Chemistry · Redox Equilibrium · NEET
Yes. Assign oxidation numbers on both sides. In H2O(s): H is +1, O is -2. In F2: F is 0. In HF: H is +1, F is -1. In HOF: H is +1, O is 0, F is -1. Oxygen goes from -2 (in water) to 0 (in HOF), so oxygen is oxidised. Fluorine goes from 0 (in F2) to -1 (in HF and HOF), so fluorine is reduced. Because one element rises and another falls in oxidation number, it is a redox reaction. This is NCERT exercise 7.4.
In HOF (hypofluorous acid), oxygen is 0. Here is why: fluorine is more electronegative than oxygen, so F takes -1. Hydrogen takes +1. The molecule is neutral, so +1 (H) + O + (-1) (F) = 0, which gives O = 0. This is a rare case where oxygen is neither -2 nor -1. It happens only because fluorine is the one element more electronegative than oxygen.
Fluorine is the strongest oxidising agent among halogens. It is so reactive that in liquid water it oxidises the oxygen of water itself: 2F2 + 2H2O -> 4HF + O2. Oxygen goes from -2 to 0 (released as O2 gas). Because fluorine attacks water, we do not run fluorine displacement reactions in water solution. Chlorine, bromine and iodine are milder, so chlorine can displace bromide and iodide from their solutions safely.
With liquid water: 2F2 + 2H2O -> 4HF + O2 (oxygen released as O2 gas). With ice (solid, cold water): H2O(s) + F2 -> HF + HOF, where the oxygen ends up in HOF instead of as O2. The cold, solid state slows the reaction and lets the intermediate HOF survive instead of going all the way to O2. Both are redox reactions where oxygen is oxidised and fluorine is reduced.
No. Disproportionation needs the SAME element to be both oxidised and reduced. Here two DIFFERENT elements change: oxygen is oxidised (-2 to 0) and fluorine is reduced (0 to -1). So it is a normal redox reaction, not disproportionation. Do not confuse it with cases like Cl2 + water where chlorine itself both rises and falls.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Hydrogen fluoride (HF) and hypofluorous acid (HOF). The balanced equation is H2O(s) + F2(g) -> HF(g) + HOF(g).
Oxygen is oxidised (from -2 in water to 0 in HOF). Fluorine is reduced (from 0 in F2 to -1 in HF and HOF).
In HOF, oxygen is bonded to the more electronegative fluorine, which takes -1. To keep HOF neutral with H at +1, oxygen must be 0. In water there is no fluorine, so oxygen keeps its usual -2.
No. Fluorine is the most electronegative element, so it always has oxidation number -1 in compounds (or 0 as free F2). This is a fixed NCERT rule and a common NEET checkpoint.
The idea is the same (fluorine oxidises oxygen of water), but the product differs. With liquid water you get O2 gas: 2F2 + 2H2O -> 4HF + O2. With ice you get HOF instead of O2.
