he goes and welcome to subtopic 4.1 on energy and specifically this is going to cover the work on fuels and combustion this is the first science understanding the complete combustion of fuels containing carbon and hydrogen produces carbon dioxide and water and energy you need to know how to write thermo chemical equations for the complete combustion of fuels in which the only products are carbon dioxide and water so thermo chemical equations is something we have covered in stage 1 chemistry they essentially outline the energy that is released or absorbed during a chemical process they do consists of a few other components as well so they include the mole ratio of the reactants and products which we achieved through balancing the equation they include physical States solid liquid gas or in solution as well as the quantity of heat energy that is released or absorbed expressed as a value known as Delta H or the enthalpy change so a general equation for combustion will consist of a fuel reacting with an oxidant typically it will be oxygen or oxygen gas this will go to produce carbon dioxide and water for one example we've got here is octane so we know octane is a liquid at room temperature you know octane can react with oxygen gas to produce carbon dioxide and water we can see the states of everything there we need to balance it so just as a reminder to balanced combustion reactions we firstly look at balancing the carbons by adding the appropriate co2 s on the right we can see that there are eight carbons on left and one carbon on the right here so we'll need eight co2 molecules or eight moles we then look at balancing the hydrogens by adding the necessary amounts of water we've got 18 hydrogen's on the left two on the right which means we need nine waters and then we finally balance the oxygens by adding the appropriate number of o2 molecules or moles so over to the right we can see that there are eight lots of 2 which is 16 plus 9 which gives us 25 oxygens on the right and two on the left and keep in mind with combustion reactions this is the only case where we can introduce fractions so instead of perhaps trying to multiply this whole equation out to get even numbers or to get whole numbers we're just going to write the coefficient as a fraction over two so that would be 25 on two lots of o2 and then this will balance the 25 oxygens on the product side the last thing that we need to include is the Delta H value or the enthalpy change for the reaction so given that there is one mole of octane undergoing complete combustion this is going to end up giving us a value of negative five thousand four hundred and seventeen kilojoules per mole the negative indicates this is an exothermic reaction meaning that it is going to release this energy for a second example we've got ethanol so another carbon-based fuel here with its formula there we can see it's a liquid at room temperature so we're just going to write out its unbalanced equation first as such we need to look at balancing this in very much the same way so firstly balancing the carbons so we would say we need to co2 s on the right follow that with balancing the hydrogen's six on the left two on the right so that tells us we need three water's and then we balance the oxygens and just a thing here is to be a little bit careful because in alcohols like ethanol there is this oxygen that we have to factor in here so we can see that there are three oxygens on the left side we've got four plus three which is seven on the right so one thing I typically say is just to remove one oxygen on both sides so that will help eliminate this oxygen here and one from here that leaves essentially six on the right two on the left which means that we then need three lots of boats who on the left the last thing is to then include the Delta H value so in this case it is negative one thousand three hundred and seventy one kilojoules per mole for the next science understanding incomplete combustion producing carbon or soot and carbon monoxide is more likely with longer chain carbon-based fuels explain why incomplete combustion is more likely with longer chain carbon-based fuels than with shorter chains discuss the undesirable consequences of incomplete combustion incomplete combustion occurs due to an oxygen deficiency combustion we have to factor in is a redox process so what this means is that we can show oxidation numbers of the atoms in our chemical equations to determine what's happening to our reactants whether they're undergoing oxidation or reduction in to what degree here we've got methane undergoing complete combustion so we've got one lot of methane reacting with two lots of o2 to produce one lot of co2 and two lots of water from here we could actually look at determining the oxidation numbers of all of the atoms in this equation so this is what I've done above here and of particular note is the oxidation number of carbon we can see that carbon in methane goes from an oxidation number of minus 4 to plus 4 when it forms co2 so this represents an increase in oxidation number which indicates that methane has undergone oxidation in this case we can see carbon exists in its maximum of oxidation state after complete combustion so carbon is unable to obtain an oxidation number any higher than +4 incomplete combustion on the other hand produces or yields carbon or soot and carbon monoxide so examples of incomplete combustion can consists of methane or methane reacting with oxygen to produce carbon monoxide and water and just confirm that these equations are balanced we could also see methane undergoing incomplete combustion to produce carbon in the form of soot as well as water from here we can look at the oxidation states of each of our atoms so with the first equation and in particularly if we look at carbon here it has gone from an oxidation state of negative 4 to +2 whereas in this equation here we can see that we've got an oxidation state change from minus 4 to now zero so these represent lower increases in oxidation number which means that the carbon in the methane hasn't undergone complete combustion another thing just to note is that when you look at the balanced equations you may have noticed that there are a lower number of moles of oxygen required to produce carbon monoxide and carbon in the form of soot so this could occur when there is a deficiency or lack of oxygen in the environment to allow for complete combustion as I mentioned before carbon has a lower oxidation state of +2 and zero in carbon monoxide and carbon respectively so this tells us that carbon can essentially undergo further oxidation and in doing so can eventually form carbon dioxide and also release more energy from the reaction one example that you should all be quite familiar with for incomplete combustion is looking at a Bunsen burner flame so we know that with Bunsen burners if we close the air hole that we end up producing a more yellowy flame and as we start to open up the air hole it converts that yellow suit flame into a more blue flame which we know is the heating flame so opening up that air hole essentially increases the availability of oxygen to combust with our methane in our Bunsen burners and it allows for a more complete combustion so we can observe that as a more bluish flame and we also know that this is going to produce a lot more heat than our yellow safety flames I did mention that these can be quite so you might see that there are black smoke particles actually rising from this yellow flame here we know that incomplete combustion is more likely as the size of the carbon chain increases but why is that the case let's have a look at a few different examples so coming back to our methane example undergoing complete combustion to form co2 in h2o and we'll contrast that we obtain here so again undergoing complete combustion we can see between the two octane is the larger molecule or it has the larger carbon chain and what we can see is that there is essentially increased demands of oxygen per mole of our fuel going from methane which only requires two moles of oxygen per one mole of methane whereas four obtained for one mole of octane you would need 25 on two moles of oxygen to undergo complete combustion another reason why incomplete combustion is more likely for larger fuels is because as the fuels increase in size so do their dispersion forces and this means that there is a reduced ability for the fuel and the oxidant which is a gas to thoroughly mix and to allow for complete combustion a simple experiment we can use to compare between completely and incomplete combustion is to place small samples of an alkane and an alkene so in this case we've got hexane and hexene in some evaporating dishes we place a piece of filter paper above it and we essentially ignite them and allow them to combust and if we do that and we look at the resulting filter papers we can see that hexane can look something like this and hexane or more specifically hex one een can look like this you can see that hex one een has the March darker colored filter paper here indicating that it's produced more so that means that the degree of unsaturation can also influence incomplete combustion we could say that increase unsaturation increases the likelihood of incomplete combustion the reason for this is because the percentage of carbon is increased per mole or per molecule of our fuel so this place is an increased demand for oxygen with these particular molecules in regards to the issues of incomplete combustion we can start off and talk about carbon monoxide so we know that carbon monoxide is a strong binder to hemoglobin which is a protein that you find within red blood cells and we know that it's responsible for the transport of oxygen through your blood and through your bloodstream carbon monoxide essentially ends up being a stronger binder to hemoglobin then oxygen itself so what this will do is limit the ability for oxygen to be transported by this hemoglobin found on red blood cells in terms of carbon in the form of soot so we can often associate it with producing these kind of black flames and we could suggest that they are a form of visual pollutant they can settle onto the surface of different structures so buildings but also including plants and so if you imagine soot is being deposited onto their leaves then this is going to reduce their ability to photosynthesize and that might affect their growth one other thing that we can consider is the fact that carbon or soot has the ability to act as a effectively as a greenhouse gas in the sense that it can trap heat within the Earth's atmosphere and this graph is just showing you some of the information from this graph is showing you the impact that things like carbon dioxide as well as methane and black carbon also have in terms of this warming effect for the Earth's atmosphere so we can see it is really second to only carbon dioxide in this slide I've just summarized the undesirable consequences of incomplete combustion so on the Left we can see carbon monoxide binds strongly to hemoglobin in red blood cells limits oxygen transport and then the concerns with this is that it can lead to headaches dizziness loss of consciousness and potentially even death carbon in the form of soot we know as a visual pollutant it can decrease the rate of photosynthesis if we in how this it could actually aggravate our respiratory system and this is more prominent for people suffering from asthma and bronchitis the inhalation of soot can actually result in the inhalation of carcinogens which bind to those particles we know that black colored objects are extremely good at absorbing light radiation so the presence of carbon or soot in the atmosphere can enhance the absorption of light and then that can result in increased warming effect in the troposphere that concludes part 1 of this series of videos I'll see you guys in the next one