$\begin{array}{1 1} (a)\;2NO_{2(g)} \to N_2O_{4(g)} \\ (b)\;2NH_{3(g)} \to 3H_{2(g)}+N_{2(g)} \\ (c)\;CaCO_{3(s)} \to CaO_{(s)}+CO_{2(g)} \\ (d)\;C_6H_6(l) \to C_6H_6 \end{array} $
