$X:2S{O_2} + {O_2} \rightleftharpoons 2S{O_3}$
$Y:PC{l_5} \rightleftharpoons PC{l_3} + C{l_2}$
$Z:2HI \rightleftharpoons {H_2} + {H_2}$
$2SO_{2(g)}+ O_{2(g)} $ $\rightleftharpoons$ $ 2SO_{3(g)}$
$SO_{2(g)} + \frac{1}{2} O_{2(g)} $ $\rightleftharpoons$ $ SO_{3(g)}$