ShmoopTube

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And here's your Shmoop du jour, brought to you by the force. [Girl using the force at a birthday party]

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Nifty party trick, but not a good explanation tactic on exams…

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Here's our question….

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The boiling point for a covalent compound can be predicted by understanding intermolecular

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forces. Which substance below exhibits the weakest intermolecular forces and possibly

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the lowest boiling point?

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Alright. Well, the question writer threw us a bone by telling us that the substance with [Men fighting over a bone]

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the weakest intermolecular forces will have the lowest boiling point.

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So all we have to do is figure out which substance has the weakest forces between its molecules.

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How do we do this?

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Use the forceeee…

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All intermolecular forces are ultimately just an attraction between a positive charge and [Positive and negative charge connected with love heart]

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a negative charge, but the type of molecule will dictate how much charge is involved.

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The more charge, the more attraction.

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There are essentially four strengths of charge interactions that we can rank.

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The strongest is ionic bonding, because it involves attractions between ions that have

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at least one full positive or negative charge. [Ions attract each other from sodium and chlorine]

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Another type of fairly strong interaction is a dipole interaction. A dipole interaction

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occurs when a molecule has a bond dipole between a very electronegative atom, and a less electronegative

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atom.

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This strong dipole creates a partial positive charge on the less electronegative atom and

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a partial negative charge on the more electronegative atom.

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The partial negative charges are attracted to the partial positive charges, making the [Example of normal dipole interaction]

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molecules stick together, but not as tightly as if they were full charges.

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So…maybe not the molecules you want on your side in a rumble… [Ionic bonds fighting a dipole]

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Any dipole in a molecule will do this, but hydrogen bonding is a special case in which

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these interactions are particularly strong. [Hydrogen bonds in a cup of water]

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A hydrogen bond is a dipole interaction that occurs in a molecule that has hydrogen atoms

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bonded to a very electronegative atom, typically oxygen or nitrogen like the example shown

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here.

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Here’s what’s on our list so far: There’s one last force to consider. Electrons

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in a molecule can move around and shift their charge density. This can cause more of the [Electrons moving around in a molecule]

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negative charges of electrons in a molecule to temporarily end up on one side of the molecule,

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leaving that side slightly negatively charged and the other side slightly positively charged

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for a brief instant in time. The negatively charged side of one molecule can attract the

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positively charged side of another, and the bigger the molecule, the greater the area

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over which this attraction occurs and the stronger it is. [Lots of molecules with negatively charged electrons]

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These attractive forces are very weak for one important reason:

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The unbalanced charge lasts less time than it takes to jump to hyperspace. [Spacecraft jumps through hyperspace]

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So here's our final lineup of forces:

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Now we need to figure out what forces are holding together the molecules in the question..

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We can get through this at light speed. [Spaceship appears]

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H2O and BF3

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both have dipoles, but H2O can make hydrogen bond, while BF3 sadly cannot.

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That means H2O has the strongest intermolecular forces, followed closely by BF3. [H2O rocket wins race ahead of BF3]

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Both CO2 and SeO2 only have London Dispersion Forces, but CO2 is smaller than SeO2, so that's that... Carbon

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is higher on the periodic table, therefore smaller than selenium, so the forces holding [Carbon and selenium circled on periodic table]

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it together are weaker.

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So to make a very long story short, the answer to the question is C. Small, but proud. [Darth Vader points to answer C]

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Much like a wise little green dude we know…

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The right answer, C is…used the force, we did… [Yoda makes Darth Vader disappear]

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