2.7.3 Singlet State of Molecular Oxygen

1. Addition of chlorine gas 2. Emission of red light
Addition of chlorine gas Emission of red light
Animation of Experiment (MPEG Format, 480 k)

Photo 1: Chlorine gas is added to a gas wash bottle containing a cooled mixture of hydrogen peroxide solution (30%, 30 ml)) and caustic soda solution (20 g NaOH in 140 ml water).
Photo 2: Dimming the lights shows that the solution glows red. Chlorine reacts with sodium hydroxide to form sodium chloride and sodium hypochlorite:

Cl2 + 2 NaOH® NaOCl + NaCl + H2O

The hypochlorite ions that are formed then react with hydrogen peroxide to form chlorine peroxide ions, which form oxygen through the separation of chlorine:

H2O2 + OCl-® ClOO- + H2O
ClOO-®1O2 + Cl-

Due to the law of conservation of spin, the oxygen produced is in an excited single state (1O2, paired valence electrons with antiparallel spin) and not a triplet state (3O2, two unpaired valence electrons with parallel spins), which is the lowest energy  state of molecular oxygen. (The existence of unpaired valence electrons in a stable molecule is very rare in nature and confers high chemical reactivity.) The emission of chemiluminescence causes  singlet molecular oxygen 1O2 to make the transition to the triplet state:

1O2®3O2h times nu