<p>Using Hess's law, can somebody explain to me how to find the change in enthalpy of a particular equation?</p>
<p>For example, CO + NO ---> CO2 + 1/2 N2</p>
<p>do you have any numerical values to go along with that?</p>
<p>Enthalpy change would be negative though</p>
<p>Sorry. Here's the equations.</p>
<p>CO + 1/2 O2 ---> CO2 H = -283.0 kj
N2 + O2 ---> 2NO H = 180.6 kj</p>
<p>answers -373.3 kJ</p>
<p>and this is how you get it....you see the values for those 2 equations?
k your overall equation is CO + NO ---> CO2 + 1/2N2 right</p>
<p>SO you have to find a way to combine those 2 equations in your second message to get that equation....so set it up like this</p>
<p>CO + 1/2O2 = CO2 you keep the -283.0kJ as that H value
you have to modify the second one...flip it around so the O2s will cancel...and you have to divide it by 2 so you'll only have 1 NO in the final equatoin
So you'll have to work with
2NO ----> N2 + O2....right? since you flipped that equation around the value for H will change signs so it'll be H = -180.6
now you have to set everything equal so you're gonna have to divide that equation by 2
(2NO ----> N2 + O2)/2 ====> NO ---> 1/2 N2 + 1/2 O2 and since you divided the equation by 2 you have to divide the H value by 2...so you end up with: -90.3 kJ</p>
<p>Now write down both the CO2 and NO equations that you have</p>
<p>CO + 1/2O2 ---> CO2
NO ---> 1/2 N2 + 1/2O2 </p>
<p>Now combine it all together.....(and add your H values)</p>
<p>CO + 1/2O2 + NO ----> CO2 + 1/2N2 + 1/2O2......you have a 1/2O2 on both sides right? so that means you can cancel the O2s!
you end up with the equation: CO + NO ---> CO2 + NO!
Look familiar?<br>
K...since you flipped your NO H value and made it a negative AND you dividied it by 2.....the H value will be -90.3 (i thnk i already said that).....add your CO2 H value by your NO H value
-90.3 + -283.0 = -373.3 kJ = Htotal</p>
<p>One question:</p>
<p>When you divided equation B by 2, did you do that just to get the O2's to be equal?</p>
<p>yeah! i mean you could multiply the other equation by 2 if you wanted and it'd come out to be the same answer</p>
<p>wait no you wouldn't....cuz it wouldn't be the same equation anymore....sorry yeah...you had to divide the equation by 2 to make the O2s equal AND to make the entire equation match the equation you had in your first post</p>
<p>Thanks, nahrafsfa. This chemistry stuff is killing me</p>
<p>Mead13: the type of problem u just posted requires practice, practice, practice</p>
<p>fei, you're right. I haven't practiced at all. I've been neglecting chemistry all semester and now its starting to bite me in the ass.</p>
<p>I'm lucky that chemistry is "my subject"...it's just that one i understand the most for some odd reason</p>