Hess' Law fractions

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Emily Glaser 1F
Posts: 156
Joined: Thu Jul 27, 2017 3:01 am

Hess' Law fractions

Postby Emily Glaser 1F » Tue Jan 23, 2018 5:02 pm

Why is it okay/normal to put fractions in these equations to perform Hess' Law? For example, H2 + 1/2O2 --> H20

jillian1k
Posts: 54
Joined: Sat Jul 22, 2017 3:00 am

Re: Hess' Law fractions

Postby jillian1k » Tue Jan 23, 2018 5:20 pm

Yes. In fact, it's often necessary to avoid assigning coefficients larger than 1 to products. For example, in the same rxn you gave, the other way to balance it is: 2H2 + O2 --> 2H20. However, this now multiplies the products (H20) by 2. Since reaction enthalpies given next to Hess's law rxns are the change in enthalpy per exactly 1 mole of product, adding a coefficient to that product would then change the rxn (and therefore the enthalpy) to producing twice as much product. Using fractions as coefficients elsewhere is the best way to avoid this problem.

AtreyiMitra2L
Posts: 169
Joined: Fri Sep 29, 2017 7:03 am
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Re: Hess' Law fractions

Postby AtreyiMitra2L » Tue Jan 23, 2018 5:34 pm

In hess's law, we are calculating the overall enthalpy change. Because enthalpy is a state function, we know it isnt pathway dependent. Therefore, it doesn't matter which way it got there. If you have fractions, as long as you changed enthalpy along with it its fine :)

Jennie Fox 1D
Posts: 66
Joined: Sat Jul 22, 2017 3:01 am

Re: Hess' Law fractions

Postby Jennie Fox 1D » Tue Jan 23, 2018 6:36 pm

Since enthalpy is a state function, it does not follow a specific pathway, and thus it it okay to use any coefficients as long as you consistently change the enthalpy along with it.


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