Gc

Emmie

Full Member
10+ Year Member
Aug 17, 2008
602
0
0
  1. Pre-Dental
When solid something is dissolved in water at 25C, the temp of the solution decreases. What is true about the signs of delta H and delta S for this process?

So since it's going from solid to liquid, I assume that delta S is neg. but it's WRONG!

Sorry it seems like an easy question but my brain doesn't wanna work today. HELP:confused:
 

sugarsting

Full Member
10+ Year Member
Aug 23, 2008
200
0
0
  1. Pre-Dental
I think I know why you're confused.

Remember increasing in entropy is favorable. This means entropy of products > entropy of reactants, so deltaS is positive.

(Whereas with free energy or enthalpy, decreasing energy is favorable. This means the energy of the products < energy of reacts, so /H is negative and exergonic/exothermic.)

So like alanan84 said, solid to liquid increases entropy so entropy of products minus entropy of reactants is positive.
 

joonkimdds

Senior Member
10+ Year Member
7+ Year Member
Jun 30, 2005
2,780
2
151
  1. Pre-Dental
When solid something is dissolved in water at 25C, the temp of the solution decreases. What is true about the signs of delta H and delta S for this process?

So since it's going from solid to liquid, I assume that delta S is neg. but it's WRONG!

Sorry it seems like an easy question but my brain doesn't wanna work today. HELP:confused:

I don't know what made you think solid to liquid is negative delta S. Can u explain to me why you thought solid to liquid is negative delta S?
 
About the Ads

UCB05

Full Member
Jun 14, 2009
800
3
0
  1. Pre-Dental
i think i asked the exact same question earlier today, except i was confused as to why deltaH was positive. solid --> liquid --> gas is always increasing in intropy (positive)

Solid > liquid > gas IS increase in entropy -> positive TdeltaS. DeltaH is positive because the system absorbs heat from its surroundings.

There's always someone that thinks too hard and asks why the reaction is spontaneous if deltaH is positive and TdeltaS is positive. "But then isn't the reaction only spontaneous at high temperature?" Those values are always relative. Who's to say that room temperature isn't "high" enough temperature for spontaneity?
 

razblo

Full Member
10+ Year Member
Nov 30, 2008
292
28
246
  1. Resident [Any Field]
When solid something is dissolved in water at 25C, the temp of the solution decreases. What is true about the signs of delta H and delta S for this process?

So since it's going from solid to liquid, I assume that delta S is neg. but it's WRONG!

Sorry it seems like an easy question but my brain doesn't wanna work today. HELP:confused:

I know the exact question you're talking about.

S = Entropy which is considered DISorder, so if you're going from a solid to liquid you're creating disorder because the solid is dissolving. Thus deltaS INCREASES.

Since the temp. of the water decreases you can assume that the reactions requires an input of heat, which is a POSITIVE deltaH.

Hope that helps! :)
 

sixkiller

Full Member
Jun 19, 2009
275
0
0
I know the exact question you're talking about.

S = Entropy which is considered DISorder, so if you're going from a solid to liquid you're creating disorder because the solid is dissolving. Thus deltaS INCREASES.

Since the temp. of the water decreases you can assume that the reactions requires an input of heat, which is a POSITIVE deltaH.

Hope that helps! :)

This is not single component phase change.....

The problem concerns the dissolution of a solid in water.

Furthermore, the dissolution of a solid in water is not always accompanied by a positive change in entropy.

For example, the dissolution of aluminium chloride in water at 1atm and 298k is accompanied by an entropy change of -315J/k

Your answer is correct, but the reasoning is flawed.

The question is best answered by taking what is given in the problem and not making the assumption with respect to entropy.

Seemingly spontaneous process and a positive enthalpy change...thus the spontaneity or tendency for dissolution must be entropic (positive change).

The dissolution of a solid in water is usually entropically favorable, but not always. In some cases, such an assumption can get you into trouble.
 
This thread is more than 12 years old.

Your message may be considered spam for the following reasons:

  1. Your new thread title is very short, and likely is unhelpful.
  2. Your reply is very short and likely does not add anything to the thread.
  3. Your reply is very long and likely does not add anything to the thread.
  4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
  5. Your message is mostly quotes or spoilers.
  6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
  7. This thread is locked.