Colourful Solutions > The covalent model > Polar molecules

IB Chemistry Shop

Standard level

Vectors are quantities that have both magnitude and direction. They can add up, or cancel out, depending on orientation. Dipoles are vector quantities.

Syllabus ref: S2.2.6

Structure 2.2.6 - Molecular polarity depends on both bond polarity and molecular geometry.

  • Deduce the net dipole moment of a molecule or ion by considering bond polarity and molecular geometry.

Guidance

  • Examples should include species in which bond dipoles do and do not cancel each other.

Tools and links

  • AHL Structure 3.2 - What features of a molecule make it “infrared (IR) active”?

Resolution of dipoles

Dipoles are similar to forces in that they can be resolved, added and cancelled out. If the dipoles within a molecule do not cancel out, then the molecule itself is polar, i.e. there is a region of positive charge within the molecule and another region of negative charge.

The carbon = oxygen bond is polarised due to the difference in electronegativity between carbon and oxygen (2.5 and 3.5 respectively).

There is a horizontal dipole on the left of the molecule, but there is also another horizontal dipole with the same magnitude, but the opposite direction on the other side of the molecule.

Both of these dipoles cancel out, leaving a molecule that is non-polar.



water

The polarity of water

Water is a polar molecule in which the dipoles do not cancel out. This is because of the shape of the water molecule.

Although the O-H bonds are both equally polar they are not exactly opposite one another and cannot cancel out. In fact they cancel out in the horizontal plane (as drawn), but not in the vertical.

The dipoles in the water molecule can be resolved into horizontal and vertical components (green bars).

The horizontal components are equal in magnitude, but opposite in direction and cancel out.

The verticle dipoles are equal in magnitude and act in the same direction and so they add up. The molecule is polar overall, with a vertical dipole, negative at the top (near the oxygen) and positive at the bottom (between the two hydrogens).

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Polarity of molecules

The polarity of a molecule can be determined by firstly considering the relative electronegativities of the component atoms to find polarised bonds (dipoles), then inspecting the molecule to see if the individual dipoles cancel out. It is important to remember that the symmetry of a molecule is important in this respect.

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Consequences of polarity

Polar molecules are 'sticky' in that they are more attracted to one another. They behave as if they are tiny magnets, although the force between them is electrostatic, not magnetic.

The more polar they are the greater the degree of intermolecular force. This is covered in greater detail in subsequent sections.

The polarity of liquids can be easily demonstrated using a stream of liquid from a burette and approaching it with a charged electrostatic rod. The stream of liquid bends towards the rod if it is polar.

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Worked examples

Q232-01 Which of the following molecules is polar?
  1. CH4
  2. CF4
  3. NF3
  4. CO2
Answer

CH4 is tetrahedral and totally symmetrical - non-polar

The C-F bonds in CF4 are dipoles, but once again the molecule is totally symmetrical - non-polar.

NF3 has three polarised bonds and the molecule is a trigonal pyramidal shape. The nitrogen atoms is 'higher' than the relatively negative fluorine atoms. NF3 is a polar molecule with a vertical dipole (as shown).
Q232-02 Which of the following bonds will have the greatest polarity?
  1. C-H
  2. C-N
  3. C-O
  4. C-C
Answer

The largest dipoles occur when there is the greatest difference in electronegativity between atoms. In this case, carbon (electronegativity = 2.5) and oxygen (electronegativity = 3.5) have the greatest difference in electronegativity.

Answer = C-O

Q232-03 Which combination of atoms can form a polar covalent bond?
  1. H and H
  2. H and Br
  3. N and N
  4. Na and Br
Answer

Polar covalent bonds form when there is a significant difference in electronegativity between atoms that are bonded together. In this question there is a significant difference in electronegativity between H (2.1) and Br (2.8). There also is a large difference between Na and Br but in this case they form an ionic bond. Therefore the answer is H and Br
Q232-04 Which of the following is/are polar?
  1. I. CH2Cl2
  2. II. CHCl3
  3. III. CCl4
  1. I, II and III are polar.
  2. I and II are polar.
  3. II and III are polar.
  4. I is the only polar molecule.
Answer

Of the three molecules given, only CCl4 is completely symmetrical and therefore non-polar.

The correct response is I and II only
Q232-05 Which molecule is non-polar?
  1. NH3
  2. BH3
  3. H2S
  4. HI
Answer

The only symmetrical molecule in which all of the dipoles of the bonds cancel out is BH3 (trigonal planar)
Q232-06 Which of the following is the best example of a non-polar molecule containing polar bonds?
  1. F2
  2. SO2
  3. CS2
  4. PCl3
Answer

For the molecule to be non-polar, but contain polar bonds it must be totally symmetrical with the polar bonds cancelling each other out.

This is the case for carbon disulfide, CS2. The C-S bonds are weakly polarised, but the molecule is linear (similar to carbon dioxide) and the two dipoles cancel out.
Q232-07 The neutral molecule XCl3 is found to have zero dipole moment. The element X is most likely to be?
  1. B
  2. N
  3. P
  4. I
Answer

For the XCl3 molecule to be totally symmetrical it must be trigonal planar.

Hence the element X must only have three electrons in its valence shell.

Therefore the correct answer is Boron
Q232-08 Which molecule is non-polar?
  1. SO3
  2. SO2
  3. CO
  4. H2O
Answer

Requirements for non-polarity are symmetry, or non-polar bonds.

In this case the molecule sulfur trioxide, SO3 is totally symmetrical, trigonal planar.
Q232-09 Which molecules would be expected to have a significant dipole moment?
  1. 1. SOCl2
  2. 2. ICl
  3. 3. FCl
  4. 4. BCl3
  1. 1 and 2
  2. 3
  3. 1, 2, and 3
  4. all of them
Answer

Of the choices given, only 4 is symmetrical and therefore all of the others have significant dipole moments. Therefore the correct response is 1, 2, and 3
Q232-10 Which molecule is polar?
  1. CO2
  2. SF6
  3. BCl3
  4. SF4
Answer

All of the molecules are symmetrical except SF4, which has an expanded octet (HL only).

Therefore the correct response is sulfur tetrafluoride, SF4.

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