so this video is going to be all about hess's law and how we can calculate the enthalpy change of a reaction using it so consider this reaction nitrogen gas reacts with hydrogen gas to produce ammonia and the enthalpy change for that reaction is going to be negative 92 kilojoules and then we have another reaction water decomposes to hydrogen gas and oxygen gas and the enthalpy change for that is going to be positive 286 kilojoules so using those two reactions how can we calculate the enthalpy change of this reaction ammonia reacting with oxygen gas producing nitrogen gas and water so hess's law states that we can use these reactions adjust them in such a way to get the enthalpy change of this reaction but how do we go about doing it so here's what we need to do we need to adjust these two reactions in such a way that when we add them we get this reaction and then we need to adjust these values accordingly and when we add those numbers we're going to get the enthalpy change for this reaction and that's how we're going to do it so we need to focus on the right stuff so in reaction 1 and 2 you want to focus on the reactants and products that appear only once in those two reactions for example nitrogen gas is only found in equation one not in equation two so you want to focus on n2 hydrogen gas it's found in both of these two equations so you do not want to focus on h2 if you do it's you're going to make your life a lot harder so notice that nitrogen gas is on the left side but we want it to be on the right side and we need a coefficient of 2 in front of it so what i'm going to do is i'm going to reverse this reaction and multiply by 2. whenever you reverse reaction you got to change the sign and if you multiply this reaction by two you need to multiply the enthalpy change by two so i'm going to have four nh3 so this is now going to be on the other side and if you multiply two nh3 by two you should get four in each three and this is going to yield two and two plus three times two is six so that's going to be six h2 and the enthalpy change is going to be positive 184 kilojoules that's 92 times 2. now let's focus on this equation should we focus on water h2 or oxygen you can focus on water or oxygen but you don't want to focus on h2 because it's found in these two equations so i'm going to focus on h2o notice that in the net reaction we have six water molecules on the right side here we only have one on the left so we need to reverse it and multiply by six so six times that is going to be 6 h2 and then 6 times a half o2 that's going to be 3 o2 and that's going to produce 6 water molecules so we got to change the sign it's going to be negative 286 and then we're going to multiply it by 6. so that's going to be negative 17 16 kilojoules so now let's add the two reactions and we need to make sure that we get the net reaction so we have four nh3 on the left that's to the left of this arrow so let's rewrite it here four nh3 and notice that the six h2 molecules they cancel because they're the same on both sides on the left we have three o2 that's the left of that arrow and then on the right side we have two n2 and six h2o which is the same as what we have here now since we add equation 1 and 2 to get this reaction to calculate the enthalpy change of that reaction according to hess's law we just need to add these values so it's 184 plus negative 17 16. so the answer is going to be negative 15 32 kilojoules and so that's how you can use hess law to calculate the enthalpy change of a reaction if you know the enthalpy changes of other reactions here's another example let's say that water decomposes into hydrogen gas and oxygen gas and let's say that that process requires positive 576 kilojoules of energy and then oxygen breaks down into atomic oxygen with an input of positive 495 kilojoules so with this information use hess's law to determine the enthalpy change of this reaction hydrogen gas plus atomic oxygen produced in liquid water so feel free to pause the video as you try that example so what's the first thing that we need to do in this example it's best to identify which species to focus on i'm going to focus on hydrogen gas notice that we have two of it on the right side but we need one of it on the left so we got to multiply the first reaction by a half and we need to reverse it so once we reverse it it's going to be h2 and you gotta divided by two so it's h2 plus one half o2 producing one water molecule now i need to make one small correction that should be a two and not a six now we need to change the sign and we have to divide that number by two 572 divided by two is 286 so this is going to be negative 286 kilojoules now notice that we have one oxygen atom on the left side and here we have two on the right side so we need to reverse that reaction and divided by two as well so it's gonna be o turning into one half o2 so it's going to be negative 495 divided by two which is 247.5 kilojoules so now let's add the two reactions so these two numbers will those two things will cancel so it's gonna be h2 plus o turns into a liquid water so negative 286 plus negative 247.5 is negative 533.5 kilojoules and so this is the answer now let's consider one more example so we're going to have two ozone molecules decomposing into three oxygen gas molecules and the energy for that is going to be negative 427 kilojoules i'm gonna have to write small because there's going to be more reactions in this one than the last one now in the next one oxygen gas decomposes into atomic oxygen and so the energy change for that is positive 400 495 kilojoules and then finally we have nitrogen monoxide reacting with ozone to produce nitrogen dioxide plus oxygen gas and so this is going to be negative 199 kilojoules so with this information go ahead and determine the enthalpy change of this reaction nitrogen monoxide plus atomic oxygen turning into nitrogen dioxide so go ahead and try it so let's call this equation one two and three so what should we focus on i'm going to focus on equation three so notice that i have nitrogen monoxide on the appropriate side i don't have to change it so i'm going to rewrite equation 3. and so the energy change is going to remain negative 199 kilojoules now notice that no2 is already in the appropriate side so the next thing i need to focus on is atomic oxygen which is only found in equation two so i need it to be on the left side so i have to reverse this reaction and multiply by half so this is going to be o turning into one half o2 and so this is going to be negative 495 divided by 2 we said it was 247.5 now notice that ozone and o2 they're found in other reactions o2 is found in equations two and three and ozone is only found in one e3 so i'm going to focus on ozone now because ozone is not in the net reaction that means i need it to cancel completely so in this reaction i have one ozone molecule on the left side if i want that to cancel i need to have one ozone molecule on the right side so i've already used equation three so i'm going to focus on equation one keep in mind this is equation three and this one is two so in order to get one ozone molecule on the right i need to reverse that reaction and multiply by half so on the left i'm going to have 3 over 2 o2 and it's going to change from negative to positive and then i need to divide 427 by 2 which is 213.5 kilojoules so now let's add the reactions so ozone will cancel now on the right side we have one oxygen gas molecule plus a half so that's 1.5 on the left side we have 3 over 2 which is 1.5 so therefore this cancels with these two on the right so what we have left over is no plus oh turns into no2 so since these three reactions add up to the net reaction the enthalpy change of the net reaction is the sum of the enthalpy changes of those three reactions so it's going to be negative 199 plus negative 247.5 plus 213.5 so that's negative 233 kilojoules so that's the energy change for this reaction so that's how you can use hess's law to calculate the enthalpy change of a reaction using the enthalpy changes from other reactions that are provided you