heat capacity q

[inaccensa]

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Substance A has a greater heat capacity than substance B.
Which of the following is most likely true concerning
substances A and B?
A. Substance A has larger molecules than substance B.
B. Substance B has a lower boiling point than substance A.
C. At the same temperature, the molecules of substance B move faster than those of substance A.
D. Substance A has more methods of absorbing energy than substance B.

Why can't the answer be B?

In a free adiabatic expansion, a real gas is allowed to spread to twice its original volume with no energy transfer from the surroundings. All of the following are true concerning this process EXCEPT:
A. No work is done.
B. Increased potential energy between molecules results
in decreased kinetic energy and the gas cools.
C. Entropy increases.
D. The gas loses heat.

i thought the ans will be A, but it is D. Since they mentioned adiabatic system, it makes sense, but why isn't it A?

Initial conditions = W= PV
W =P change in V = P (2V-V) = PV

Is it cause the initial = final condition?

 

[Isoprop]

Substance A has a greater heat capacity than substance B.
Which of the following is most likely true concerning
substances A and B?
A. Substance A has larger molecules than substance B.
B. Substance B has a lower boiling point than substance A.
C. At the same temperature, the molecules of substance B move faster than those of substance A.
D. Substance A has more methods of absorbing energy than substance B.

Why can't the answer be B?

Best way is to use an example. Water has a higher heat capacity than mercury or rubbing alcohol. While rubbing alcohol has a lower bp, mercury has a (much) higher bp.

In a free adiabatic expansion, a real gas is allowed to spread to twice its original volume with no energy transfer from the surroundings. All of the following are true concerning this process EXCEPT:
A. No work is done.
B. Increased potential energy between molecules results
in decreased kinetic energy and the gas cools.
C. Entropy increases.
D. The gas loses heat.

i thought the ans will be A, but it is D. Since they mentioned adiabatic system, it makes sense, but why isn't it A?

Initial conditions = W= PV
W =P change in V = P (2V-V) = PV

Is it cause the initial = final condition?

By definition, free expansion is when no work is done.

 

[ChemEngSoonMD]

Best way is to use an example. Water has a higher heat capacity than mercury or rubbing alcohol. While rubbing alcohol has a lower bp, mercury has a (much) higher bp.



By definition, free expansion is when no work is done.

x2, metals have insanely high boiling points but have ridiculously low specific heats.

 

[Bond03]

Substance A has a greater heat capacity than substance B.
Which of the following is most likely true concerning
substances A and B?
A. Substance A has larger molecules than substance B.
B. Substance B has a lower boiling point than substance A.
C. At the same temperature, the molecules of substance B move faster than those of substance A.
D. Substance A has more methods of absorbing energy than substance B.

Why can't the answer be B?

What is the answer to this question? D?

 

[Isoprop]

Yes. Heat capacity is a measure of how much temp changes when you add energy. Remember that temperature is a measure of the average kinetic energy of the molecules. If you are adding the same amount of energy, and the kinetic energy of the molecules aren't increasing as much (because temperature isn't rising as much), the substance must be able to hold the "extra" energy in other ways.

 

[wong89138]

For the free adiabatic expansion question, why couldn't it be B?

So increased potential energy between molecules increases as the molecules move apart, but in the book it says molecular potential energy has no effect on temperature.

Thanks!

 

[IntelInside]

For the free adiabatic expansion question, why couldn't it be B?

So increased potential energy between molecules increases as the molecules move apart, but in the book it says molecular potential energy has no effect on temperature.

Thanks!

I would have thought it was B as well because free adiabatic processes dont give off heat (q=0) and work = 0. Therefore internal energy is the same, but even though the internal energy is remaining the same, for a real gas, the temperature actually will fall. That's because as the gas expands and the molecules get farther apart, since they have an attraction for each other, separating them must increase the microscopic PE of the gas. So if the total internal energy remains the same, but the microscopic PE is increasing, the microscopic KE must be decreasing. Therefore the temperature decreases.

 

[wong89138]

hmm, that makes sense on an intuitive level, but i just don't like contradictions, even when you're supposed to look past them. Thanks for the reply!

 

[SKation]

Hi Guys!

I didn't quite get why if Substance A has a higher heat capacity than B then why won't substance B have a lower boiling point than substance A. Are those not related? Can someone explain please??

 

[inasensegone]

Hi Guys!

I didn't quite get why if Substance A has a higher heat capacity than B then why won't substance B have a lower boiling point than substance A. Are those not related? Can someone explain please??

To quote The Berkeley Review:

"Phase change processes have little to do with heat capacity. They both relate to intemolecular forces to an extent, bu they are not related to one another."

As an example, solids generally have a lower heat capacity than liquids. This is because heat capacity has to do with how well a substance is able to absorb the kinetic energy (heat). A solid's constituent molecules are unable to exhibit translational motion (only vibrational) and therefore they cannot absorb this energy as well as a liquid can (whose molecules do exhibit translational motion)

 

[SKation]

To quote The Berkeley Review:

"Phase change processes have little to do with heat capacity. They both relate to intemolecular forces to an extent, bu they are not related to one another."

As an example, solids generally have a lower heat capacity than liquids. This is because heat capacity has to do with how well a substance is able to absorb the kinetic energy (heat). A solid's constituent molecules are unable to exhibit translational motion (only vibrational) and therefore they cannot absorb this energy as well as a liquid can (whose molecules do exhibit translational motion)

Hello,
Thanks for replying, however in the second portion where you say that solids have a lower heat capacity and then go on to say that they cannot absorb this energy as well as liquids then wouldnt that mean that the solid has a higher heat capacity because it will take longer for it to heat up due to the fact that it can't absorb heat as well as a liquid?

Thanks!

 

[inasensegone]

Hello,
Thanks for replying, however in the second portion where you say that solids have a lower heat capacity and then go on to say that they cannot absorb this energy as well as liquids then wouldnt that mean that the solid has a higher heat capacity because it will take longer for it to heat up due to the fact that it can't absorb heat as well as a liquid?

Thanks!

Heat capacity is basically a measurement of storage capacity of absorbed heat for a substance. The key words are absorbed and storage. If the solids are not absorbing the energy, then it isn't stored.

If substance A has a higher heat capacity than substance B, when equal masses of both absorb the same amount of heat, substance A will experience a smaller rise in temperature than substance B. They both absorbed the same amount of energy, and thus stored the same amount, but substance A responded differently as to how that energy affected it. You can think of this as substance A being more efficient at storing the energy than substance B is. In this regard, solids are less efficient at storing absorbed energy than liquids, because the molecules of solids don't have that many degrees of motion.

 

[SKation]

O ok i think I get it so you are saying that solids have only vibrational motion while liquids have both vibrational as well as translational therefore when the liquids absorb the heat they have like more ways to disperse it therefore it dosent lead to that much of an increase in temperature giving it the higher heat capcity?

 

[inasensegone]

O ok i think I get it so you are saying that solids have only vibrational motion while liquids have both vibrational as well as translational therefore when the liquids absorb the heat they have like more ways to disperse it therefore it dosent lead to that much of an increase in temperature giving it the higher heat capcity?

Yeah pretty much... although technically solids can have rotational motion as well, although to my knowledge it isn't much and isn't often considered a great contribution.