Let's do this Hess's law problem using the heats of formation of these particular substances. So I guess this problem is 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. As you can see on this table, it's got water gaseous and water liquid that have different heats of formation. Why? Because they're different states. So it takes more energy to take it from a solid to 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 that heats of formation of these different particles. Is everybody okay with that? I guess the first thing we should do is balance this equation. You guys should have reminded me. So what did we got here? So what did we say? What? So it was 3, 2, 2. Okay, so it's very important to balance because if you don't, you won't be able to do this problem. 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 effylene here? So plus 52.3. What about oxygen? Gas. Zero, alright? Okay, so make sure you guys get that. Why is it zero kilojoules? Because it's in its most stable form at atmospheric conditions. And it's in its elemental form. So that's why I wanted to do this problem with you guys. So I could give that kind of treatment there. So carbon dioxide, what is it? 1.5. And water gashes. Negative 2.41 point A. So how do we do this? So the heat of the reaction is 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. Fix the one small one. So what do we do? So the products are... So we've got a multiply by the coefficient too. So it's going to be 2 times negative 3.93.25 Sylligials per mole plus 2 times negative 2.41 point A. Sylligials per mole minus 1. Time's 15.2.3 kilojoules per mole plus 0. Sorry. Plus 5. Sorry. No. That's 3 times 0 kilojoules per mole. Everybody okay with that? Be patient. Okay. So now all you do is just plug that into your calculator. So... Okay. So this is the number I got. And hopefully everybody got the same number as me. So we'll say 0. Negative 1, 3, 2, 2, 1, 9. So that is the heat of the reaction. Is this an exothermic or endothermic reaction? Exothermic reaction. Exothermic reaction. So hopefully you guys can see how this is very similar to... In fact this is, as a law, an action. Okay. Any questions on this one? Pretty straightforward. I guess the thing to remember is the standard state thing. Okay. And you have to be given all those numbers. So.