Hess's Method

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Hess's Method

Postby 905085650 » Sun Jan 27, 2019 5:35 pm

Can someone explain to me the Hess's method of calculating overall bond enthalpies? Thank you!!

Srikar_Ramshetty 1K
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Re: Hess's Method

Postby Srikar_Ramshetty 1K » Sun Jan 27, 2019 5:57 pm

Hess's Method does not deal with the breaking and forming of bonds. Hess's Law simply states that total enthalpy of a reaction is equal to the net change of enthalpy for the series of reactions/steps to arrive at the net equation. Typically, you will be given several reactions that will will work in series to create an end product, similar to the Nitrogen Dioxide example that Professor Lavelle showed us.
N2 + O2 -> 2NO change in enthalpy = 180KJ
2NO + O2 -> 2NO2 change in enthalpy = -112KJ
N2 + 2O2 -> 2NO2(Net Equation) Net change in enthalpy = 180KJ + ( - 112KJ) = 68KJ

In future examples you may need to derive the net equation by removing "intermediates" from the process. In the example above the intermediate is 2NO, which you can see is left out of the net equation.

Hope that helps!

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Re: Hess's Method

Postby bonnie_schmitz_1F » Sun Jan 27, 2019 6:04 pm

Hess's method is where you look at multiple different reactions and their standard reaction enthalpies to determine the standard reaction enthalpy of you desired reaction.

This is an example from class:
We want the standard enthalpy for N2 + 2 O2 --> 2 NO2
N2 + O2 --> 2 NO ∆H = 180 kJ
2 NO + 02 --> 2 NO2 ∆H = -112 kJ

Then add the two reactions and their enthalpies to get
N2 + 2 O2 --> 2 NO2 ∆H = 68 kJ

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

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