Inhibitors

Inhibitors are substances that decrease the rate of a chemical reaction -- the opposite effect of a catalyst. Two examples are given. (i) Chloroform (CHCl3) was once used as an anaesthetic, but on standing it slowly undergoes air oxidation to form the poisonous gas carbonyl chloride (phosgene): 4 CHCl3(l) + 3 O2(g) -> 4 COCl2(g) + 2 H2O(l) + 2 Cl2(g). Adding about 2% ethanol to the chloroform suppresses this formation of COCl2 -- the ethanol acts as an inhibitor, retarding the oxidation reaction, which is why chloroform is traditionally stored with a small amount of ethanol added. (ii) Hydrogen peroxide decomposes according to 2 H2O2(l) -> 2 H2O(l) + O2(g); this decomposition is inhibited by adding dilute acid or glycerol, both of which act as inhibitors and slow the reaction, which is why commercial hydrogen peroxide solutions are often stabilised this way to extend their she …