Question

Which of the following processes would you expect to have a negative value for entropy? NaCl(s) Na+(aq) + Cl- (aq) H2O(s) H2O(l) NaCl(s) NaCl(l) 2 Al(s) + 3Br2(l) 2AlBr3(s) C2H5OH(l) C2H4(g) + H2O(g)

Answer 1

Negative changes in entropy is associated with reactions that go from a more disordered state to a more ordered state. In this case, it will be the chemical reaction:

2 Al(s) + 3Br2(l) -> 2AlBr3(s)

This is because we have a mixture (high entropy) being reacted to form a solid (lower entropy).

Answer 2

Answer: $2Al(s)+3Br_2(l)\rightarrow 2AlBr_3(s)$

Explanation:  Entropy is the measure of randomness or disorder of a system. A system has positive value of entropy if the disorder increases and a system has negative value of entropy if the disorder decreases.

Solids have highest order followed by liquids and gases have least order.

1. $NaCl(s)\rightarrow Na^+(aq)+Cl^-(aq)$

As a solid is dissociating into ions, the randomness is increasing and the entropy is also increasing.

2. $H_2O(s)\rightarrow H_2O(l)$

As solid is changing to liquid phase, the randomness is increasing and the entropy is also increasing.

3.$NaCl(s)\rightarrow NaCl(l)$

As solid is changing to liquid phase, the randomness is increasing and the entropy is also increasing.

4. $2Al(s)+3Br_2(l)\rightarrow 2AlBr_3(s)$

As one solid and liquid is changing into solid only, the randomness is decreasing and the entropy is also decreasing.

5. $C_2H_5OH(l)\rightarrow C_2H_4(g)+H_2O(g)$

As liquid is changing to gaseous form,  the randomness is increasing and the entropy is also increasing.