Learn about Senior Chemistry, Energetics 5, in this comprehensive video by bannanaiscool.
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Rob Lederer: So if you add enough energy to the molecule, you can make a vibrate rotate translate all but you can also if you add enough energy, break apart the atoms from each other. Now we're talking about the energy that it takes to either make or break bonds. That's potential energy, that's bond energy. So Ep is talking about or Ep and else the potential energy, that is talking about bond energy. Now this bond was inside the molecule. Here is an atom, here is an atom. This bond is inside this molecule, within the molecule, and so this bond is called an intramolecular brand. Intra means within. So when you disturb an intramolecular bond, we are talking about the bond that keeps the atoms together in a molecule, but you know? Molecules can actually be attracted to one another to form solids and liquids. So that type of bonding is also significant. We don't have those sticks to put these together, but if you pretend that was Hcl and here is another molecule of Hcl, well because it's a polar molecule, that's from chemistry junior, then these two which have partially positive and negative regions can attract each other. When they do, you can make a liquid or solid. Now when you do that, there is a bond that's establishes. It's the force of attraction, that's what bond is. That's called an intermolecular bond because inter means between. When you play interscholastic sports, that's between other schools, but when you play intramurals, that's like playing games inside your school. So this force of attraction here between molecules is intermolecular, and it's very significant in terms of phase changes of solids, liquids, gases. So now we go to phase change. But before we do move onto phase change, we should talk about heat capacity first. Now the specific capacity is the amount of energy that can be absorbed or released by any substance with 1 gram of that substance to be able to raise it 1 degree celsius. So I think some of you are familiar that water has what we say is a high heat capacity compared to most substances at 4.19 joules to raise 1 gram of water 1 degree celsius, so heat capacities are going to be important for the formulas that we are going to be doing pretty soon. I wanted to touch upon this because actually it has something very clever to do with the kinetic energy definitions of vibrational, rotational, translational energies that we've talked about. Okay now, here is hydrogen and here is its heat capacity from any kind of data chart that you can find. So hydrogen has a specific heat capacity of 14.304 joules per gram degree celsius. Now we've written helium's here and neon's heat capacity here as well. Hydrogen has a higher heat capacity than either of these two. It seems like when the molecular masses increase of these chemicals, the heat capacities go down. Well you know what? We are not really comparing here apples to apples, if you know what I mean, because 1 gram of hydrogen actually does have in it more molecules of H2 than 1 gram of helium does; their molecular masses are different. So you can't compare specific heat capacities and really get any kind of information out of them but you can, if you take the molecular masses of the chemical, like hydrogen for instance at 2.02 grams per mole and you multiply it by that heat capacity, then you will get this. Now look, I do a -- analysis, unit cancellation, factor labeling, whatever you want to call it, it's cancelling units and it is the best way to be able to do any type of calculation. As a matter of fact, your teachers are going to be very - they are just going to really stress the fact that you need to cancel units all the time when you do calculations. So it is my class. So if you multiple a molar mass times a specific heat capacity, the grams cancel, and you are left with joules per mole degree celsius, which is now called not a specific capacity but a molar heat capacity. Now when you do the molar heat capacity calculation for hydrogen, you get 28.9.