Question

For each process, predict the sign on the entropy change and write a sentence or two to explain how to make this prediction without completing any mathematics. a.A solid melts. b.A vapor is converted into a solid. CH 117 Recitation Name: _________________________________ ~ 196 ~ c.A liquid freezes. d.A solid sublimes e.A vapor condenses to a liquid. f.A liquid boils. g.Dissolving a tablespoon of salt in water. h.Combustion of gasoline.

Answer 1

Answer: a. A solid melts: $\Delta S=+ve$

b. a vapor is converted to solid: $\Delta S=-ve$

c. a liquid freezes:  $\Delta S=-ve$

d. A solid sublimes:  $\Delta S=+ve$

e.  a vapor condenses to liquid :  $\Delta S=-ve$

f. a liquid boils:   $\Delta S=+ve$

g. dissolving a tablespoon of salt in water:  $\Delta S=+ve$

h. combustion of gasoline:  $\Delta S=+ve$

Explanation:

Entropy is the measure of randomness or disorder of a system. If a system moves from  an ordered arrangement to a disordered arrangement, the entropy change is negative and vice versa.

a. A solid melts: The solid is converting to liquid, thus the randomness is increasing as the molecules are moving away and the intermolecular forces are getting weaker. Thus the entropy change $(\Delta S)$ is positive.

b. a vapor is converted to solid: The gas  is converting to solid, thus the randomness is decreasing as the molecules are moving close and the intermolecular forces are getting stronger Thus the entropy change $(\Delta S)$ is negative.

c. a liquid freezes: The liquid is converting to solid, thus the randomness is decreasing as the molecules are moving close and the intermolecular forces are getting stronger. Thus the entropy change $(\Delta S)$ is negative.

d. A solid sublimes: The solid is converting to gas, thus the randomness is increasing as the molecules are moving away and the intermolecular forces are getting weaker. Thus the entropy change $(\Delta S)$ is positive.

e.  a vapor condenses to liquid : The gas  is converting to liquid, thus the randomness is decreasing as the molecules are moving close and the intermolecular forces are getting stronger. Thus the entropy change $(\Delta S)$ is negative.

f. a liquid boils: The liquid is converting to gas, thus the randomness is increasing as the molecules are moving away and the intermolecular forces are getting weaker. Thus the entropy change $(\Delta S)$ is positive.

g. dissolving a tablespoon of salt in water: The solid is converting to ions , thus the randomness is increasing as the ions can move freely. Thus the entropy change $(\Delta S)$ is positive.

h. combustion of gasoline: The liquid is converting to gas, thus the randomness is increasing as the molecules are moving away and the intermolecular forces are getting weaker. Thus the entropy change $(\Delta S)$ is positive.