we're now going to have a look at how we can calculate the standard enthalpy change of combustion uh by doing an experiment so firstly i'm going to run through what the definition is again and then how we set up the experiment then i'm going to work through um a question where we are calculating the standard 3 change of combustion and then lastly we're going to look at how accurate that experiment is so first things first let's have a look at the definition again and so the standard three change of combustion um is the enthalpy change that takes place when one mole of a substance so that's the important part here reacts completely with oxygen and with everything being under standard conditions and all reactants and products being in their standard states as usual okay so in the example i'm gonna go through today the uh fuel that we're gonna be talking about is methanol um so let's now write out the enthalpy change of combustion equation um for methanol so the formula for methanol is ch3oh don't worry too much at the moment if you don't know that and is a liquid fuel so it's going to be the liquid state symbol and if we are doing the combustion reaction as always we will then react it with oxygen gas and because it's complete combustion with oxygen we are going to form carbon dioxide gas and we are going to form water and in the liquid state because it's going to be in a standard state and then to match the definition we've got to make sure we only have one mole of the methanol being burnt and so in order to balance this we need to have two waters and we therefore need to have three over two oxygens present okay let's now have a look at the setup of the experiment so we will hopefully get a chance to do this experiment um in the classroom uh but for now we have to run through how it's set up um on the screen so over here at the bottom we have our spirit burner now this spirit burner usually has this wick in it and and our liquid fuel is going to be found in here so let's imagine that we have our methanol in this spirit burner over here and then at the top over here we over here says copper can it can be anything uh so we could have used a beaker and anything that's obviously not flowable um so we've got this copper can over here in this copper can we put our water and you need to know what the volume of the water is because that's important when it comes to our calculation okay so we are burning our fuel um down here and the water is essentially acting as the surroundings okay so we want to see how much energy is being transferred from this chemical system here to the water here so going from our chemical system to the surroundings and in within that surrounding we are putting our thermometer and that's going to give us our reading okay so when we do this experiment we have to write before we even start burning anything we have to write down a few um values before we can get started okay so the first thing that you need to do is make sure you know what the volume of water is okay so i mean it depends on how you want to do it but um usually the experiment method will tell you how much water to add and we also then need to take the initial temperature of the water okay so before we start burning anything and you have to write down the volume of the water and the initial temperature of the water okay so we can actually keep the thermometer in there once we start burning we can keep stirring it with that thermometer and we also before starting the experiment we do also need to take down the mass of our spirit burner okay so um three measurements we need to take down volume of water temperature of water and the mass of the spirit burner okay and then we can start our experiment so we light our spirit burner and what we do is we let that burn for a little while um you know usually about three to five minutes the time here isn't important okay so that's not a variable that we're measuring uh it just needs to be for a considerable amount of time um for our water to heat up slightly okay once we've decided to end the experiment we extinguish the flame and then very very quickly um as i said you should be stirring your water and during the experiment uh what you need to do very quickly is write down the final temperature of the water okay so we've got initial temperature we've got a final temperature and then also we should re-weigh our spirit burner we want to basically see how much fuel has been burnt um during that process okay and then um once we've done that we are going to i'm just going to move to a results table so we're going to run through um what to do with all these values once we've done our experiment how from this we're going to calculate the enthalpy change of combustion okay so um as i said we've got the mass of the spirit burner before burning and the mass of the spirit burner after burning and taking away um the mass before burning and so taking the mass of the spirit burner after burning away from the mass of the spirit burner before burning uh we can essentially get the mass of the fuel burn okay so that's going to be important at one point um as i said the volume of water is also important so we've got that we've got our initial temperature of water and our final temperature of water and from that if we do um the initial the final temperature takeaway the initial temperature we can get our temperature change so this essentially in our equation is going to be our delta t okay so our change in temperature um so if you remember the equation to work out the energy change we're going to work out the energy change first that is sorry that is q equals m c delta t okay so firstly we need to decide what m is what c is and what delta t is delta t we've already calculated by doing the final temperature minus the initial temperature so that is 41. um just bear in mind um i'm just going to mention this again the temperature here has been measured in degrees celsius and you don't here need to convert to kelvin because we're working out the difference uh the difference is going to be the same whether these values are in kelvin or in degrees celsius okay so don't worry at this point of converting anything to kelvin uh we can just keep it in degree celsius because we're working out the difference um so that stays as 41 degrees celsius um and over here the um specific heat capacity which is our value c uh we've been told that for water uh the specific heat capacity is 4.18 and the mass now this is very important so we've actually got two um things we can possibly have the mass for okay we've worked out the mass of fuel burnt um the volume of water is 150 um centimeters cubed we've been told that the density is one gram per centimeter cube so that tells us and that this volume because essentially what that means is that one centimeter cubed equals one gram and so this over here equals 150 grams of water okay so we've got two masses here now it's very important use the correct mass here and as we said this m in q equals m c delta t is the mass of the surroundings okay and earlier i said that the water is part of the surroundings here we want to see how much energy is transferred from the system to the water which is our surroundings so it's very important here that we use the mass of the surroundings which in this case is the water so here it's 150 grams okay if you use the mass of the fuel burn that's irrelevant here and you'll get the wrong answer okay right if i now plug all those values in into my q so that's going to be mass of water which is 150 and multiplied by specific heat capacity 4.18 multiplied by delta t which is 41. and you should get a value for q of 25 thousand seven hundred and seven and remember q here is in joules okay we still haven't worked out the enthalpy change uh because q is the energy change okay if you remember um enthalpy change so in this case it's enthalpy change of combustion and this is given to us by doing um so if you remember the units for this are kilojoules per mole okay at the moment we've got something in joules as we convert that into kilojoules first so i'm going to do my um q in kilojoules and divided by the mole so this is where we're going to use the fuel uh because we're going to calculate the moles of the fuel burnt okay so um let's just quickly convert our q into kilojoules all i need to do is divide this remember to go from joules to kilojoules we divide by a thousand so i'm just going to divide this number by a thousand so to get my q in kilojoules i'm going to do my joules divided by a thousand and that's going to give me 25.707 kilojoules okay so now i've got my q in kilojoules i just need to work out the moles of the fuel burn so i'm just going to move down here and so the mass of my fuel is 1.6 now if i want to work out moles from mass it is mass in grams divided by the mr okay my mass here is 1.6 grams my mr so methanol is ch3oh and i've already worked out the molar mass so that is 32. if i do that um i get 0.05 moles okay so i've now worked out my moles of fuel burnt so now if i go back to this equation here um my q in kilojoules was 25.707 kilojoules if i now divide that by my moles which is zero point zero five that's five um this answer is now going to be um 504 0.14 and my units for this are kilojoules per mole okay so this over here is the answer i get through my calculation okay but this here is not my final answer okay i haven't decided what the sign should be now this is not something you're going to get through the calculation this is what you're going to get by looking at the temperature change okay if we look over here during the experiment the temperature increase went from 21.5 to 62.5 that was a temperature increase so that tells you it's an exothermic reaction okay and as we know the delta h for an exothermic reaction should be negative so we have to now decide that sign okay and as we know combustion reaction is always going to be negative so that should be fairly straightforward um but just by looking at the temperature change we can see it was a hot reaction and therefore exothermic um and so our final answer is actually minus 514.14 kilojoules per mole okay and it's very important that you put that sign in because you lose the mark if you don't put that negative sign in so our final answer uh for the standard entropy change of combustion of methanol is minus 514.14 kilojoules per mole okay let's now have a look at how accurate this experiment is um because obviously it's an experiment there may be lots of reasons why and that value that we get is not the same as the one in the data book okay obviously the data book one is considered more accurate so there may be reasons why it's not that exact value and there are actually four reasons why and you need to know these four reasons really well so i'm going to talk through each of them um the first one um is due to a major error in this experiment um in this experiment not all of the heat in this um set up over here is going to the water okay as we can see a lot of the heat can escape to the surroundings um so a major source of error in this experiment is the heat loss to the surroundings okay um in this experiment um because we are using a liquid fuel when liquid fuels get hot they can evaporate quite easily okay um any fuel that's left in the wick um after we extinguish the flame that can evaporate and therefore take away some of the mass um and essentially if the fuel evaporates when we take our mass it's going to be less than expected okay um so that fuel that's evaporated hasn't been burnt um and therefore when we include the mass of that um it's actually giving us a more inaccurate value of the mass of fuel being burnt okay so with these fuels they can evaporate and that can lead to um our experimental value being inaccurate another thing and that can lead to inaccuracies is the fact that we are assuming that complete complete combustion is occurring okay that's an assumption we're making but the truth is we don't know that we it could be that some incomplete combustion is occurring okay and as we know incomplete combustion actually uses more energy and therefore that can make our final change value to be um inaccurate okay um another thing is if we're doing this in a lab it could be that we don't actually have stunned conditions in the lab okay we might not be at 298 kelvin um we might not be at 100 kilopascals um so that could be another source of error in this experiment okay when you're doing or if you're asking an exam question you know what the sources of error or why is that value not accurate um you've got to make it specific to the experiment okay and these four points here are specific to standard enthalpy change of combustion um and therefore you need to remember these four