Question

Match the transformations in Column I with appropriate options in Column II.
(2011)

	Column I		Column II
A.	$C{O}_2\left(s\right)\to C{O}_2\left(g\right)$	p.	Phase transition
B.	$CaC{O}_3\left(s\right)\to CaO\left(s\right)$	q.	Allotropic change
C.	$2H\bullet \to H_2\left(g\right)$	r.	$△$H is positive
D.	$P_{(white,solid)}\to P_{(red,solid)}$	s.	$△$ S is positive
		t.	$△$ S is negative

• A. A-p,r,s;
  B-r,s;
  C-t;
  D-p,q,t
• B. A-p,s;
  B-r;
  C-q,t;
  D-p,t
• C. A- s;
  B-r,s;
  C-t;
  D-q,t
• D. A-p,r,s;
  B-r,s;
  C-q,t;
  D-p,q,t

Answer 1

The correct option is A A-p,r,s;

B-r,s;
C-t;
D-p,q,t
(A) $C{O}_2\left(s\right)\to C{O}_2\left(g\right)$
It is just a phase transition (sublimation) as no chemical change has occurred. Sublimation is always endothermic. Product is gas, more disordered. hence $△$ S is positive
(B) $CaC{O}_3\left(s\right)\to CaO\left(s\right)+C{O}_2\left(g\right)$
It is a chemical decomposition, not a phase change. Thermal decomposition occur at the expense of energy, hence endothermic. Product contain a gaseous species, hence, $△$ S > 0.
(C) $2H\to H_2\left(g\right)$
A new H-H covalent bond is being formed, hence, $△$ H < 0. Also, product is less disordered than reactant, $△$ S < 0.
(D) Allotropes are considered as different phase, hence $P_{white,solid)}\to P_{red,solid}$ is a phase transition as well as allotropic change.
Also, red phosphorus is more ordered than white phosphorus, $△$ S < 0.