Using Reaction Enthalpies

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Guillermo Vega 1H
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Using Reaction Enthalpies

Postby Guillermo Vega 1H » Sat Jan 14, 2017 8:06 pm

When is it appropriate to solve an equation using Hess's Law, Bond Enthalpies, or Standard Enthalpies of Formation?

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Re: Using Reaction Enthalpies

Postby Xinyi_Yan_3L » Sat Jan 14, 2017 9:38 pm

You can decide which method to use by looking at what information you are given in the question.
When you know the reaction enthalpies of reactions related to your given reactions, you use Hess's law. For example, if you are given N2+ O2= 2NO ΔH1°=180.5 kJ and N2+2O2=2NO2 ΔH2°=66.4 kJ, and want to find the ΔH°for 2NO+O2=2NO2 you can find ΔH°=ΔH2°- ΔH1°
If you are given the enthalpies of each bonds, use bond enthalpies. For the same example, you can calculate the reaction enthalpy if you know the energy to break NO double bond, OO double bond, and NO single bond. The reaction enthalpy would be: E(N=O)-E(N-O)-E(O-O)
And if you know the standard enthalpy of NO formation, O2 formation, NO2formation, use Standard Enthalpies of Formation. The enthalpy for this reaction is the ΔHf°(NO2)-ΔHf°(NO)-ΔHf°(O2)

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