Answer
Carbon monoxide is the only one that has a standard heat value that is not 0 so that is the exception of the six.
Work Step by Step
Find standard heat of formation values: Na(s) - 0 kJ/mol, O2(g) - 0 kJ/mol, Br2(l) - 0 kJ/mol, CO(g) - (-110.5) kJ/mol, Fe(s) - 0 kJ/mol
