btec applied science unit 3 energy content of fuels how can we measure how much energy different fuels contains so what is a fuel again a fuel is a substance that releases heat energy when it is burned when you burn a fuel combustion takes place and chemical potential energy is converted into heat energy it's transferred into heat energy energy is measured in joules which is capital j and a thousand joules is one kilojoule that is little k capital j one kilojoule now that could be heat energy or any type of energy is measured in joules there is another unit of energy that it mentions in the syllabus and that is killer calories now one kilo calorie is about 4184 joules or 4.2 kilojoules a kilocalorie is about 4.2 kilojoules if you look at food labels then they will tell you for any particular food that you buy uh how many kilojoules of energy or how many kilocalories of energy it contains okay one confusing thing is that on most food labels it actually just says calories if it says calories then it actually means killer calories if it says this biscuit is 250 calories it means killer calories these are the fuels that are mentioned in the specification you might get a question involving any of these petrol paraffin food as a fuel cooking oil methanol ethanol propanol butanol pentanol and wax if it's a liquid fuel then the experiment will involve burning it in something called a spirit burner and what you do is you weigh the spirit burner before the experiment then you weigh it after and then the difference in mass will tell you the mass of the fuel which has been burned uh if it's a solid fuel such as food it is a common experiment to to be asked to actually find out how much energy food contains you use a a needle with a wooden holder you set fire to the to the food for example it might be a peanut or a what's it or whatever you set fire to it in a bunsen flame then you put it under the boiling tube until it stops burning you measure its mass before and after and you note the temperature rise okay how do we work out the energy released well actually before that uh burning fuels can be hazardous what are the problems with burning fuels well fuels are flammable yeah they burn uh you've got to be careful with them you don't want to burn your house down they can be toxic in themselves which means they can be poisonous very often there's a risk of explosion with liquid and gases there's always a risk of explosion particularly gases harmful effects of products of incomplete combustion if you burn a fuel such as natural gas and there's not enough oxygen then it can produce carbon monoxide which is very poisonous instead of carbon dioxide you can get carbon monoxide so if the fuel doesn't burn properly you can get some pretty nasty stuff given off and there is pollution okay pollution from soot uh pollution from well carbon dioxide is a greenhouse gas but also if it's something like uh oil or coal a fossil fuel which contains sulfur then oxides of sulfur can cause acid rain so pollution so we can compare the energy content of fuels by measuring how much heat energy a certain amount of fuel gives to a certain amount of water so we know how much fuel we've used up because we weighed our spirit burner before and after we can work out how much energy the water gains if we know the mass of the water if we know the volume of the water then we know its mass and we know the temperature rise of the water and we know something called the specific heat capacity of the water this is the energy needed to raise the temperature of a kilogram of water by one degree centigrade the specific heat capacity of water you'll be given it in the exam and you use this equation e equals m c delta t is that new dj rapidi mc delta t okay e is the energy given to the water in joules m is the mass of the water now i'll show you in a minute you can do it in kilograms or you can do it in grams uh we said the specific heat capacity of the water if we're doing the mass in kilograms then it's 4200 that's joules per kilogram degree centigrade that's probably what you'll be given in the exam you'll probably do the mass in kilograms you can do it in grams and use 4.2 to raise the temperature of a gram of water is 4.2 joules to raise it by 1 degree and then delta t is the temperature rise of the water delta this triangle delta means the change the change in temperature the temperature rise here's an example the temperature of 100 mils of water rises from 20 degrees to 34 degrees centigrade how much heat energy has been given to the water pause the video have a go the answer is there you go e equals m c delta t i'm doing the mass in kilograms so 100 mils is 100 grams it's not 0.1 kilograms so i'm using 4200 for the specific heat capacity and my temperature rise is 14 so my answer is 5880 joules or 5.88 kilojoules okay so as i said 100 mils of water is 100 grams is not 0.1 kilograms the value of c you'll be given in the question the 4200 and then the temperature rise that's pretty obvious so we got 5.88 kilojoules what we will be asked to do probably what you will be asked to do is to work out the energy released per gram and then from that how much energy is released per mole so we're working out the energy per mole of the fuel now one mole of ethanol is 46 grams you should be able to work that out the molar mass of ethanol is 46. yeah one mole is 46 grams now if 1.2 grams of ethanol were burned how much energy did each gram get well that would be 5.88 divided by 1.2 is 4.9 kilojoules per gram then how much energy would you get from burning a mole of ethanol well if it's 4.9 kilojoules per gram you would multiply that by 46 and that would give 225 kilojoules per mole so that's how much energy we would get according to our experiment from burning a mole of ethanol if we go on tinternet or look up in the data book okay that says a reference on the internet says that the energy content of ethanol is 1367 kilojoules per mole it's an awful lot more than we got why do you think we got a much smaller value than that think about it i'm not going to tell you it should be pretty obvious now look at this table and remember this pattern these are some different fuels and you'll notice that the different fuels have different numbers of carbon atoms in their molecules the methanol just has one carbon atom atom ethanol has two etc etc and then the energy released in kilojoules per mole okay we've already talked about ethanol but can you see what the pattern is the pattern should be pretty obvious and can you explain that pattern you