how can you tell if the net dipole moment of a molecule is zero? (polar or non-polar)

[mrh125]

cases like BF3 and PCl5 that are non-polar just confuse me and I know you can do it with trig and vectors (i proved that in gen chem) , but there's gotta be an easier and more efficient way to do that, help?

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[Cmdr_Shepard]

Generally you would need to know the molecular geometry (T-shape, see-saw, etc) in order to estimate whether it will be non-polar or polar. Look for radial symmetry in flat molecules (if all the bonds are the same) and that should indicate a net-dipole of zero. If the molecule is not flat, look for bonds that "cancel" each other out across lines of symmetry - anything remaining would contribute to a net-dipole.

Practice makes perfect!

 

[Czarcasm]

cases like BF3 and PCl5 that are non-polar just confuse me and I know you can do it with trig and vectors (i proved that in gen chem) , but there's gotta be an easier and more efficient way to do that, help?

Yeah, like @Cmdr_Shepard explained above, knowing the molecular geometry and shapes are essential to answering those polarity questions -- or even for those boiling point, melting point comparison questions -- which is related back to the same concept.

Most of the time however, questions typically involve the most frequent shapes: trigonal planar, linear, bent, tetrahedral, trigonal pyramidal, etc. but you do occasionally get the oddball question asking you to compare the polarity of less frequent molecular shapes (typically in physical sciences section). Usually the examples they provide are fairly obvious.

One other then to note is that Boron BF3 does not have a lone pair (it's one of the odd exceptions to octet), and so its molecular geometry is trigonal planar. In this case, the fluorine atoms are 120 degrees apart and all pulling with equal magnitude in 3 different directions. Because there is no net pull in any particular direction, BF3 is nonpolar. If however, you incorrectly assumed there was a lone pair present in BF3, then you might assume it occupies a tetrahedral geometry, in which case there would indeed be a net force in one direction. So knowing how many bonds and lone pairs an atom has is also important, but this is essentially related back to the basics of molecular geometry.

Hope this helps.

 

[dudewheresmymd]

Yeah, like @Cmdr_Shepard explained above, knowing the molecular geometry and shapes are essential to answering those polarity questions -- or even for those boiling point, melting point comparison questions -- which is related back to the same concept.

Most of the time however, questions typically involve the most frequent shapes: trigonal planar, linear, bent, tetrahedral, trigonal pyramidal, etc. but you do occasionally get the oddball question asking you to compare the polarity of less frequent molecular shapes (typically in physical sciences section). Usually the examples they provide are fairly obvious.

One other then to note is that Boron BF3 does not have a lone pair (it's one of the odd exceptions to octet), and so its molecular geometry is trigonal planar. In this case, the fluorine atoms are 120 degrees apart and all pulling with equal magnitude in 3 different directions. Because there is no net pull in any particular direction, BF3 is nonpolar. If however, you incorrectly assumed there was a lone pair present in BF3, then you might assume it occupies a tetrahedral geometry, in which case there would indeed be a net force in one direction. So knowing how many bonds and lone pairs an atom has is also important, but this is essentially related back to the basics of molecular geometry.

Hope this helps.

Does carbon dioxide have dipole-dipole forces? If there is no net dipole in CO2, can dipole-dipole IMFs exist?

 

[DrknoSDN]

Does carbon dioxide have dipole-dipole forces? If there is no net dipole in CO2, can dipole-dipole IMFs exist?

CO2 is non-polar and it does not have dipole-dipole forces.
It does have momentary or instantaneous dipoles that are created for fractions of a second allowing for intermolecular van der waals forces.

Even non-polar alkanes have van der waals IMF's. That is the major reason longer chain alkanes have a higher boiling point. The IMF's are stronger as chain length increases but much weaker than IMFs between polar molecules.

 

[Czarcasm]

Does carbon dioxide have dipole-dipole forces? If there is no net dipole in CO2, can dipole-dipole IMFs exist?

The distinction you want to make is between polar BONDS and polar MOLECULES. A molecule can have polar bonds (a dipole) however, collectively, the molecule itself can be non-polar. This is the case for CO2. Individually, both oxygen atoms are more electronegative than carbon and there's a net dipole (pull in either direction). However, because they are both C=O bonds attached to the same carbon, it is a planar molecule and therefore each bond pulls with equal magnitude and opposite direction. This is why it's a nonpolar molecule.

Dipoles are vectors and just like in Physics, the geometry of the structure needs to be considered to determine if the forces cancel each other out. This is the essense of determining molecular polarity.

 

[dudewheresmymd]

The distinction you want to make is between polar BONDS and polar MOLECULES. A molecule can have polar bonds (a dipole) however, collectively, the molecule itself can be non-polar. This is the case for CO2. Individually, both oxygen atoms are more electronegative than carbon and there's a net dipole (pull in either direction). However, because they are both C=O bonds attached to the same carbon, it is a planar molecule and therefore each bond pulls with equal magnitude and opposite direction. This is why it's a nonpolar molecule.

Dipoles are vectors and just like in Physics, the geometry of the structure needs to be considered to determine if the forces cancel each other out. This is the essense of determining molecular polarity.

So to clarify if a molecule has NO NET dipole it does not have dipole dipole IMF as in the case of co2?molecules like nitrogen gas and oxygen gas do they also have no net dipole?

 

[Czarcasm]

So to clarify if a molecule has NO NET dipole it does not have dipole dipole IMF as in the case of co2?molecules like nitrogen gas and oxygen gas do they also have no net dipole?

Well no. To have a intramolecular dipole would mean that there's a difference in electronegativities. In some cases, these differences are small (C-H bonds) and can be ignored. But in other instances, they are considerable difference, such as in a C=O bond. To have NO intramolecular force or dipole would essentially mean the electronegativities of all bonded atoms are the same/equal. N2 gas (N triple bonded to N) and O2 gas (O double bonded to O) are both non-polar because they have the same electronegativities (there is no net pull towards either atom). In this instance, you wouldn't even need to consider the geometry of the molecule.

Also consider CCl4 which has 4 polar C-Cl bonds. However, because they are dipoles pulled in equal and opposing directions (it's has tetrahedral geometry), the molecule overall is nonpolar. So you can have a nonpolar molecule that internally has intramolecular dipoles due to differences in electronegativites (but cancel each other out). You can also have a nonpolar molecule that has no internal dipoles (examples: O2 and N2). And on the other extreme, you can have a polar molecule with polar bonds (ex. H2O) because these dipoles don't cancel each other out.

 

[dudewheresmymd]

Well no. To have a intramolecular dipole would mean that there's a difference in electronegativities. In some cases, these differences are small (C-H bonds) and can be ignored. But in other instances, they are considerable difference, such as in a C=O bond. To have NO intramolecular force or dipole would essentially mean the electronegativities of all bonded atoms are the same/equal. N2 gas (N triple bonded to N) and O2 gas (O double bonded to O) are both non-polar because they have the same electronegativities (there is no net pull towards either atom). In this instance, you wouldn't even need to consider the geometry of the molecule.

Also consider CCl4 which has 4 polar C-Cl bonds. However, because they are dipoles pulled in equal and opposing directions (it's has tetrahedral geometry), the molecule overall is nonpolar. So you can have a nonpolar molecule that internally has intramolecular dipoles due to differences in electronegativites (but cancel each other out). You can also have a nonpolar molecule that has no internal dipoles (examples: O2 and N2). And on the other extreme, you can have a polar molecule with polar bonds (ex. H2O) because these dipoles don't cancel each other out.

got it. thanks. so how would you compare dipole dipole strength of something like CCl4, CO2, and Hcl in terms of boiling points etc?