 Hello Science30s! I'm going to help you through this Hess's Law handout. In order to do this you're going to need three things. You're going to need the Hess's Law handout that's on ldindustries.ca. You're also going to need your data booklet and there's a copy of the data booklet on my website as well if you need it. The data booklet page you're going to need is page five so get that out and ready and you're also going to need a calculator. So get your calculator out we're going to be doing some calculation work here today. So what's Hess's Law? It's basically a formula that's going to allow us to figure out how much heat energy is released in a chemical reaction. So we've been looking at energy conversions, we've been looking at burning fossil fuels like coal or natural gas or petroleum for energy and as heat energy the main form that the energy is released in we're going to see how to calculate how much. Now there's a big fancy looking formula on page five of your data booklet and let me tell you it does look confusing it has got a lot of weird symbols in it so let's break it down. First thing we see is on the left hand side this little triangle that's the Greek symbol delta it means a change in delta r h and in your data booklet it'll tell you that delta r h is the energy change of reaction. So in regular terms that's like how much energy is released in this sort of reaction or absorbed in this these reactions. So that's sort of like our answer how much energy gets released when you do the reaction or absorbed. Now to calculate that what we need to do is take the sum of now this Greek symbol here that's the symbol sigma it means the sum of or to add together n now n is the number of moles okay the amount and moles of the substance you've got to deal with and we're going to multiply that by delta f h naught. Again there's a pretty weird looking symbol there but those values the delta f h naughts are these energies associated with each of the different compounds on page five of your data booklet. So all these numbers here that you're seeing on this list on the far right hand column are those delta f h naught values and what we're going to do with these is we're going to total up the product delta h f naughts and the reactants and then subtract them. Now trust me this is a lot easier to do than to talk about so so let's actually jump into one and get started. Now when you look at that table you'll notice that some of the values are positive and some of them are negative. So if it's a negative value what that's going to mean is that energy is going to be released when that compound is formed and if it's a positive it means energy is absorbed. We call reactions where energy is released exothermic and we call reactions where energy is absorbed endothermic. All right so I've got a worked out example I've had to solve one of these Hess's law problems. The example we're going to do is the combustion of methane so here's the reaction that we're dealing with and you can kind of see it's it's got a left hand side and right hand side so the left hand side is the reactants and the right hand side is the products and that is my first step for solving one of these problems. It's just figure out where the reaction certain products are just kind of separate them out in your brain or you can put these cool little arrows in there if you'd like to sort of make them more obvious to you. The next thing I want you to do is to use page five your data booklet and the equation and do a little labeling. We're going to write out underneath of each compound how many moles of that compound there is so I wrote n equals one for the first compound of NH4. Now the reason I put n equals one is because if you don't see any number in front of the chemical you can assume the number of moles n is one so there was no number here in front of the methane the CH4 so that tells me I'm going to put a one in for the number of moles. Oxygen got a two so there's where the two came from the carbon dioxide again a one and the water gas a two so I'm just going to write down the values of n underneath of each compound next I'm going to go and write down the heats of formation for each of these so I'm just grabbing inside of the data booklet so for methane there it is right in the middle of the page negative 74.6 so I put negative 74.6 down there the units for those are kilojoules per mole and I'm not putting the units in here just to save ourselves a little bit of space now one thing you're going to notice is if you try to find something like oxygen on your table on page five it's not there and that's an important note to remember when you're doing this elements don't get a value of delta 8 delta f H naught they're zero they don't take any energy to form because they're not compounds so I went through and did that for the other two compounds that I had here CO2 and H2O the interesting thing about this is all the reactions we're going to look at in science 30 are combustion reactions for this so you're always going to have these same two products so for every one of the examples we're looking at you should have the same two products on the right hand side all right that's all of the grunt work of getting the numbers off the data booklet the next thing we're going to do is some multiplication so I'm just going to multiply together the number of moles by the energies of formation a lot of times this is pretty easy because you'll either have one times a number or a number times zero and maybe just you have to double the odd number so you can use your calculator to do that here's what I get when I've done all of my multiplications the formula tells us we had to do that multiplication because the formula says you should take n and delta h uh f or delta f h naught and you should multiply them together that's what it means when it just puts those two terms right next to each other side by side now the next thing you have to do is total up both sides of the equation i'm going to add together the two numbers on the reactant side and the two numbers on the product side so I total those up I add them together keeping in mind I have to include the negative signs if they're there and in the very very last step step five I have to subtract the products number from the reactants number and again you don't have to memorize to do that the formula says take the products and subtract the reactants so here's what I get when I take the products of negative 877.1 and subtract the reactants of negative 74.6 I get negative 802.5 kilojoules that's how much energy would be released for that first reaction let's go and try another one here if we flip the page we have a couple more examples we can work out together first one here is we're going to figure out how much energy is released if you burn propane which is in your barbecue or if you live in the country in your furnace so I'm going to start off by working out the values of n for each of these so for the first one there's one mole because there's no number written in front for oxygen there's five for carbon dioxide there's three for water vapor there's four now I'm going to go look up all the numbers that I have for the enthalpies of formation so let's see if we can find propane on here there it is negative 103.8 so I'll write that in make sure you put in the negative sign if it's there oxygen gets a value of zero because it's an element and I'm just going to look up in my previous page for the carbon dioxide and water so carbon dioxide was negative 393.5 those will always be the same so you'll get to just kind of copy and paste them from question to question and the water was negative four uh oops it was negative 241.8 there we go so I've got all that worked out now I'll do my multiplications so if I need to multiply one times negative 103.8 that's easy enough doesn't change it any uh five times zero is just zero so nothing more to do there and now on the other side here just kind of put my arrow to show the two sides I'm going to grab my calculator and I'm going to multiply together three and negative three 93.5 so that's going to give me negative 1180.5 and I'm going to do four multiplied by negative 241.8 so that's negative 967.2 all right now I've got to total up the two numbers here on the product side so let's see what I get when I add those together I'm going to keep the negative in there when I add them it's going to give me negative 2147.7 and my total from the reactant side was just negative 103.8 now to finish it all off my total energy is going to be my reactants uh products minus my reactants just like the formulas telling us to do up here the delta rh is products total minus reactants total so my products total was negative 2147.7 minus my reactants negative 103.8 let's work it out notice when I'm putting this into my calculator I have to subtract these two numbers so I have the negative number and then minus the negative of 103.8 so be really careful you don't drop any of the minus sides so I get negative 2043.9 now we haven't been doing much with units but we can put our units in here at the very end now that is in kilojoules so there's a Hess's law calculation the rest of the calculations this page are very similar the only difference being you are going to start off with a different hydrocarbon so you're going to have a different uh initial amount on your reactant side but your two product values will be the same and you'll have different numbers of moles to multiply by through if you want to check your work on this you can check out the key on ldindustries.ca