Chemistry · Redox Equilibrium · NEET
No. A combination reaction (A + B → C) is redox ONLY when at least one reactant is a free element. A free element has oxidation number 0, so it must change when it goes into a compound. Example: C + O₂ → CO₂ is redox because C goes 0 → +4 and O goes 0 → −2. But CaO + CO₂ → CaCO₃ uses two compounds and no element changes, so it is combination but NOT redox. NCERT states this rule directly, and NEET loves testing it.
CaCO₃ → CaO + CO₂ makes no free element. Check the numbers: Ca stays +2, C stays +4, O stays −2 on both sides. No oxidation number changes, so there is no oxidation and no reduction. NCERT clearly says 'all decomposition reactions are not redox reactions' and gives CaCO₃ as the example. This is a very common NEET trap.
Look at the products. If at least one product is a FREE element (like O₂, Cl₂, N₂, a metal, or H₂), it is a redox decomposition, because that element ended at oxidation number 0. If every product is still a compound, check oxidation numbers — usually nothing changes and it is not redox. Fast rule: free element on the product side → redox.
This is one reactant splitting, so it is a disproportionation-style decomposition. Chlorine goes from +5 in KClO₃ down to −1 in KCl (reduced). Oxygen goes from −2 in KClO₃ up to 0 in O₂ (oxidised). Potassium stays +1 the whole time and does not change. So oxygen is the reducing part and chlorine is the reduced part in this same compound.
Combustion reactions are combination redox reactions. They use free O₂ (oxidation number 0), so an element is always present and something is always oxidised. Example: CH₄ + 2O₂ → CO₂ + 2H₂O. Carbon goes −4 → +4, oxygen goes 0 → −2. NEET counts all combustion using elemental dioxygen as redox.
Which of the following reactions is a metal displacement reaction? (This item lists two thermal decomposition reactions as distractors that you must recognise.)
What is the change in oxidation number of carbon in the reaction CH₄(g) + 4Cl₂(g) → CCl₄(l) + 4HCl(g)?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A combination reaction joins two or more substances into one product (A + B → C). A decomposition reaction is the reverse: one compound breaks into two or more products. Combination needs a free element in the reactants to be redox; decomposition needs a free element in the products to be redox.
CaO + CO₂ → CaCO₃. Both reactants are compounds, and Ca stays +2, C stays +4, O stays −2. No oxidation number changes, so it is combination but not redox.
2KClO₃ → 2KCl + 3O₂. Chlorine drops from +5 to −1 and oxygen rises from −2 to 0. A free element (O₂) appears, so it is a redox decomposition.
Because it tests one clear idea in disguise: a reaction is redox only if an oxidation number changes. NEET often shows a combination or decomposition and asks 'is this redox?' The safe check is to look for a free element (oxidation number 0) on one side.
No. Potassium stays +1 in both KClO₃ and KCl. Only chlorine (+5 to −1) and oxygen (−2 to 0) change. NCERT specifically points out that K does not change here.
