dipole moments

Dipole moments in molecules

If a molecule contains polar covalent bonds then there will be a bond dipole present, that is the molecule will contain atoms with partial charges. As long as the molecule is non-symmetrical then these bond dipoles will result in the molecule being a polar one. If the molecule is highly symmetrical it is likely the bond dipole will cancel each other out due to the geometry and shape of the molecule, this means that the molecule will be non-polar.

If a polar molecule is placed between two electrically charged plates they will align themselves up in such a way that the partially positively charged end of the polar molecule (δ+) will align with the negatively charged plate and the δ- end of the molecule will align itself with the positively charged plate. Non-pola molecules will simply arrange themselves in a random way if placed between two charged plates.

The overall net polarity of a molecule is called the dipole moment. It is given the Greek symbol µ (pronounced mu). The dipole moment of a molecule depends on the magnitude (size) of the charges at end end of the polar bond and on the distance between these two oppositely charged atoms in the polar covalent bond.
As an example consider the hydrogen halides: HF, HCl, HBr and HI; these are shown in the table below. You can clearly see that as the difference in electronegativity increases, which means a larger partial positive charge (δ+) on the hydrogen atom a negative partial charge (δ-) on the halide atom, then the dipole moment increases in size. It is worth mentioning that all these molecules are non-symmetrical and have permanent bond dipoles and therefore the molecules will be polar.

Substance structure difference in electronegativity dipole moment
hydrogen fluoride 4.0-2.1= 1.9 1.8
hydrogen chloride 3.0-2.1= 0.9 1.1
hydrogen bromide 2.9-2.1= 0.8 0.8
hydrogen iodide 2.6-2.1= 0.5 0.44

Calculating dipole moments


The dipole moment of a molecule is calculated using the formula:

µ= Q x r

where: Dipole moments are usually recorded in units of debyes(D), where 1 debye is 3.34 x10 -30 coulomb-metres. As an example consider an electron, charge 1.6 x 10-19 coulombs; moving a distance of 1 x 10-10 metres, which is around the length of a typical covalent bond. Then moving this amount of charge would create a dipole moment of:

Example

Hydrogen chloride is a polar molcule with a dipole moment of 1.1D, the H-Cl bond length is 1.27 x 10-10m. Calculate the size of the charge on the hydrogen and chlorine atoms in this polar molecule. So simply start with the equation above for calculating the dipole moment and re-arrange for Q.

We can easily extend this calculation to work out the % ionic character in the H-Cl bond:

Testing the size of the dipole moment in liquids.

You may have carried out a practical investigation in class where you can compare the size of the dipole moment present in various liquids simply by measuring how far they are deflected by a charged rod.
The set-up is very simple. The liquids under test are placed in a burette. The tap in the burette is opened and a slow stream of liquid is allowed to flow out. Next the plastic rod is charged by rubbing it with a dry cloth or fur rag. The rod will become either positively or negatively charged. The charged rod is then brought up to the slow flowing liquid from the burette and using a ruler you can estimate how far the liquid is deflected.

Deflection of polar liquids - an explanation

We can offer an explanation as to why some liquids will be deflected by the charged rod. In the example below the first 2 burettes are filled up with water. Water is a polar molecule with a dipole moment. This means that when the charged rod is placed near the stream of water, the water molecules will orientate themselves in such a way that the oppositely charged portion of the molecule is directed towards the charged rod. If you look at the diagram closely you will see that the water molecules are orientated differently in each of the first 2 water streams from the burette. This is simply because the charge on the rod in each example has been changed. In the final example, it does not matter whether you use a rod with a positive or negative charge. Tetrachloromethane is a non-polar molecule and will not be attracted to the rod, no matter what charge is on it.

Key Points

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