Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Topic started by: jennielynn_1980 on September 24, 2006, 03:59:03 PM

Title: which systems are at equilibrium?
Post by: jennielynn_1980 on September 24, 2006, 03:59:03 PM
Hi all.   Here is the question:

Consider the following systems:

a) a container that is half filled with alcohol, stoppered and allowed to stand for several days

b) crystals of potassium permanganate that are dissolved in water until the water becomes a uniform purple colour and a constant amount of cyrstal remains in the container

c) an ice cube in a beaker of water in a normal classroom

d) moth balls that are place in a an empty flask that is stoppered and allowed to stand for several days

Which of the above are at an equilibrium state?
i) a and b only
ii) a, b and d only
iii) b and c only
iv) a, b, c, and d


So I know that a) is in an equilibrium state.  Beyond that I am a bit stumped.  I don't think b) or c) are because they are not in a closed system.  Option d) is in a closed system but I don't think there is a reaction or phase change occuring in this case.  That means none of the possible options agree with me.  Where am I going wrong?
Title: Re: which systems are at equilibrium?
Post by: Yggdrasil on September 24, 2006, 05:15:33 PM
I don't think the system being closed is necessarily the best criteria to use to determine whether the system is at equilibrium.  If a system is at equilibrium, the system's properties are time independent (i.e. the system will not change over time).  Maybe this perspective will help you find the right answer.
Title: Re: which systems are at equilibrium?
Post by: jennielynn_1980 on September 24, 2006, 06:06:04 PM
With that perspective I would say A and B because in C the temp in the room is different than the temp in the beaker meaning there would be a change there that is not consistent with equilibrium.  And again I don't think D is a chemical or phase change of any sort.
Title: Re: which systems are at equilibrium?
Post by: Dan on September 24, 2006, 06:57:11 PM
Quote
And again I don't think D is a chemical or phase change of any sort.

So are the properties of D time dependent or time independent?

Title: Re: which systems are at equilibrium?
Post by: jennielynn_1980 on September 24, 2006, 09:10:46 PM
I think D's properties are independant of time because no matter how long you leave it there, there will be no change.  So this system is at equilibrium?  I see your point but at the same time, with everything I have just studied about equilibrium, it doesn't seem to meet the criterion because there is no forward or reverse reaction or phase change so how can it be at equilibrium?  From what I have studied, there must be forward and reverse reactions occurring simultaneously at the same rate for a system to be at equilibrium.
Title: Re: which systems are at equilibrium?
Post by: Yggdrasil on September 24, 2006, 09:45:43 PM
Moth balls are made of naphthalene which sublimes at RTP.  So, you have a solid/gas phase equilibrium.
Title: Re: which systems are at equilibrium?
Post by: jennielynn_1980 on September 25, 2006, 11:28:41 AM
Okay, thanks!  I wouldn't have known that.  I thought it would be the same as leaving a plastic ball in a closed container.