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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Nano on November 06, 2004, 02:14:04 AM

Title: Thermochemistry-Enthalpy Problem
Post by: Nano on November 06, 2004, 02:14:04 AM

Hi,

we are learing thermochemistry right now, and I don't get what enthalpy is, in our book, it says enthalpy is H. When we are calculating Energy, we use Delta E=q+w. I get that part, it means Energy equals heat plus work. But I don't get what enthalpy means, how it works and how to calculate it, i'll apreciate your help

James

Title: Re:Thermochemistry-Enthalpy Problem
Post by: Demotivator on November 08, 2004, 09:39:45 AM

Delta E=q+w (where E is internal energy).
If a process takes place at constant pressure, then pressure-volume work results if the volume of the system changes. Rewriting the above:
deltaE = q_p - P(deltaV)
or,
  q_p = deltaE + P(deltaV)

By definition, H=E+PV. Thus, for a process which takes place at constant pressure,
q_p = deltaH
This says that the heat exchanged between system and surroundings at constant pressure is the enthalpy. Since most chemical processes occur at constant pressure, enthalpy is a useful function because we only need to measure the heat absorbed or evolved without worring about the pressure-volume work.

Title: Re:Thermochemistry-Enthalpy Problem
Post by: Nano on November 09, 2004, 06:24:10 PM

thanks a lot  :D

Title: Re:Thermochemistry-Enthalpy Problem
Post by: Mitch on November 09, 2004, 06:54:56 PM

Remember Thermodynamics is about what can and can not happen. Kinetics is about how fast it happens.

Title: Re:Thermochemistry-Enthalpy Problem
Post by: Nano on November 14, 2004, 10:20:03 PM

roger that