Question

Match the transformations in Column I with appropriate options in Column II:

Column I		Column II
A	$C{O}_2(s)\to C{O}_2(g)$	P	Phase transition.
B	$CaC{O}_3(s)\to$ $CaO(s)+C{O}_2(g)$	Q	Allotropic change.
C	$2H\to H_2(g)$	R	$\Delta H$ is positive.
D	$P_{\text{(white, solid) }}\to P_{\text{(red, solid) }}$	S	$\Delta S$ is positive.
		T	$\Delta S$ is negative.

• A. $(A)\to (P),(Q),(S);(B)\to (R),(S);(C)\to (T);(D)\to (P)$ $(R),(T)$
• B. $(A)\to (P),(R),(S);(B)\to (R),(S);(C)\to (T);(D)\to (Q)$, $(R),(T)$
• C. $(A)\to (P),(R),(S);(B)\to (P),(S);(C)\to (T);(D)\to (Q)$ (S), (T)
• D. $(A)\to (P),(Q),(S);(B)\to (R),(S);(C)\to (R);(D)\to (Q)$ $(R),(T)$

Answer 1

Answer: (B)

For the given changes:
(A) $C{O}_2(s)\stackrel{\Delta }{\to }C{O}_2$ (g) Phase transition,
$\Delta H=+$ ve and $\Delta S=+$ ve
(B) $CaC{O}_3(s)\stackrel{\Delta }{\to }CaO(s)+C{O}_2(g)$
No phase transition, $\Delta H=+$ ve and $\Delta S=+ve$
(C) $2H\to H_2(g)$
Two particles give one gaseous particle,
so $\Delta S=-ve$
(D) $P_{\text{(white, solid) }}\stackrel{\Delta }{\to }{P}_{\text{(red, solid) }}$
This is an allotropic change. $\Delta H$ is positive, as white phosphorus converts to red phosphorus on heating.
$\Delta S$ is negative, as red phosphorus is a polymeric solid, less reactive than white phosphorous.