Perchlorate - Reactivity As An Oxidant

Reactivity As An Oxidant

The perchlorate ion is the least reactive oxidizer of the generalized chlorates. This appears to be a paradox, since higher oxidation numbers are expected to be progressively stronger oxidizers, and less stable. A table of reduction potentials of the four chlorates shows that, contrary to expectation, perchlorate is the weakest oxidant among the four in water.

Half-reaction	E° (V)
H+ + HOCl + e– → ½Cl2(g) + H2O	1.63
3H+ + HOClO + 3e– → ½Cl2(g) + 2H2O	1.64
6H+ + ClO3– + 5e– → ½Cl2(g) + 3H2O	1.47
8H+ + ClO4– + 7e– → ½Cl2(g) + 4H2O	1.42
½Cl2(g) + e– → Cl–	1.36
ClO– + H2O + 2e– → Cl– + 2OH–	0.89
ClO2– + 2H2O + 4e– → Cl– + 4OH–	0.78
ClO3– + 3H2O + 6e– → Cl– + 6OH–	0.63
ClO4– + 4H2O + 8e– → Cl– + 8OH–	0.56

This shows that the chlorates are stronger oxidizers in acidic conditions than basic conditions, while chlorine is unchanged.

Gas phase measurements of heats of reaction (which allow computation of ΔHf°) of various chlorine oxides do follow the expected trend wherein Cl2O7 exhibits the largest endothermic value of ΔHf° (238.1 kJ/mol) while Cl2O exhibits the lowest endothermic value of ΔHf° (80.3 kJ/mol).

The central chlorine in the perchlorate anion is a closed shell atom and is well protected by the four oxygens. Hence, perchlorate reacts sluggishly. Most perchlorate compounds, especially salts of electropositive metals such as sodium perchlorate or potassium perchlorate, are slow to react unless heated. This property is useful in many applications, such as flares, where the device should not explode, or even catch fire spontaneously.

Mixtures of perchlorates with organic compounds are more reactive. Although they do not usually catch fire or explode unless heated, there are a number of exceptions. Large amounts of improperly stored ammonium perchlorate led to the PEPCON disaster, in which an explosion destroyed one of the two large-scale production plants for ammonium perchlorate in the US.