NaOH as a Reagent: Disproportionation, Industrial Byproducts and Applications

Several nonmetals undergo disproportionation (simultaneous oxidation and reduction of the same element) when treated with hot concentrated NaOH. Boron reacts with hot NaOH to give sodium borate plus hydrogen gas (2B + 2NaOH + 2H2O → 2NaBO2 + 3H2, sometimes written toward Na3BO3-type products depending on conditions) — boron is oxidized while some hydrogen is released. White phosphorus (P4) disproportionates in hot NaOH solution to give sodium hypophosphite (NaH2PO2, phosphorus oxidized to +1) and phosphine gas (PH3, phosphorus reduced to -3) — a classic reaction. Sulfur disproportionates in hot concentrated NaOH to give sodium thiosulfate (Na2S2O3) and sodium sulfide (Na2S) together with water — again splitting the same starting element into a more-oxidized and a more-reduced product simultaneously. All three of these are genuine, textbook-correct disproportionation reactions of NaOH with a nonmetal.

A boron trifluoride-type compound (BF3, as the compound Z) reduced by excess lithium aluminium hydride gives diborane, B2H6 (compound X), releasing LiF and AlF3 as by-products of the reduction. Diborane then hydrolyses readily in water to give boric acid, H3BO3 (compound Y), releasing hydrogen gas in the process (B2H6 + 6H2O → 2H3BO3 + 6H2). Separately, diborane burns readily in oxygen (it is highly reactive and pyrophoric-adjacent) to give boron oxide, B2O3, and water, releasing a large amount of energy — this combustion reactivity is part of why diborane has been explored as a high-energy fuel additive. Reading a sequence like this means tracking each compound through one transformation at a time: reduction of a fluoride gives the hydride, hydrolysis of the hydride gives the oxyacid, and combustion of the hydride gives the oxide.

Industrially, sodium hydroxide is manufactured by the chlor-alkali process: electrolysis of concentrated aqueous NaCl (brine), which produces NaOH at the cathode compartment while chlorine gas, Cl2, is released as the co-product at the anode (hydrogen gas is also released at the cathode alongside NaOH, but among the given classic options, Cl2 is the standard byproduct answer). Two other sodium compounds show up in specific applications worth remembering by name: sodium thiosulfate, Na2S2O3 (commonly called hypo), is the standard photographic fixer, dissolving away unexposed silver halide from photographic film after development; and, in a linked industrial reaction sequence starting from Na2CO3 treated with water and CO2 to give NaHCO3, then combined with ZnCl2 and heated, the resulting zinc compound eventually decomposes to ZnO, which — being amphoteric — dissolves in excess NaOH to form sodium zincate, Na2ZnO2, the final product in that sequence.