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Heat of reaction: Hess's Law

Hellow everyone!
Can someone tell me, if I use a 0.5 mole solution will I only have half the energy produced when it comes to the heat of a reaction?

Cory Gowen
Teacher - Kyle, Saskatchewan, Canada
March 18, 2008

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I think I'd answer yes, Cory. One reason we can't achieve perpetual motion is that you get the same amount of energy change from a reaction or a change of state regardless of the path you take. The energy change depends only on the start point and end point, and my understanding is that Hess' Law is a formal way of saying that, using the proper language of enthalpy. So if you half the amount of the reactant, I'd say you get half the heat change.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 28, 2008

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First rule of chemistry: follow the units.

Enthalpy change values are stated in terms of kJ/mol. To get to the energy (in kJ) you multiply by the number of moles reacted. In which case yes, reacting 0.5 moles will give half the energy of reacting 1.0 moles.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

March 28, 2008