Video instructions and help with filling out and completing What Form 2220 Respectively

Instructions and Help about What Form 2220 Respectively

Professor Dave here let's talk about intermolecular forces Dave explains what is happening when a liquid boils wider different liquids boil at different temperatures to answer these questions we have to learn about intermolecular forces these are the electrostatic interactions between molecules so atoms within a molecule make covalent and ionic bonds with each other but molecules also participate in interactions with other molecules let's look at the different types first we have ion-ion interactions large ionic solids are held together by these networks of ionic bonds which are the strongest intermolecular force because they involve formal charges after that we have ion dipole interactions so first we must understand what a dipole is the covalent bonds in a water molecule are polar because oxygen is more electronegative than hydrogen and will pull the electrons in the bond towards itself because of the bent shape of the molecule when we combine these vectors we see water has an overall dipole or a side of the molecule with some electron excess and aside with electron deficiency dipoles can make electrostatic interactions because the partially negative side is attracted to positive charges and the partially positive side is attracted to negative charges so when sodium chloride dissolves in water the sodium ions make ion dipole interactions with the negative side of waters dipole and the chloride ions make ion dipole interactions with the positive side of waters dipole each ion can make several of these interactions which store a lot of energy which is why sodium chloride will dissociate in water in the first place next we have dipole-dipole interactions as you can guess this is when dipoles interact with each other as with pure water when in liquid form water molecules will move in such a way so as to always be making electrostatic interactions between the negative end of one dipole and the positive end of another dipole in this case these dipole-dipole interactions qualify for a special title hydrogen bonds this is when dipoles generated by NH o H or fh bonds interact with each other these are just especially strong dipole-dipole interactions they are especially strong because these are the most electronegative elements so they will create the most strongly polarized bonds resulting in a very strong dipole and therefore very strong dipole-dipole interactions we can almost think of partial charges as some fraction of a formal charge so the greater the partial charge the stronger the interaction though never quite as strong as interactions between formally charged particles lastly we have the Vander Waals or the London dispersion force these names refer to the same force and are completely interchangeable so I will arbitrarily refer to them as van der Waals forces this is the consolation prize of the intermolecular forces because any substance can do it only ions make ion-ion interactions and only covalent molecules with a dipole can make dipole-dipole interactions but absolutely anything can do van der Waals for example take a look at helium helium is a noble gas and due to a full valence shell it does not make bonds with other atoms so a sample of helium is just a bunch of helium atoms well the electron cloud around a helium atom will at any time be slightly lopsided or skewed towards one direction this will result in something called a momentary dipole this means one side of the atom is ever so slightly partially negative and the other side is slightly partially positive this is much weaker than a formal dipole but it still exists and can be measured if a momentary dipole approaches another atom it can generate an induced dipole meaning the slight partial negativity repels this electron density over to the other side of the atom so it will also have a slight dipole and then there can be a momentary dipole induced dipole interaction that is the van der Waals force this is a weak and fleeting attraction but this is all that mono atomic species and nonpolar covalent compounds can do and for very large molecules like some hydrocarbons the force can become significant so the ion ion force is strongest because it involves interactions between formerly charged particles ion dipole is next because it involves a formal charge and a partial charge then dipole dipole which is between partial charges and Vander Waals which is between tiny induced dipoles to see how intermolecular forces dictate phase change let's do a thought experiment first recall that a solids particles are rigidly packed and not moving a liquids particles are moving but they are still close together and interacting gaseous particles are moving and they are far away from each other so compared to liquids they basically don't interact so let's pretend we have three substances helium water and sodium chloride we will place them at zero Kelvin or absolute zero which is the lowest temperature possible a complete absence of heat energy where there is no energy available for motion here everything even helium is a solid in order to go from the solid to the liquid to the gas phase heat energy has to go into the sample and overwhelm the intermolecular forces that are occurring in a liquid there is some energy stored in electrostatic interactions and whatever amount that is that is precisely the amount of heat energy that has to be provided to liberate the molecules into the gas phase where they are not interacting and not storing energy because nature will not tend to go to a higher energy spontaneously so the energy still bored in these interactions has to be provided in some other way this means that the stronger the forces between the molecules the more heat energy we will have to provide to melt and boil the sample so let's take our three substances slowly raise the temperature and see what happens helium as it is only participating in incredibly weak van der Waals forces needs