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Question

Match the transformations in Column I with appropriate options in Column II. 
(2011)
  Column I   Column II
A. CO2(s)CO2(g) p. Phase transition 
B. CaCO3(s)CaO(s) q. Allotropic change
C. 2HH2(g) r. H is positive 
D. P(white,solid)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
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B
A-p,s;
B-r;
C-q,t;
D-p,t
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C
A- s;
B-r,s;
C-t;
D-q,t
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D
A-p,r,s;
B-r,s;
C-q,t;
D-p,q,t
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Solution

The correct option is D A-p,r,s;
B-r,s;
C-t;
D-p,q,t
(A) CO2(s)CO2(g)
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) CaCO3(s)CaO(s)+CO2(g)
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) 2HH2(g)
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 Pwhite,solid)Pred,solid is a phase transition as well as allotropic change. 
Also, red phosphorus is more ordered than white phosphorus, S < 0.

Chemistry

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