 Let's do this Hess's law problem using the heats of formation of these particular substances, so I Guess it's this problem the same Figure out the heat of The reaction of this reaction using its particular or its various heats of formation So we've got ethylene here, and it's really important that you know the state of matter of these things Okay, it's like as you can see on this table. It's got water gashes and water liquid They've got different heats of formation Okay, why because they're different states, you know, so it takes more energy to take it from a you know Solid to a liquid or a gas or whatever Okay, anyways, so the first thing we want to do so what are we looking for we're looking for Delta heat of this reaction And we know The heats of formation of these different Okay, is everybody okay with that? I guess the first thing we should do is balance this equation guys should have reminded me right so What do we got here? So What do we say it was it was three two two, right Okay, so it's very important to balance because if you don't you won't be able to do this problem Okay, so heat of formation is in units of kilojoules per mole So help me out by looking at the table if you guys could what is the heat of formation of ethylene here? Plus 52.3 What about oxygen? Gas Zero right, okay, so make sure you guys get that why is it zero kilojoules because it's it's in its It's in its most stable form at atmospheric conditions. Okay, and it's in its elemental form Okay, so I that's why I wanted to do this problem with you guys so I could give give that So carbon dioxide, what is it and water gashes? Okay, so how do we do this so the heat of the reaction? It's going to be the sum You guys know the summation symbol, right? the sum of the total so heat of formation of the products minus the sum of the heat of Formation of the reactants, so now let's see if we can fit all of that in one line Okay, so So the products are so we've got to multiply by the Coefficient two so it's going to be two times negative three ninety three point five kilojoules per mole times 241.8 kilojoules per mole minus 22.3 kilojoules per mole plus zero I'm sorry plus five Times zero Everybody okay with that equation? Okay, so now all you do is just plug that into your calculator So that's the same number as me So we'll say zero Negative one three two two point nine kilojoules so that is Is this an exothermic or endothermic reaction? Exo I can see how this is very similar to in fact this is has this law in action, okay? Any questions on this one? Pretty straightforward, I guess the thing to remember is the standard state okay, and you have to be given all those numbers