Video: How to Do Hess's Law Problems With Fractions

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Video Transcript


Hi, my name is Subhah Agarwal, and I'm a math expert, and today we're going to go over how do to Hesses law problems with fractions. So Hesses law states that the total enthalpy of a reaction is independent of the pathway. So it doesn't matter all the little steps you go through to get to the final reaction, it's going to be the same process regardless of if you go through one pathway or if you go through another pathway. So basically what that tells us is if we have the different steps we can add up the changes in enthalpy from like step 1, step 2, and step 3, to get the final change of enthalpy and that end reaction that we're interested in. So basically when you're dealing with fractions it's the exact same process. You're still going to want to add up the individual steps to get to some reaction that you're looking for. It's just that it might get a little bit more complicated because it is a fraction. The addition, that's all that's going to change is the addition and subtraction, that might be a little bit more complicated. So one option is just to kind of grit your teeth and just do it the same way as you would a regular Hesses law problem, or you can multiply by the greatest common denominator and this will remove all fractions and turn it into just an ordinary Hesses law problem. To give you a quick example of what that might look like, R1+R2 leads to R3 and then say we had some R4 leads to R2+R3 and the final equation we're looking for was R1+R4 leads to 2R3. And we had each change in enthalpy which I'm denoting by Dh sub 1 and Dh sub 2. So these are the little enthalpies we were given. Now if I just want to find the enthalpy for this reaction I can see right here these terms cancel out, I don't have to mess with any fractions, it's really simple, I would just add everything up. R1+R4 leads to 2R3. And then my answer would just be Dh1+Dh2. But say instead this was given to me as 1/4+1/4+1/4. I mean I could either, I would just have to multiply everything by 4 and then add it together. So do the greatest common denominator, simplify the fractions, same process, just a little bit more tricky in the work. And that's how you do a Hesses law problem with fractions. My name is Subhah Agarwal and thank you for taking an interest in math.
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