Hess's law

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Hess's law

Postby Andrea_Torres » Sun Jan 26, 2020 7:02 pm

How do we add reactions to obtain net reaction? For example during lecture two of the reactions were N2 and 2NO equal N2 so how does that work?

Jonathan Gong 2H
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Re: Hess's law

Postby Jonathan Gong 2H » Sun Jan 26, 2020 7:08 pm

In lecture, the two given reactions were as follows. (1) N2(g) + O2(g) --> 2NO(g). (2) 2NO(g) + O2(g) --> 2NO2(g). Since the product of the first reaction becomes a reactant in the second reaction, it can be canceled out on both sides of the reaction. Then, you would simply combine/add reactants with reactants and products with products to determine the net reaction. (3) N2(g) + 2O2(g) --> 2NO2(g).

Chris Tai 1B
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Re: Hess's law

Postby Chris Tai 1B » Sun Jan 26, 2020 7:54 pm

If we want to apply Hess's law to this situation, we're typically given the change in enthalpy (deltaH) for both of the reactions independently.
For the reaction N2(g) +O2(g) -> 2NO(g), deltaH = 180kJ
For the reaction 2NO(g) + O2(g) -> 2NO2 (g), delta H = -112 kJ
So, using the logic above to combine the two reactions, we get a net reaction of N2(g) + O2(g) + 2NO(g) + O2(g) -> 2NO(g) + 2NO2(g). The 2NO(g) cancels out on both sides, since it's used as a reactant in one reaction and is a product in another, so the net reaction becomes N2(g) + 2O2(g) -> 2NO2(g).
However, we have to also add the two changes in enthalpies to get our overall deltaH of the entire reaction: 180kJ -112kJ = 68kJ. Thus, using Hess's law to state that enthalpy changes are additive, we can calculate the overall deltaH for nitrogen dioxide formation.

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