Decomposition of Sodium Hypochlorite: The Uncatalyzed Reaction

Abstract

'The decomposition of sodiu~n hypochlorite has been re-examined. The results show that Foerster and Dolch's mechanism of the decomposition to chlorate and chloride is correct; they postulated a slow bimolecular reaction to chlorite and chloride, followed by a faster reaction of chlorite with more hypochlorite. Values of the rate constants of both steps are reported; they make the activation energies 24.8 l;cal./g~n-molec~~le for the first step and 20.8 kcal./gm-molecule for the second. The rates are such that a t 40" C. a solution of sodium hypochlorite will contain about lyo as much chlorite as hypochlorite. The rate is strongly affected by changing ionic strength; a t low ionic strengths it is nearly constant or falls slightly; above about 0.8, the rate rises and a t high ionic strengths the rise is quite rapid. No signs of specific catalytic effects of sodium chloride, hydroxide, or carbonate could be observed, and it seems probable that earlier reports of this were due to variations in ionic strength. The decomposition to chloride and oxygen has been measured and is a unimolecular reaction, which is possibly, but not certainly, uncatalyzed. Values of its rate constant are reported; they also are much altered by changing the ionic strength.Although this reaction was first investigated a considerable number of years ago, there are several matters connected with it that are still not entirely clear. Briefly, the position seems to be this. The best early work on the subject was that of Foerster and his co-worlters, particularly Foerster and Dolch (2). They found it to be a second order reaction, and consequently deduced that the mechanism was: 2NaOC1 + NaClOz + NaCl NaOCl + NaCIO? + NaC103 + NaC1 .The first step is the slower. They showed by an independent experiment that the second step was indeed faster, and their rate constants gave activation energies of 224 and 20% kcal./gm-molecule for the first and second step respectively. In addition to the reactions given above it has long been known that decomposition to sodium chloride and oxygen also occurs. Foerster and Dolch's rates are the total reaction rates, though they knew that the reaction to chlorate was very much the more important. They also found that the rate increased as they added sodium hydroxide. Later Giordani (3) investigated the reaction, and concluded that it was a combinatiori of a termolecular reaction giving chlorate, and a bimolecular reaction giving oxygen. He also found a marked effect of sodium hydroxide, but in the opposite direction from that fou~lci by Foerster; and, a t least from about 0.5 to 1.0 M sodium hydroxide, obtained rate constants proportional to (Na0H)-?. H e therefore assumed that hypochlorous acid was essential to the reaction, and that it went in one step, as follows:OCI-+ 2HOC1+ C103-+ 2HC1 He proposed that the reaction to oxygen was 20C1--, 2C1-+ 0,. 'Manuscript

Decomposition of Sodium Hypochlorite: The Catalyzed Reaction

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