Change in internal energy during water vaporization

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SUMMARY

The discussion centers on the internal energy change during the vaporization of water at 100°C, referencing the first law of thermodynamics. It establishes that while the temperature remains constant, the change in internal energy (dU) is not zero due to the phase change. The heat of vaporization, quantified as 540 cal/gram, is primarily attributed to the change in internal energy (ΔU), indicating that the differential heat capacity equation does not apply during this phase transition.

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  • Understanding of the first law of thermodynamics
  • Familiarity with phase changes and their thermodynamic implications
  • Knowledge of heat capacity concepts
  • Basic principles of calorimetry
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Students and professionals in thermodynamics, physicists, and chemists interested in understanding energy changes during phase transitions, particularly in water vaporization.

Ian Baughman
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According to the first law of thermodynamics,

dQ = dU + dW and you can find dU = nCvdT
If this is the case then when water at 100°C vaporizes to steam at 100°C shouldn't the change in internal energy be zero because it is dependent on temperature change?

 
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It's a change of phase at that temperature, so that the differential heat capacity equation is not applicable. It could be looked at as a discrete jump in the heat capacity as a function of temperature. I believe the 540 cal/gram number is a ## \Delta H ##, but most of this is due to the ## \Delta U ##.
 

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