Question

In which of the following reactions do you expect to have a significant decrease in entropy?

Fe(s) ? Fe(?)

Fe(s) + S(s) ? FeS(s)

2 Fe(s) + 3/2 O₂(g) ? Fe₂O₃(s)

HF(?) ? HF(g)

Answer 1

we know that

the order of entropy is given by

solid < liquid <<< gas

1)

consider the eqaution

Fe(s) ---> Fe (l)

here

solid is converted into liquid

so there is an increase in entropy but that is no significant

2)

consider the equation

Fe(s) + S(s) ---> FeS (s)

two solids combine to give one solid

so they is a increase in entropy

but still is not that significant

3)

consider the reaction

2 Fe(s) + 1.5 02 (g) ---> Fe203 (s)

here

1.5 moles of gas is converted to solid

so

there is a decrease in entropy

4)

consider the reaction

HF (l) ----> HF (g)

here

a liquid is converted to a gas , which has very high entropy

so

there is a significant increase in entropy in this reaction

so the answer is

option D)   HF(l) ---> HF (g)