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Topic: Relationship between Enthalpy and Kinetic Energy  (Read 1564 times)

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Offline cvc121

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Relationship between Enthalpy and Kinetic Energy
« on: January 12, 2018, 02:34:36 AM »
Hi there.

I am encountering some problems with a general question about the various thermodynamic variables. The question is as follows:

Which of the following relationships is correct?
a) ΔG > 0 represents a spontaneous process.
b) ΔS is the amount of energy available to do work.
c) ΔH < 0 represents a nonspontaneous process.
d) ΔH > 0 represents an increase in kinetic energy.
e) None of the above are correct.

I have already ruled out a), b), and c) but I am having difficulty trying to understand the relationship between enthalpy and kinetic energy. I know that ΔH > 0 signifies an endothermic reaction and that heat energy is taken in from the surroundings (increase in temperature), and turned into potential energy in the products. Since the temperature increases within the system, wouldn't kinetic energy increase making statement d) correct?

The answer however is e) so I am clearly not on track with my thinking.

All clarification/guidance in the right direction is very much appreciated. Thank you!

Offline mjc123

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Re: Relationship between Enthalpy and Kinetic Energy
« Reply #1 on: January 12, 2018, 04:38:11 AM »
Temperature doesn't increase within the system. Under isothermal conditions, heat is taken in from the surroundings to maintain the system at constant temperature. Under adiabatic conditions (no heat exchange), heat is taken from the system itself and the temperature decreases. Either way, heat (kinetic energy of molecules), from inside or outside the system, is converted to chemical potential energy. Thus kinetic energy decreases.

Offline Enthalpy

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Re: Relationship between Enthalpy and Kinetic Energy
« Reply #2 on: January 12, 2018, 09:45:28 AM »
I wouldn't say "represents" an increase in kinetic energy. For sure, the kinetic energy is not the only form of energy in H. Searching some exotic conditions, it may be possible that U contains no kinetic energy at all.

There is more. The enthalpy of some amount of matter is not fully contained in that matter, as opposed to the internal energy: PV results from the surroundings. So, attempts to represent H by attributes of the amount of matter, like for instance the kinetic energy of atoms of that matter, will fail.

The nearest relationship is in an adiabatic expansion of gas, where enthalpy fully converts in macroscopic kinetic energy. But that's only one situation.

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