Chemistry · Redox Equilibrium · NEET
The classical concept is the oldest and simplest way to spot oxidation. It says a substance is oxidised if oxygen is ADDED to it, or if hydrogen is REMOVED from it. You do not need electrons or oxidation numbers here — you just watch the atoms. In 2Mg + O2 -> 2MgO, oxygen joins magnesium, so magnesium is oxidised. In 2H2S + O2 -> 2S + 2H2O, hydrogen is pulled out of H2S, so H2S is oxidised.
It feels odd at first, but there is a good reason. Look at NCERT reaction CH4 + 2O2 -> CO2 + 2H2O. In going from CH4 to CO2, the carbon LOST its hydrogen atoms and GAINED oxygen. Both changes happen together and mean the same thing chemically. So chemists realised that 'losing hydrogen' has the same effect as 'gaining oxygen'. That is why the definition of oxidation was widened to include removal of hydrogen.
Ask two simple questions. (1) Did the substance gain oxygen atoms? (2) Did it lose hydrogen atoms? If YES to either one, that substance was oxidised. If it lost oxygen or gained hydrogen instead, it was reduced. Remember the hook 'Oxygen IN or Hydrogen OUT = Oxidation'.
We say H2S is oxidised. The hydrogen is removed from H2S, leaving free sulphur (S). Removal of hydrogen is oxidation, so H2S is oxidised to S. At the same time, oxygen is added to hydrogen to form water — but the classical focus for oxidation here is 'H2S lost its hydrogen'.
The classical idea only watches oxygen and hydrogen atoms — easy but limited. The electronic idea says oxidation is LOSS of electrons (OIL RIG: Oxidation Is Loss). The electronic idea is more powerful because it works even when there is no oxygen or hydrogen in the reaction. NEET often starts with the classical idea, then upgrades you to electrons and oxidation numbers.
No, and that is its weakness. It only works when oxygen or hydrogen is present in the reaction. For a reaction like 2Na + Cl2 -> 2NaCl, there is no oxygen and no hydrogen, so the classical rule cannot label it. That is exactly why NEET later teaches the electron-transfer and oxidation-number definitions, which cover all redox reactions.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Classical oxidation is the addition of oxygen to a substance, or the removal of hydrogen from it. For example, Mg gains oxygen to form MgO, and H2S loses hydrogen to form S — both are oxidation.
2Mg(s) + O2(g) -> 2MgO(s) and S(s) + O2(g) -> SO2(g). In both, oxygen is added, so magnesium and sulphur are oxidised.
2H2S(g) + O2(g) -> 2S(s) + 2H2O(l). Hydrogen is removed from H2S, so H2S is oxidised to free sulphur.
In CH4 + 2O2 -> CO2 + 2H2O, carbon both gains oxygen and loses hydrogen. Since both happen together, chemists saw that removing hydrogen has the same meaning as adding oxygen, so the definition was broadened.
No. It only works when oxygen or hydrogen is present. NEET builds on it with the electronic concept (loss/gain of electrons) and the oxidation-number method, which cover all redox reactions.
