Thermodynamics question

[AA|FCB|DOC]

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I ran into a question where it said there is steam and water both at 100 degrees celcius. then it said which one has more internal energy than the other. Although intuitively it makes sense to me that the steam has more energy, I chose that they both have the same internal energy since I thought internal energy is directly proportional to temp and since both are at the boiling point then they have the same internal energy. Does anyone have an explanation to this? thanks in advance.

 

[MedPR]

I ran into a question where it said there is steam and water both at 100 degrees celcius. then it said which one has more internal energy than the other. Although intuitively it makes sense to me that the steam has more energy, I chose that they both have the same internal energy since I thought internal energy is directly proportional to temp and since both are at the boiling point then they have the same internal energy. Does anyone have an explanation to this? thanks in advance.

At 100c the two phases are in equilibrium with each other. I would assume gas has greater internal energy because the gas molecules are moving faster and have more KE.

 

[Testosterone]

Temperature is directly proportional to kinetic energy. Internal energy consists of kinetic energy and potential energy. If the system is isolated (no external heat is added or lost and no work is done on or by this system), the internal energy stays the same.

 

[AA|FCB|DOC]

ok, so you would say that the gas has more internal energy than the liquid because the gas has more kinetic energy, but less potential energy while the liquid has more potential energy but less kinetic energy? i guess what confuses me now is how do you determine the potential energy of one phase compared to another?

 

[Testosterone]

Yes. Conceptually, there are many different types of potential energy but the kind most relevant to phase changes would probably be chemical potential energy. To illustrate, from a gas to liquid phase transition, what is lost as kinetic energy is gained as potential energy toward intermolecular attractions, etc.

The only way to specifically quantify the change in potential energy is through empirical data and in an MCAT passage, they would have to provide that information to you.

 

[AA|FCB|DOC]

one other question, so would it be correct to say an isothermal process is one where the internal energy is constant so that in E=w+q we can set e=0 so that we get w=--q?

 

[NonTraditional3]

one other question, so would it be correct to say an isothermal process is one where the internal energy is constant so that in E=w+q we can set e=0 so that we get w=--q?

Correct.

 

[MedPR]

one other question, so would it be correct to say an isothermal process is one where the internal energy is constant so that in E=w+q we can set e=0 so that we get w=--q?

Yup, if internal energy is constant, then heat in = work out or vice versa.

 

[mynamehere]

I ran into a question where it said there is steam and water both at 100 degrees celcius. then it said which one has more internal energy than the other. Although intuitively it makes sense to me that the steam has more energy, I chose that they both have the same internal energy since I thought internal energy is directly proportional to temp and since both are at the boiling point then they have the same internal energy. Does anyone have an explanation to this? thanks in advance.

i agree with your response to your original question .. but as kaplan explained it(and forcibly justified to myself): gas has more internal energy since it takes energy for liquid -> gas ..
and breaking bonds require energy = endothermic

liquid + energy/heat -> gas
so gas has more energy

 

[LoLCareerGoals]

i agree with your response to your original question .. but as kaplan explained it(and forcibly justified to myself): gas has more internal energy since it takes energy for liquid -> gas ..
and breaking bonds require energy = endothermic

liquid + energy/heat -> gas
so gas has more energy

This is the correct and imo most intuitive asnwer.
Average molecular speed (representative of Ekinetic) u = sqrt(3RT/molar mass). Achieving the same average molecular speeds (i.e. temp) does not mean internal energy is the same.
Think back to temp vs heat added diagram. Temp rises vs heat, then stays the same as the phase change occurs, WHILE HEAT CONTINUES TO BE ADDED, then raises again, then again stays the same while heat continues to be added till it is all vapor. Heat added during those flatline is the difference.