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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Kuahji on November 23, 2007, 10:34:38 PM

Title: When are delta E & delta H equal?
Post by: Kuahji on November 23, 2007, 10:34:38 PM

The question is "Under what circumstances are delta E & delta H essentially equal.  Using the equation delta H = delta E + PV.  Delta E & H will essentially be equal when there is no change in volume.  This part I understand, but the solutions manual adds

"Delta H & delta E are nearly equal when there are no gases in a chemical reaction..."  Why is it only with gases?  In a reaction, solids and liquids also change forms (like ice to water) & have a change in volume.  So am I just getting confused with concepts, or is the book trying to point out something I'm not seeing?

Title: Re: When are delta E & delta H equal?
Post by: Yggdrasil on November 24, 2007, 12:20:17 AM

The change in volume between solids and liquids is negligible.  For example, when 1 mol of liquid water becomes 1 mol of ice at 1 atm pressure, pΔV is about 0.15J/mol, whereas most ΔH and ΔE values are in the hundreds of kJ/mol.