[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 im 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

[Yggdrasil]

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?

If the temperature of the solution decreases, does that mean the reaction (NH₄NO_{3 (s)} --> NH₄⁺_(aq) + NO₃^-_(aq)) is exothermic or endothermic?  What does this tell you about the ΔH of the reaction?

Now to determine the sign of ΔS, you can either think about the definition of S and how it would relate to the reaction, or you can think about ΔG.  Since the reaction happens spontaneously, you know that ΔG < 0.  Now, how does ΔG relate to ΔH and ΔS?  Based on the sign of ΔH and ΔG can you deduce the sign of ΔS?

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

Think about the meaning of H and S.  H is basically a measure of the energy of a system.  Therefore, when ΔH > 0, the system is absorbing heat to raise its energy and when ΔH < 0, the system is losing energy and releasing heat.

S is a measure of the "disorder" of the system.  If ΔS > 0 the system is becoming more disordered, while ΔS < 0 means the system is becoming more ordered.

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

How is molarity defined?  How is molality defined?

[katrina007]

Hi,

Thanks for your help. I understand question three now. The answer is (d).
Though I'm not sure of the answers to question one and two but here is what I think. Please let me know if i'm correct or wrong.

Q1:
- Since the temperature is decreasing, the H must be negative therefore its an exothermic process. Noting this, we can say that S is positive when the reaction is in exothermic process.

Q2: Both H and S are positive?

 

[Yggdrasil]

Q1:  In thermodynamics, you need to distinguish between your system and your surroundings.  In this example, your system is the equilibrium between ammonium nitrate, ammonium ions, and nitrate ions.  The surroundings is the water.

So, the reaction causes the temperature of the surroundings (the water) to fall, meaning that the system takes heat away from the surroundings.  If the system takes heat away from the surroundings, then heat is going into the system.  This means that heat is going into your reaction (i.e. your system) and therefore, the reaction is endothermic (ΔH > 0).

Q2:  Absolutely correct.  Since you must heat a liquid in order for it to evaporate, evaporation is an endothermic process (ΔH > 0).  Also, since a gas is more disordered than a liquid, evaporation causes an increase in entropy (ΔS > 0).

Q3.  Correct as well.  Good job.

[katrina007]

Ok so in Q1, its endothermic therefore the (H) is positive and the the (S) is negative. correct?

Thank-you for helping me with these questions. Really appreciate your time and effort in helping me understand the material. If you don't mind or if anybody else would have any opinions for the following two questions, then please help. Again, thanks in advance.

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

As you can see, I'm not good with vocabulary of chemistry terms. Don't know these kinds of questions and need help. Though I'm able to do calculations. Anyways, again if anyone can help please let me know. Thanks in advance.

- Katrina!
P.S. - I've reposted these two new question as new topic for easy access.

[Yggdrasil]

Ok so in Q1, its endothermic therefore the (H) is positive and the the (S) is negative. correct?

Recall that ΔG = ΔH -TΔS.  The reaction occurs spontaneously, so ΔG must be negative.  But, ΔH is positive.  So, for ΔH - TΔS to be negative overall, TΔS must be positive (and greater than ΔH), meaning that ΔS is positive.

This makes sense from a physical standpoint as well.  Solid ammonium nitrate is a well-ordered crystal whereas when ammonium nitrate dissociates into ammonium ions and nitrate ions in solutions, it becomes significantly more disordered.  Therefore, you gain disorder as ammonium nitrate dissolves and ΔS > 0.

I will leave the other questions for the other thread you created.