| View previous topic :: View next topic |
| Author |
Message |
corinne_ry6054
Joined: 12 May 2009
Posts: 1
|
 Posted: Fri Aug 21, 2009 10:09 pm Post subject: GS-7, Physical Sciences, #18 |
 |
|
| Doesn't entropy ALWAYS increase? |
|
| Back to top |
|
 |
mcat_premed3832
Joined: 19 Oct 2006
Posts: 413
|
| Posted: Sun Aug 23, 2009 8:56 pm Post subject: Entropy |
|
|
Did you mean to ask about question 18?
Entropy of the universe increases in any real system. The issue is: what if you are not taking into account "the universe" or what if you are limiting the scope of "the system"?
For example: If I gave you a glass of water and asked you to make ice. Perhaps you would put the water in the freezer. What happened to the entropy of the water? Clearly the water went from liquid (relatively random moving molecules) to solid (crystalline, regular array, very organized). If the system in defined as the glass of water then there is no doubt, entropy (randomness) has decreased.
Now if the system is defined as your kitchen, then you will notice the heat generated at the back of your freezer which is creating randomness in the movement of molecules in the air, etc., in order to produce some orgnanization in the glass of water (not a good idea to put a glass of water in the freezer though!).
If the system is just the molecules, then in Q18 clearly the system (as defined) is more organized (less random, less entropy). |
|
| Back to top |
|
|
mikecl7138834
Joined: 03 Feb 2011
Posts: 1
|
| Posted: Tue May 03, 2011 12:44 pm Post subject: bad link |
|
|
| the link leads to the title question 15. not 18 |
|
| Back to top |
|
|
mohsinalam5969
Joined: 18 Apr 2011
Posts: 50
|
| Posted: Mon Jun 20, 2011 2:50 am Post subject: |
|
|
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when converting energy to work. During this work, entropy accumulates in the system, but has to be removed by dissipation in the form of waste heat
ENTROPY OF THE SYSTEM MAY INCREASE OR DECREASE...some examples of the processes in which enthalpy changes are given as follows
purification-decreases
dissolving-increases
crystallization-decreases
mixing-increase
freezing-decreases |
|
| Back to top |
|
|
mcat_premed3832
Joined: 19 Oct 2006
Posts: 413
|
| Posted: Mon Jun 20, 2011 8:53 am Post subject: |
|
|
| Quote: | | ENTROPY OF THE SYSTEM MAY INCREASE OR DECREASE |
The statement you made can lead to mistakes on the real MCAT because it is precisely this point that the AAMC occasionally tests. I will repeat what I said above about the issue of properly identifying the system. I used the expression "real system" or another precision can be isolated, closed or entire system (the universe).
"the entropy of an isolated system always increases or remains constant."
http://en.wikipedia.org/wiki/Entropy
"- The entropy of a closed system (isolated such that no matter/energy enters or leaves the system) always increases. This is the Second Law of Thermodynamics.
- The entropy of the universe (the ultimate closed system) always increases.
- The entropy of an open system (a system that exchanges matter/energy with its environment) can increase or decrease, but if it decreases, the entropy of its total environment must increase such that the entropy of the universe increases."
http://www.roperld.com/science/minerals/EntropySustain.htm |
|
| Back to top |
|
|
Kama'Aina86
Guest
|
| Posted: Tue Jul 19, 2011 11:50 pm Post subject: Enthalpy for GS7 Phys Sect Quest 15 |
|
|
For this problem isn't the equation= Bonds broken minus Bonds formed?
This would mean that for Bonds broken: 394kj +602 kj
minus
Bonds formed: (-1096kj)
Enthalpy of Rxn should be: (394 + 602 - (-1096 kj)) = 2092 kj |
|
| Back to top |
|
|
|
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
