Calculating the enthalpy of a reaction

[HenryH]

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Hey guys,

This is a fairly simple question dealing with enthalpy calculation, but I'm stuck on one little part. Here is the question:



"Given the following reactions:
N2 + O2 ==> 2NO enthalpy = +180.7 kJ
N2 + 2O2 ==> 2NO2 enthalpy = +66.4 kJ

Calculate the enthalpy of reaction for

2NO + O2 ==> 2NO2"


1.) I reversed the first reaction so that I would have 2NO ==> N2 + O2. The enthalpy also becomes -180.7 kJ.

2.) I left the second reaction unaltered.

With the changes made, I would now have:

2NO ==> N2 + O2
N2 + 2O2 ==> 2NO2

The N2 elements would cancel out -- I understand that much. However, what would happen with the 2O2 and O2? Would one of the O2 elements cancel out, leaving me with just one O2 (which is what I need to get to the final reaction and calculate the enthalpy)?

 

[laee3]

Yes, you're correct. N2 will cancel out. O2 will cancel out and leave just one O2 in the final equation. Also don't forget to flip the sign of the reaction you just flipped.