Topic 3 · page 14 of 15
Polar bonds and polar molecules
A bond can be polar while the molecule is not. A molecule is polar only if it has polar bonds and their dipoles do not cancel, so the shape decides. A jet of liquid near a charged rod tests this in the laboratory.
Read first: Electronegativity and bond polarity, Lone pairs and the shapes of NH3 and H2O
Polar bonds are not the same as polar molecules 3.15
A polar bond has unequal sharing of the bonding pair, so one atom is δ+ and the other is δ− (see electronegativity and bond polarity). A polar molecule has an overall separation of charge, with one side of the molecule δ+ and the other δ−.
A molecule can have polar bonds and still be non-polar. The effect of each bond is like a push in a particular direction. If the pushes are equal and in opposite directions, they cancel. If they do not cancel, the molecule has a net dipole and is polar.
In the figure, each teal arrow points along a bond towards the δ− atom. The orange arrow shows the net dipole of the molecule, where there is one.
Two questions decide each case:
- Are the bonds polar? (Is there a difference in electronegativity?)
- Is the shape symmetrical, so that the dipoles cancel?
| Molecule | Polar bonds? | Shape | Dipoles cancel? | Molecule is |
|---|---|---|---|---|
| HCl | yes | linear, two atoms only | no | polar |
| CO₂ | yes (C=O) | linear | yes | non-polar |
| H₂O | yes (O–H) | bent | no | polar |
| NH₃ | yes (N–H) | pyramidal | no | polar |
| BCl₃ | yes (B–Cl) | trigonal planar | yes | non-polar |
| CCl₄ | yes (C–Cl) | tetrahedral | yes | non-polar |
The same test works for molecules from the other pages:
- SF₆ (octahedral) and PCl₅ gas (trigonal bipyramidal) are symmetrical, so they are non-polar.
- NCl₃ has the pyramidal shape of NH₃, so it is polar.
- CH₄ is symmetrical, so it is non-polar. The C–H bond itself is almost non-polar.
- Cl₂ has a bond with no difference in electronegativity, so it is non-polar.
Worked exampleCCl₄ and NCl₃
Is a molecule of CCl₄ polar? Is a molecule of NCl₃ polar?
CCl₄: Cl is more electronegative than C, so each C–Cl bond is polar. C has 4 bonding pairs and no lone pairs, so the molecule is tetrahedral. The four dipoles are symmetrical and cancel, so CCl₄ is non-polar.
NCl₃: N has 3 bonding pairs and 1 lone pair, so the molecule is pyramidal. The bond dipoles do not cancel, because the shape is not symmetrical, so NCl₃ is polar.
Practical: jets of liquid and a charged rod 3.15
The deflection of a jet of liquid near a charged rod is a test for polar molecules.
PracticalJets of liquid and a charged rod
Method
- Fill a burette with the liquid, and run it into a beaker so that a thin, steady jet falls from the tap.
- Hold a charged rod close to the jet and watch whether the jet bends.
- Repeat with each liquid, using water, ethanol and cyclohexane, with the same rod and the same distance.
- (Optional) Repeat with a rod of the opposite charge.
Safety. Ethanol and cyclohexane are flammable. Keep naked flames away, and collect the liquids in a beaker under the burette.
| Liquid | Observation | Conclusion |
|---|---|---|
| Water | jet bends towards the rod | polar molecules |
| Ethanol | jet bends towards the rod | polar molecules |
| Cyclohexane | jet is not deflected | non-polar molecules |
Why a polar jet is attracted: polar molecules can turn. For a positive rod the δ− ends turn to face the rod, and for a negative rod the δ+ ends turn to face it. Either way the nearer end is attracted to the rod, so there is a net attraction and the jet bends towards the rod.
For the explanation in the exam, write the conclusion as well as the observation: deflected means polar, not deflected means non-polar.
Check yourself
- State what is meant by a polar bond and by a polar molecule.
- Explain why CO₂ is a non-polar molecule although it contains polar bonds.
- Explain why H₂O is a polar molecule.
- Predict, with a reason, whether each of BCl₃, NH₃ and CCl₄ is polar.
- In the jet experiment a rod is held near jets of water and of cyclohexane. State what is seen for each and what each shows.
- A student writes: "the jet of water bends because the water is positive, so it is attracted to a negative rod". Give two corrections.
Answers cover these until you have tried the questions
- A polar bond is a covalent bond in which the bonding pair is shared unequally because of a difference in electronegativity, giving δ+ and δ− ends. A polar molecule is one with an overall (net) dipole, because it has polar bonds whose dipoles do not cancel.
- The two C=O bonds are polar, because O is more electronegative than C. CO₂ is linear, so the two dipoles are equal and in opposite directions and cancel. There is no net dipole, so the molecule is non-polar.
- The two O–H bonds are polar. Water has 2 bonding pairs and 2 lone pairs, so it is bent. The dipoles do not cancel, so there is a net dipole and the molecule is polar.
- BCl₃: polar bonds, but the shape is trigonal planar and symmetrical, so the dipoles cancel and it is non-polar. NH₃: polar N–H bonds, and the shape is pyramidal (one lone pair), so the dipoles do not cancel and it is polar. CCl₄: polar C–Cl bonds, but the shape is tetrahedral and symmetrical, so the dipoles cancel and it is non-polar.
- The water jet bends towards the rod, showing that water has polar molecules. The cyclohexane jet is not deflected, showing that its molecules are non-polar.
- (1) Water molecules are neutral overall. They are polar, with δ+ and δ− ends, not positive. (2) The jet is attracted to either a positive or a negative rod, because the polar molecules turn so that their oppositely charged end faces the rod.