How Do Changes in Temperature and Phase Affect Enthalpy and Entropy?

[katrina007]

Hello,

I've tried working out these three problems on my homework but I'm not sure how to start it. I looked in the book but it doesn't explain clearly and I am getting confused. If anyone here knows how to work these problems please let me know. Thanks in advance for any help. It will be much appreciated.

Question One
- When the solid NH4NO3 is dissolved in water at 25 C, the temperature of the solution decreases. What is true about the signs of enthalpy (H) and (S) for this process?

Question Two
- When a liquid evaporates, which is true about the signs of the enthalpy and entropy changes?

Question Three
- A temperature change causes the volume of a solution to increase. This change will cause:

a) the molarity and molality to increase
b) the molarity and molality to decrease
c) the molarity to inscrease but not change the molality
d) the molarity to decrease but not change the molality
e) neither the molarity nor molality to change

Again. Thanks in advance for any help that is given.

 

[symbolipoint]

Q1: Enthalpy is heat energy if I remember properly. Does your book discuss exothermic and endothermic processes? Endothermic processes require energy from the environment (therefore positive) and exothermic ones release energy to the environment (thereforen, negative). In case I stated those backwards, check your book.

Q1 & 2: S is always increasing; physical chemists and physicists understand this better.

Q3: You need to know the definitions of molarity and molality. If volume increases but mass stays the same, which if any of those concentration units change, and in which direction? Do not be upset for my not directly answering this one. Think!
'symbolipoint'

 

[katrina007]

Hey,

Thanks for your help. I think I kind of understand it better though I'm still confused on Q 1 & 2. But I get question three now. The answer is (d) because as the volume increases and the moles stays the same, the molarity must be decreasing - the number gets smaller and smaller with a high volume.

Yes for question one, our book does talk about endo and exo thermic processes. I understand what they mean, but I'm confused how that is going to help me solve for the question? So was the positive and negative the answers to the question?

And for question two, you're saying that both H & S will increase (it will be positive) ?

 

[Vagrant]

Shouldn't it be signs of ∆H and ∆S in the first question?

 

[katrina007]

yes that's correct. it should be signs...but which way? which ones are positive and negatives?

 

[Vagrant]

∆E is +ve when the total entropy increases. When you dissolve an ionic solid in water what do you get? Will the entropy increase or dec?
∆H is +ve for endothermic reactions and -ve for exo. Is the reaction in ques exo or endothermic reaction?

 

[katrina007]

in question one its exothermix reaction becuz of the temperature decreasing, therefore the H is negative... and I don't know how to figure out the S enthaply? but i assume its positive? is this correct

 

[Vagrant]

Re-read your own question the temperature of the surroundings dec.What does that tell you about the system?

 

[Vagrant]

Entropy is measure of disorder. Does dissolving solid in water inc the disorder in the system or dec it?

 

[katrina007]

it increases?
i really don't know. I am confused with this whole chapter. all this stuff is new.

 

[Vagrant]

Since the temp of the surrounding dec that means that the system has absorbed heat from the surrounding, i.e the system needs heat for the reaction to take place. So is the reaction endo or exo?

 

[katrina007]

its exothermic?

 

[Vagrant]

Going into the molecular picture solids are the most organized, hence their disorder is the least, followed by liquids and gases. So when you dissolve a solid in water will the disorder inc or dec?

 

[Vagrant]

its exothermic?

How did you reach to this conclusion?

 

[katrina007]

Because def. of exothermic is the reaction that releases energy in the form of heat. Also you can tell because of the temperature decreasing. since it is therefore the sign of th enthaply must be negative (H) and the negative means its exothermic.

I understand that the (H) is exothermic and its negative sign but what about the (S) ?

 

[Vagrant]

Temp is dec.
so heat is not being released but absorbed. That means the reaction is endothermic.

I have got to get back to my own studies now. Why don't you go back and read up more on this until you understand the concepts. I'll check on this when I come back tonight.

 

[Vagrant]

Meanwhile to give you something more to figure out entropy is inc. in this case.

 

[katrina007]

Yea i do need to read the chapters again. But thanks for your help.
I'll figure it out soon. :)

Good night

 

[Vagrant]

Actually its 12:30 pm here. But I need to study all the same.

 

[katrina007]

its 3AM here. I know I know. Staying up this late? lol can't help it. Need to finish this homework and study for exam coming up.

Ok so I think figured it out why its endothermic. And the sign of the (H) is positive while the sign for the (S) is negative.

I've two more questions that I need help with also: If you, or anyone could take a look at it and help that will be great. Much appreciated! (I'll repost these 2 questions as new topic for easy access)

Question 4
- What is the approximate pH at the equivalence point when a weak base, like NH4OH (aq), is titrated with a strong acid like HCL (aq) ?

a) 7
b) 5
c) 9
d) 2
e) None of the above. It is...

Question 5
- Metals have loosely held electrons. This explains why metals usually...

a) have high ionization energies
b) have low thermal conductivity
c) form anions
d) are oxidized, rather than reduced
e) form few compounds

- Katrina

 

[Vagrant]

the sign for the (S) is negative.

Why is ∆S -ve?

 

[Oaksinstructor]

remember, H is enthalpy (amount of heat in the system) and S is entropy (amount of disorder in the system).

The first one:
This reaction is ENDOTHERMIC. It is absorbing heat from its surroundings, which results in a decrease in temperature. (think of it like sucking out the heat, so it gets colder). Since the system is absorbing heat energy ∆H is going to be POSITIVE (nearly all endothermic reactions have a positive ∆H)
The positive enthalpy in this reaction (endothermic reaction), causes the S(surroundings) to be negative:
S(surroundings) = -H/T
(because the surroundings lose energy (they get colder) their atoms aren't moving around quite as quickly and thus become more ordered, resulting in a decrease in entropy)

the second one, a liquid is evaporating: in order to go from a liquid to a gas what do you have to do? you have to add heat to it. so once again, it is absorbing heat from its surroundings (incidentally this is why sweating keeps us cool, as it evaporates, it absorbs heat from the surroundings...us!) and since it is absorbing heat once again it is ENDOTHERMIC, and the signs will be the same as the first one: +H -S