over the next few videos we'll be looking at organic chemistry structure and physical properties otherwise known as intermolecular forces and you'll be asked questions such as this one in your exam where you compare different types of compounds and explain why their melting points differ their boiling points or even something like their vapor pressure so you need to explain why there are these differences that we observe over here now first of all when I say physical properties what do I mean all of our organic compounds or compounds in general exhibit different physical properties so they have unique boiling points melting points vapor pressure densities and so on and the reason they have different boiling points melting points Vapor pressures and so on is something that we'll be discussing in this lesson but when I speak about physical properties this is what I'm talking about so before we get into why would they have different physical properties what about them makes them have different boiling points melting points Etc let's first focus on what is a boiling point a melting point and vapor pressure now these three terms that you see on the screen over here these are essential terms that you need to know for your metric exam and when I say you need to know it I mean you need to know the definition off by heart they ask these definitions at least one of them in almost every single chemistry exam and you need to give the definition almost word for word now boiling point melting point vapor pressure that's not a lot to learn out of the different physical properties these are the three that we'll be focusing on so yes we might touch on density in a practical or viscosity but these are the three that we are going to examine so what is a boiling point the boiling point is the temperature at which the vapor pressure of a substance equals the atmospheric pressure this is the definition that's what you're going to give word for word when you are asked to Define boiling point if I ask you the boiling point of pure water of pure H2O you would tell me that the boiling point is 100° C take note how you are stating the boiling point as a temperature boiling point is a temperature now a big mistake that often students make when they are stating boiling point is they say boiling point is the point at which a vapor pressure equals atmospheric pressure no boiling point is a temperature so you have to say the word temperature in your definition now what do they mean when the vapor pressure equals the atmospheric pressure when we heat a substance so in this example let's talk about water when we heat water what we're doing is we're giving more energy to the little water molecules they start to vibrate Foster in their F fixed positions and eventually you learned this in grade 10 the kinetic molecular theory they have energy to overcome their intermolecular forces that hold them together and it allows them to change phase from a liquid to a gas so that is what's happening when we boil a substance and the boiling point is the temperature that we measure when the vapor pressure equals the atmospheric pressure so you need to understand what is happening when we are boiling we have a liquid substance over here we provide heat the molecules over here they gain kinetic energy and they gain energy they get hotter and hotter and hotter then we reach a point where enough energy has been gained in order to enable the substance to change phase the energy is no longer causing the substance to increase in temperature but that energy is causing the intermolecular forces that is basically connecting the different molecules together for lack of a better way of saying it the intermolecular forces that's attracting those molecules together it's allowing those forces to be overcome and when they overcome when they are overcome they're no longer in a liquid phase we overcome those intermolecular forces and the particles can escape as gas particles becomes Vapor that's where vapor pressure comes from so the little particles build up here and that buildup of the vapor particles causes vapor pressure when the vapor pressure is equal to the atmospheric pressure which is about 1013 Kil pascals or one atmosphere so when the vapor pressure is equal to this this atmospheric pressure whatever temperature that is that is the boiling point and what's important for you to remember with boiling points and it makes sense is the stronger the intermolecular forces so that is the force of attraction between water molecules for example so the stronger the intermolecular forces the higher the boiling point because you need more energy more more more more more energy to overcome those intermolecular forces so the boiling point will be a lot higher and the weaker the intermolecular forces less energy is needed therefore lower boiling points here's the definition for melting point the temperature at which the solid and liquid phases of a substance are at equilibrium now in your exam you cannot tell me that the melting point is the temperature at which a solid turns into a liquid that is what is happening but according to exam guidelines that is not the official correct definition of melting point so yes the melting point is the temperature so for example for water pure water at 0° C what is happening at that temperature is that solid is turning into a liquid it's melting but this is the actual ficial definition okay those phases are at equilibrium and just the same with boiling point stronger the intermolecular forces higher melting points weaker the intermolecular forces lower melting points now linked to boiling point is this concept known as vapor pressure we spoke about Vapor as being gas so when a substance turns from a liquid from its liquid phase into its gas phase the liquid particles turn into gas particles into vapor and if you look at a closed system so closed container like this one over here when I heat the substance over here and the particles Escape into their gas phase remember those particles they attracted to one another by intermolecular forces when we add enough energy to overcome those intermolecular forces we basically are going through a phase change from liquid into gas so we're going to have Vapor gas particles and that Vapor gathers in the Clos system over here and it exerts pressure onto the liquid so vapor pressure is the pressure exerted by a vapor here's the vapor the gas particles at equilibrium with its liquid in a closed system now vapor pressure and boiling points do the opposite thing so if you have a high boiling point you have a low vapor pressure and if you have a high vapor pressure you have a low boiling point so opposites okay just think of it like that and the reason why is because the stronger your intermolecular forces okay very strong forces of attraction but between let's say your water molecules you need a lot of energy to overcome those intermolecular forces to go from liquid phase to gas phase so strong intermolecular forces difficult to convert essentially into gas phase you need a lot of energy so therefore there's not going to be a lot of gas there's not going to be a lot of vapor so there'll be a low vapor pressure so strong intermolecular forces not a lot of vapor low vapor pressure it makes sense weak intermolecular forces you don't need a lot of energy to overcome those intermolecular forces you can overcome very easily so there'll be a lot of vapor easy to go from liquid to gas phase a lot of vapor High vapor pressure okay what influences physical properties here are the four things essentially that influence physical properties and I'll be speaking about number one and number two cuz they basically kind of go together then three and four I'll do separate videos in all of these but just to give you an idea if we compare these two compounds that you see on the screen okay let's call it compound A and B I hope you can see that the difference between them is that this one is an cane only carbons and hydrogens single bonds and B is an alcohol so because they have different functional groups this one has a hydroxy functional group this one is just carbon single Bond carbon they have different homologous Series this one is an alkan this one is an alcohol what that means is because they have different functional groups and they're completely different homologous series they have different types of intermolecular forces this one has hydrogen bonding and London forces this one just Landon forces and because of those differences in intermolecular forces they have different boiling points melting points and Vapor pressures so this one B will have a higher boiling point melting point and a lower vapor pressure than a and I will get into this in more detail in its own video but just to show you it's type of intermolecular force number and type of functional group another thing that that can affect physical properties like boiling points Etc would be the structure of the compound so the number of carbon atoms or chain length or Surface area so compound a has a much shorter chain length three carbons in the main chain so it's propane versus B over here which has six carbons in the main chain much longer chain length bigger chain length bigger surface area and also a larger molecular mass although they are both alkanes they have the same type of intermolecular forces this one over here has more of the intermolecular forces so therefore it has stronger intermolecular forces again I I will go through chain length properly in more detail in its own video and then the next thing that affects physical properties also has to do with structure but it has to do with the shape of the chain or the degree of branching so this one here on the left is a straight chain alkane pentane no branching the one on the right although it has the same number of carbons so these two are chain isomers they're both C5 h12 both of them so they are isomers of one another but this one has three carbons in the main chain and it has branches so because of the degree of branching because of the shape that influences the physical properties okay so the less branched or the straighter the chain like this then this actually means that you have stronger intermolecular forces than something that looks like this but again I will do this in its own separate video but how do you answer questions on physical properties now I have the recipe that you can follow and if you follow this recipe and you do all the steps in the recipe all the time you will always get full marks for this question so let's take a look at the recipe the first step in the recipe is to State the homologous series and intermolecular forces of each compound so if I'm comparing a versus B you need to tell me the homologous series of a and therefore the intermolecular forces of a and then the homologous series of B and the intermolecular forces of B you must mention that both have London forces I will discuss this in the next video but everything has London forces then you must compare the compounds so maybe a has a shorter chain length length and B has a longer chain length or maybe B is more branched and a is less branched so what makes them different or maybe B has hydrogen bonding and London forces but a only has London forces so you're going to compare them what they have that's different and say which compound has stronger intermolecular forces or more intermolecular forces okay so one of them is going to win in terms of strength of intermolecular forces you have to mention strength so Point 2 is always strength then based on strength comes energy so the one that has stronger intermolecular forces needs more energy to overcome the intermolecular forces and then you're going to make your conclusion therefore it has a higher boiling point or lower boiling point or whatever the case is so this is essential and just a side note you are not allowed to abbreviate the word intermolecular forces you have to write it out in full intermolecular forces you can lose marks if you say IMF and another thing very important students of mine every year when I Mark lose marks if they don't mention strength and then they don't mention energy you have to follow the recipe to a tea it's like if you're baking a cake if you accidentally skip one of the steps in baking a cake your cake is going to be a flop okay you don't want to lose out on marks in this question so let's take a look at an example so ethane versus ethanol which has a higher boiling point and why so your first step is to state the homologous series and the intermolecular forces in both so ethane is an alkane that's the homologous series with only London forces that's the intermolecular forces ethanol is an alcohol it has polar it's a polar molecule with hydrogen bonding and London forces remember everything is London forces how I know that this one has London forces and this one has hydrogen bonding I'll go over in the next video but this is just to show you the recipe question so you mention the intermolecular forces and the homologous series of both then compare them so what makes them different so if you take a look at the picture they both have the same number of carbons basically it's not like this one is two carbons and this one is 10 carbons in the main chain but what makes them different is the functional group this is a carbon carbon double bond nothing else and this has got a o so hydrogen bonding in ethanol is stronger than London Oni forces you compare the compound and state which one has stronger intermolecular forces then based on that you will say because EOL has stronger intermolecular forces it needs more energy to overcome the intermolecular forces so you're going to speak about energy to overcome the intermolecular forces and while we're on this point over here you never ever ever ever say break we don't break bonds we're not breaking bonds we're not breaking into molecular forces that is not correct if you say that you will lose marks we are not breaking bonds a lot of students accidentally say that no we are overcoming or weakening intermolecular forces intermolecular forces as you should remember are very different to bonds intermolecular forces are between molecules and we never break intermolecular forces we overcome them so please make sure your wording is like mine and then because ethanol needs more energy to overcome the intermolecular forces it has a higher boiling point so there's your conclusion so I hope that this video has given you a good overview of this section of organic chemistry but please make sure to watch the sub videos because I'll be going over in a lot more detail about how we know how types of intermolecular forces affect physical properties and I go over pass papers we also look at chain length and go over pass papers and about the shape and degree of branching it's essential that you watch those videos watching this one only won't be enough to get you top marks I'll see you in the next one bye everyone