Thermodynamics and Thermochemistry - Result Question 55

####55. Match the transformations in Column I with appropriate options in Column II.

(2011)

Column I Column II
A. $\quad CO _2(s) \longrightarrow CO _2(g)$ p. Phase transition
B. $\quad CaCO _3(s) \longrightarrow CaO(s)+CO _2(g)$ q. Allotropic change
C. $\quad 2 H \bullet \longrightarrow H _2(g)$ r. $\Delta H$ is positive
D. $\quad P _{\text {(white, solid) }} \rightarrow P _{\text {(red, solid) }}$ s. $\Delta S$ is positive
t. $\Delta S$ is negative

Fill in the Blanks

Show Answer

Solution:

  1. (A) $CO _2(s) \longrightarrow CO _2(g)$

It is just a phase transition (sublimation) as no chemical change has occurred. Sublimation is always endothermic. Product is gas, more disordered, hence $\Delta S$ is positive.

(B) $CaCO _3(s) \longrightarrow CaO(s)+CO _2(g)$

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, $\Delta S>0$.

(C) $2 H \longrightarrow H _2(g)$

A new $H-H$ covalent bond is being formed, hence, $\Delta H<0$. Also, product is less disordered than reactant, $\Delta S<0$.

(D) Allotropes are considered as different phase, hence $P _{\text {(white, solid) }} \longrightarrow P _{\text {(red, solid) }}$ is a phase transition as well as allotropic change.

Also, red phosphorus is more ordered than white phosphorus, $\Delta S<0$.



NCERT Chapter Video Solution

Dual Pane