Name: Structure of Xenon-oxygen Compounds
Uploaded: 2019-07-19
Duration: 6 min 14 s
Description: Noble gases, also called inert gases because of their least reactivity, still react with one of the most reactive elements, i.e., oxygen, to form different c...

Question

The table lists the formulae and boiling points of some alkanes. Explain the trend in terms of instantaneous dipole-induced dipole forces.

Accepted Answer

The trend in these boiling points increases on going down table given; from methane down to butane, because the Instantaneous dipole-induced dipole forces of the respective alkane also increase down the table.

Instantaneous dipole-induced dipole forces (id-id forces) increase with:

increasing the number of electrons (and protons) in the molecule.

increasing the number of contact points between the molecules: contact points are places where the molecules come close together.

Down the table given, not only the number of electrons increases but also the number of contact points between the respective alkane molecules increases. The molecules in butane can line up beside each other. So there are many contact points in butane than that of the smaller alkane above it in the given table. Thus, the id-id forces are higher in butane. So the boiling point is higher in the case of butane.

The molecules of alkane with lesser carbon atoms than butane will be more compact than butane. The surface area available for coming into contact with neighbouring molecules will be smaller and hence, the id-id forces are relatively lower for smaller alkanes. So the boiling point is lower for alkanes with lesser carbon atoms. Thus, methane will have the least boiling point, while butane will have the highest boiling point with respect to the table given.

Note: Instantaneous dipole-induce dipole forces (id-id forces): The electron charge clouds in a non-polar molecule (or atom) are constantly moving. It often happens that more of the charge cloud is on one side of the molecule than the other. This means that one end of the molecule has, for a short time, more negative charge than the other. A temporary dipole is set up. This dipole can set up (induce) a dipole on neighbouring molecules. As a result of this, there are forces of attraction between the $δ^{+}$ end of the dipole in one molecule and the $δ^{-}$ end of the dipole in a neighbouring molecule. These dipoles are always temporary because the electrons clouds are always moving. Instantaneous dipole-induced dipole forces are sometimes called temporary dipole-induced dipole forces. These forces are also called London dispersion forces. It is the weakest intermolecular attractive force.

Hydrogen halide	HF	HCl	HBr	HI
Boiling point / °C	+20	–85	–67	–35

Hydrogen halide	HF	HCl	HBr	HI
Boiling point / °C	+20	–85	–67	–35

The table lists the formulae and boiling points of some alkanes. Explain the trend in terms of instantaneous dipole-induced dipole forces.

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