EK 1001 Chem. Question 630

[G1SG2]

How would you answer the following question:

If a solid at its melting point is heated, the added energy:

I. Breaks Bonds
II. Increases the translational KE of the molecules
III. Increases the vibrational KE of the molecules

A. I only
B. II only
C. III only
D. I and II only

I thought the answer was A, but it turns out to be D. Wouldn't an increase in the translational KE correspond to an increase in temperature, which is not supposed to happen during a phase change? Thanks.

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[G1SG2]

Anyone??

 

[dualityim]

I would say that the answer is D. First of all, added energy will definitely break bonds, be they ionic, hydrogen, van der waals, etc., so I is correct. Increasing the temperature increases the average kinetic energy of the molecules, shifting the boltzmann distribution and thus increasing the translational KE. So D is the answer.

Vibrational KE is not certain since it is not clear if the solid is atomic or molecular - if the solid could be pure carbon, for example, the vibrational KE contribution would be zero since there are no intramolecular bonds.

 

[G1SG2]

Increasing the temperature increases the average kinetic energy of the molecules, shifting the boltzmann distribution and thus increasing the translational KE. So D is the answer.

But there is not supposed to be a temperature increase during phase change...

 

[dualityim]

True, in an ideal, reversible situation. Unfortunately, in real systems, intermolecular bonds tend to decrease overall kinetic energy (think about random walk/brownian motion) - hence why breaking bonds releases heat energy (enthalpy) as well as increasing entropy.
Another indication of this - the problem didn't exactly say how much energy was added to the solid, so you know that some general principle must be involved.

 

[Marjan Islam]

I think question is very very unfair. Because, and this is stressed in EK, that all energy does to breaking bonds during a phase shift. Sure in REALITY, some of it will increase molecular kinetic energy, but unless they explicitly say non-ideal conditions, we're supposed to assume ideal conditions!

 

[G1SG2]

I think question is very very unfair. Because, and this is stressed in EK, that all energy does to breaking bonds during a phase shift. Sure in REALITY, some of it will increase molecular kinetic energy, but unless they explicitly say non-ideal conditions, we're supposed to assume ideal conditions!

I know right! They stress it a lot, especially in AO. Also, the next question says:

If a liquid at its boiling point is heated, the added energy:

I. Breaks Bonds
II. Increases the translational KE of the molecules
III. Increases the vibrational KE of the molecules

A. I only
B. II only
C. III only
D. I and II only

I picked A again, and this time I got it right...their explanation says:
"Energy added to a liquid at its boiling points goes into breaking bonds. Increasing translational KE of any molecule would be accompanied by a temperature increase, which doesn't occur (ideally) until all of the substance has vaporized"

And yes, we're supposed to assume ideal conditions unless told otherwise. EK frustrates me sometimes

 

[dedicate]

Maybe its just a bad question? I went with A also and Im using EK books. I know there is a question very similar to this in the chapter and the logic follows the text, that it behaves "ideally" and that all the E goes into breaking inter. bonds, and not increase in T.

 

[G1SG2]

Yeah, I think its a mistake.