?
• Define displacement reaction.
• Is photosynthesis exothermic or endothermic?

Open the Practice tab for graded questions on Types of Chemical Reactions.