Equilibrium (water and ethanol)

[ND3G]

Ethanol and water are completely miscible; that is, they dissolve in each other in all proportions

a) When two substances are mixed, is randomness increased or decreased?

b) Heat is liberated in the disolving process. Does the tendency toward minimum energy favour the separated pure substances, or the solution of one in the other?

c) In view of the tendencies listed in (a) and (b) above, explain why water and ethanol are completely miscible.

This question as been asked before but the replies do not all agree with each other.

https://www.physicsforums.com/showthread.php?t=74957

and

https://www.physicsforums.com/showthread.php?t=65859

For those of us that are a little confused, can someone please clarify the situation? (BTW this is for grade 12 Chemistry. They are looking for basic answers not forumulas...)

My take is:

a) When changes occur in a system particles are more likely to achieve a disordered condition than an ordered one. Therefore, randomness is increased.

b) The tendency towards minimum energy favour the solution of one in the other.

My reason for this is: Since heat is liberated this lowers the energy of the solution. The systems answer to the mixing of the solutions is not to unmix them but to release heat. The heat and therefore randomness of the surrounds increase.

c) By answer to part c whould then be that both processes favour the mixing of the two products as long as the system is not a closed one.

 

[chemisttree]

Good answer!

Concerning GCT's response that the mixing is actually a decrease in entropy, I don't believe that entropy is decreasing.

 

[Blueshift5]

Your responses are correct. Let me explain further:

a) When two substances are mixed, the particles are able to move more freely and interact with each other. This increases the randomness or disorder of the system. This is because there are more possible ways for the particles to arrange themselves in a mixed solution compared to a separated mixture. Therefore, randomness is increased.

b) In the dissolving process, heat is liberated because energy is released when the particles interact and form new bonds. This tendency towards minimum energy favors the solution of one substance in the other, rather than keeping them separated. This is because when the substances are mixed, the particles are able to interact and form bonds, which releases energy and lowers the overall energy of the system. This is more favorable than having the substances separated with no interaction between them.

c) Based on the tendencies mentioned in parts (a) and (b), water and ethanol are completely miscible because they are able to form strong bonds with each other. Water molecules have a partial negative charge and ethanol molecules have a partial positive charge, so they are attracted to each other and can form strong hydrogen bonds. This allows them to mix and dissolve in each other in all proportions, resulting in a completely miscible solution. In contrast, substances that are not miscible, such as oil and water, do not have strong enough interactions to mix and form a solution.